Measuring Aperture Locations in the HST Focal Plane
DR Colin Cox(Space Telescope Science Institute)
A knowledge of the positions of the HST cameras to within a few arc-seconds is clearly needed for targeting, but a more accurate knowledge is valuable for several reasons. First, the COS aperture is only 2.5 arcseconds in diameter and so requires accurate pointing to be sure not to miss a target. Additionally, multi-mission archives, multi-wavelength campaigns, coordinated observations and image mosaic generation all benefit by having the most accurate possible positions. Normal HST operations rely on the guide star catalog, (GSC2) in which stellar positions are known relatively to about 0.18 arc-seconds across an HST field of view. To obtain more accurate positions, we use as a target the open cluster M35 and an associated HST PASSOPS catalog. This catalog contains positions and proper motion measurements leading to a positional accuracy of 30 mas. By using stars from this catalog for both guiding and target we can expect to define each position to within about 50 mas. For the larger apertures we measure several stars simultaneously, statistically obtaining a further improvement in accuracy and coarsely tracking orientation. Since SM4 we have measured aperture locations for ACS, STIS, WFC3 and COS and show that positions are stable to within a few tenths of an arc-second and that, in general, overall pointing and astrometry for HST is dominated by GSC2 catalog errors. COS exhibits a trend in position not unusual for a newly flown instrument. Nevertheless we have established the position of the COS aperture well enough so that, for many observations, a blind pointing may be used in place of the full target acquisition procedure.
Dither and drizzle strategies for Wide Field Camera 3
MR Max Mutchler (Space Telescope Science Institute)
Observing strategies and data reduction methods for the ultraviolet-visible (UVIS) channel of Hubble's Wide Field Camera 3 (WFC3) inherit much from the Advanced Camera for Surveys (ACS), since they are similar cameras. The infrared (IR) channel of WFC3 is an extraordinary new capability for Hubble, and there is less conventional wisdom established for it's usage. The author presents the relevant issues and strategies for obtaining optimal results from both the UVIS and IR channels of WFC3. This is based on experience with the Early Release Observations (ERO), the Early Release Science (ERS) program, Hubble's 20th Anniversary mosaic of HH901 in the Carina Nebula, and moving-target Solar System observations. The author will further illustrate these topics, and assist users with their own HST data sets, in the MultiDrizzle course being offered on the Tuesday afternoon before the HST Calibration Workshop.
COS FUV Flat Fields and Signal-to-Noise Characteristics
DR Thomas Ake (STScI/CSC)
The COS FUV channel employs a detector comprised of two microchannel plate (MCP) segments with cross delay line anodes. The detector shows several types of non-uniformities due to the hexagonal and moire patterns in the MCPs, dead spots, gain variations, and shadows from the wire grid installed in front of the MCPs to increase quantum efficiency. These features induce fixed-pattern noise in FUV spectra. The effects of these artifacts can be reduced by dividing the data by a flat field and combining exposures taken at different grating settings. A spectral iteration technique, similar to that used for GHRS and FOS, shows that S/N > 100 can be achieved in extracted spectra. Although flat field observations were obtained during SMOV using white dwarfs, a two dimensional flat field of sufficient quality for standard CALCOS processing was not achieved. Other methodologies are being explored for flat field correction and are expected to be installed in CALCOS to improve the S/N of data incrementally. As an initial step, CALCOS currently ignores grid wire regions when creating a summed spectrum from exposures taken at different FP-POS positions. Average one-dimensional flats, first removing the grid wires, then taking out the other fixed-pattern noise, are being investigated to correct individual exposures. These may require separate flat fields for different cross-dispersion locations. Two-dimensional flats will be examined to determine if additional observations are warranted.
COS Data Processing Improvements Based on HST SMOV Results
After the Cosmic Origins Spectrograph (COS) was installed onboard the Hubble Space Telescope (HST) in May 2009, it underwent an extensive calibration and characterization check-out during the Servicing Mission Observatory Verification (SMOV) period. The results from this program were used to update reference files and make changes to CALCOS, the COS data processing software. Improvements to the standard data products will be discussed. For the FUV channel, we have begun development of a flat field correction. As an intermediate step, grid wire shadows are now ignored when combining FP-POS exposures. Pulse-height filtering has been activated to reduce background features. For the NUV channel, vignetting corrections are incorporated in the flat field file. For both channels, improvements were made in the wavelength scales, flux calibration, and data quality flagging. Additional data are included in the FITS products to allow users to perform customized processing.
Removing the herring-bone pattern-noise from all HST/STIS Side-2 CCD data: a factor ~3 enhancement in sensitivity
DR Rolf Jansen (ASU, School of Earth & Space Exploration)
When STIS resumed operations in July 2001 using its redundant "Side-2" electronics, the read-noise of the CCD detector appeared to have increased by ~1 e- due to a superimposed and highly variable "herring-bone" pattern-noise. For the majority of programs aiming to detect signals near the STIS design limits, the impact of this noise is far more serious than implied by a mere 1 e- increase in amplitude of the read-noise, as it is of a systematic nature and can result in ~8 e- relative deviations (peak-to-valley). On this poster, I will summarize a method to cleanly remove this pattern-noise from raw STIS CCD frames (Jansen et al. 2002; Brown 2001), report on an Archival Calibration Legacy program to (semi-)automatically remove the herring-bone pattern-noise from *all* raw, un-binned, Side-2 STIS/CCD frames taken between Jul 2001 and Aug 2004, and present trends in characteristics of the noise pattern. This work was funded by grant HST-AR-11258 from STScI, which is operated by AURA under NASA contract NAS5-26555.
An insight on the Gaia BP/RP and G-band flux calibration
XX Elena Pancino (INAF-OABO)
I will quickly present the Gaia mission, focussing on those technical aspects that are necessary to understand the details of its external (absolute) flux calibration. On board of Gaia there will be two (spectro)photometers, the blue one (BP) and the red one (RP) covering the range 330-1050 nm, and the white light (G-band) imager. Given the fact that the focal plane of Gaia will be constituted by 106 CCDs and the sources will cross the the focal plane at constant speed, at different positions in each of the foreseen passages (on average 80, but up to 350) in the mission lifetime, the "simple" problem of calibrating the integrated BP/RP and G-band magnitudes and the low resolution BP/RP spectra flux turns into a very delicate and complicated issue, including CTI effects, LSF (the linear version of the PSF) variations across the focal plane and with time, CCD gating to avoid saturation and the like.
JWST Science Operations: the Phase I & II Process
DR Tracy Beck (STScI)
The Space Telescope Science Institute will serve as the main Science and Operations Center for the James Webb Space Telescope, which is a segmented 6.5m telescope that will operate in deep space at L2. There are some telescope characteristics that will make observing with JWST different from using the Hubble Space Telescope. In this talk, I will highlight some of the operational observing constraints on JWST. I will also summarize the plans for the JWST proposal Phase I and Phase II process, including the use of templates to request science observations using the Astronomer's Proposal Tool (APT).
New NICMOS Flat-fields
XX Deepashri Thatte (Space Telescope Science Institute)
The A-to-D conversion, or gain, of the NICMOS detectors depends on a combination of detector temperature and bias voltage. The response of the three NICMOS detectors changed slightly due to changes in the A-to-D conversion over both the era before the installation of the NICMOS Cooling System (NCS) 1997-1998 and after the installation 2002-2008. This change can been expressed as a change in an effective temperature (or equivalently A-to-D conversion) referred to as biastemp, which is calculated directly from the bias level of the science exposures. Here we discuss the creation of new flat-field reference files used to correct for changes in the structure of flat- fields caused by the change in A-to-D conversion. These reference files (*_tdf.fits) populate the keyword TDFFILE in the headers of pre-NCS data retrieved from the OTFR after January 23, 2009 and post-NCS data retrieved after November 19, 2008 and are used by the FLATCORR step in calnica version 4.4 and later. Each *_tdf.fits file consists of five different extensions each valid at a different biastemp range. During calibration, the pipeline calculates the biastemp of the science image and then uses the flat-field extension with the best matching biastemp. For consistency check we also created epoch dependent flat-fields for post-NCS data and compared them with the biastemp dependent flat-fields.
HST Cycle 19 Exposure Time Calculators
DR Rosa Diaz (STScI)
The Exposure Time Calculator (ETC) is a web-based application that assists users in calculating the exposure time needed for their HST observations, or the Signal-to-Noise Ratio (SNR) they can attain with a given HST observing time. These quantities are key for the preparation of proposals and observations during Phase I and Phase II of the proposing cycle and therefore have to be sufficiently accurate for each of the supported observing modes of all the HST instruments. Developing a general tool that shares communality among the different instruments is complicated, not only form the point of view of attaining accuracy of the calculations but also regarding reliability, portability, and maintainability. We are currently in the final stages of the development of the new ETC for Cycle 19 in Python. This new version improves these qualities and meets the needed level of reliability for the ETC. It also provides a basis for JWST Exposure Time Calculators; in preliminary development, with a limited-functionality prototype to be available in Spring 2011. This poster and demo shows the new tool, its improvements over the previous ETC and the current status of the new version.
NICMOS Reference Files in the NCS Era, 2002-2008
DR Tomas Dahlen (STScI)
During the era after the installation of the NICMOS Cooling System (NCS) 2002-2008, the NICMOS calibration reference files have been regularly monitored to examine their stability over time. This includes repeated observations of dark frames, flat-fields images, and photometric standard stars. Over the seven-year span, a slight change in the A-to-D conversion, or gain, has been detected. This change can been expressed as a change in an effective temperature (or equivalently A-to-D conversion), referred to as biastemp, which is calculated directly from the bias level of the science exposures. The most probable cause for this behavior is a change in the detector bias voltage with time since the installation of the NCS. It is possible to estimate the A-to-D conversion at the time of observation for each science image by calculating the biastemp. Thereafter, reference files that are valid at biastemp matching that of the observations can be used. In this talk I will quantify how the change in A-to-D conversion has affected the reference files and describe how corrections have been implemented in the NICMOS calibration software calnica to account for this effect.
The Space Telescope Imaging Spectrograph After SM4 Repair
DR Charles Proffitt (STScI/CSC)
The Space Telescope Imaging Spectrograph (STIS) was originally installed in the Hubble Space Telescope (HST) in February of 1997. In May of 2001, the primary Side-1 electronics failed, but STIS continued operations using the redundant Side-2 electronics until August of 2004, when an electrical malfunction in a power supply forced cessation of operations. On May 17, 2009, during the fourth EVA of SM4, astronauts Michael Good and Mike Massimino replaced the STIS LVPS-2 circuit board containing the failed component, successfully repairing the Side-2 electronics and returning STIS to operations. STIS after this 2009 repair operates in much the same way as it did during its previous 2001-2004 period of operations with the Side-2 electronics. Internal and external alignments of the instrument are very similar to what they had been in 2004, and most changes in performance are modest. The STIS CCD detector continued to experience radiation damage throughout the hiatus in operations, leading to decreased charge transfer efficiency and an increased number of hot pixels. The sensitivities for most modes are surprisingly close to that expected from simple extrapolation of the 2003-2004 trends, although the echelle modes show somewhat more complex behavior. The biggest surprise was that the dark count rate for the NUV MAMA detector after SM4 has been much larger than had been expected; it is currently about three times bigger than it had been in 2004 and only slowly decreasing. We discuss how these changes will affect science with STIS now and in the future.
The On-Orbit Performance of the Cosmic Origins Spectrograph
DR Alessandra Aloisi (Space Telescope Science Institute)
The Cosmic Origins Spectrograph (COS) was installed on board the Hubble Space Telescope (HST) in May 2009 as part of the most recent servicing mission 4, and is a third-generation instrument that has significantly extended HST UV spectroscopic capabilities. This contribution highlights the current instrument performance, with particular emphasis on the initial on-orbit characterization during the Servicing Mission Observatory Verification and the subsequent calibration during Cycle 17. Plans for future improvements in the COS on-orbit calibration are also included.
Observing with HST below 1150 Å: Extending COS/G130M and G140L coverage to 905 Å
XX Steve Osterman (CASA)
The FUV channel of the Cosmic Origins Spectrograph is designed to operate between 1150Å and 1800Å, limited at shorter wavelengths by the reflectivity of the MgF2 protected aluminum used on the optical telescope assembly and on the COS FUV diffraction gratings. However, because the detector for the FUV channel was windowless, it was recognized early on that there was the possibility that COS would have some sensitivity at shorter wavelengths due to the first surface reflection from the MgF2 coated optics. Preflight testing of the G140L grating suggested on order 5% efficiency at 1066Å, and early on-orbit observations verified that COS was sensitive down to at least the Lyman limit with 10-20 cm2 effective area between 912Å and 1070Å, and rising rapidly to over 1000 cm2 beyond 1150Å. We have also explored narrower band observations using the G130M grating out of band for coverage from 1020Å to 1170Å and from 905Å to 1055Å. We present ray trace simulations and calibration results for these observing modes, and explore alternative configurations to increase resolution and signal to noise.
SNDICE, a calibrated multi-wavelength light source for optical telescope calibration with a stability and a precision of 10-4
DR Kyan Schahmaneche (LPNHE-IN2P3-Paris 7 University)
The need for a precise photometric calibration has grown over the last few years. Several projects based on high precision photometry, like the measurement of the dark energy equation of state using type Ia supernovae have now reached a point where the photometric calibration must be better than the one percent. To achieve such a precision, usual astrophysical calibration procedures relying on star observations are limited by the knowledge of the emission spectra of the reference standards (Vega or the HST white dwarf calibrators). More precisely, as the cosmological parameters are measured comparing fluxes of nearby supernovae to fluxes of distant highly redshifted supernovae, the key-point is the inter-calibration between different bandwidths. Instrumental devices have been developed to study, monitor and model the transmission curves of the astronomical detector (telescope + camera) with an precision of the order of one per-thousand. Such instrumental devices are a first step in the setup of a new calibration procedure for astrophysical photometry measurements. In this paper, we present such a device called SNDICE (SuperNovae Direct Illumination Calibration Experiment) based on the concept of the direct illumination of the instrument by LEDs whose emitted light can be controlled with an precision of 10-4. The opportunity offered by the progress of LED technologies for supporting CCD photometry was underlined in a first paper which described SNDICE-type systems. The light beams emitted by these LED are monitored by photodiodes located along the path of light and the all calibration device has been first calibrated on test bench with respect to a NIST photodiode. The SNDICE project has started in the Spring 2007 and was installed in January 2008 at the Canada France Hawaii Telescope (CFHT) in Hawaii in order to complement the study of the photometric response of the extra wide field camera MegaCam in the SNLS experiment. We present here first results obtained with this prototype.
The JWST Tunable Filter Imager and its Calibration
DR Andre Martel (STScI)
The JWST Tunable Filter Imager (TFI) is a sensitive camera that shares the optical bench of the Fine Guidance Sensor (FGS). It provides JWST with the capability of imaging a 2.2'x2.2' field-of-view through narrow-band filters over a wavelength range of 1.5 – 5.0 um, with a non-functional region between 2.5 and 3.2 um. The narrow-band filters are characterized by spectroscopic resolving powers of ~75 to ~120 for central wavelengths selected by the user with the aid of a tunable filter. The TFI is also capable of high-contrast imaging with a choice of four coronagraphs and a non-redundant mask. Here, we provide a brief summary of the technical specifications and scientific capabilities of the TFI and present an overview of the upcoming ground calibration and on-orbit commissioning activities.
WFC3 UVIS and IR channel flat fields
DR Elena Sabbi (STScI)
Flat fielding is a standard calibration of step for astronomical data, which allows us to correct for variations in the local response of a detector and improve the accuracy of photometric analysis. As for ACS, ground based flat-fields (Sabbi et al., ISR 2008-46, Bushouse, ISR 2008-28), acquired during the last thermal vacuum campaign (TV3), are, and will be, the base to remove the pixel-to-pixel variations in the WFC3 data. Tests performed during the Servicing Mission Observatory Verification (SMOV4), that followed the installation of WFC3 on Hubble, indicate that ground based flat-fields do not completely remove local variation in the response of the detector, but that low-frequency structures are still present on both the UVIS (Sabbi., ISR 2009-20) and IR (Hilbert et al., IRS 2009-93) data. As part of the standard calibration of WFC3 we have used observation of the rich globular cluster Omega Centauri to compare the flux of the same stars on different position of the detector and derive a correction to remove the remaining low-frequency structure. Here we will present the characteristics of the ground-based (better knows and P-flat) and the low-frequency (or L-flat) flat fields, how these files are created and their impact on the final astronomical data. At the end we will also present a summary for the future observational plan to further improve the quality of the WFC3 flat-fields.
COS Sensitivity Trends in Cycle 17
DR Rachel Osten (STScI)
After the initial on-orbit determination of the absolute flux calibration of the Cosmic Origins Spectrograph was performed, we have been monitoring the instrument's spectroscopic sensitivity regularly. Observations of spectroscopic white dwarf standard stars have shown a decline in sensitivity in the bare Aluminum gratings G225M and G285M on the NUV channel, while the gratings with Al+MgF2 coatings (G230L and G185M) appear stable. The change in sensitivities of the G225M and G285M gratings are weighted means of -3.1 and -11.0 percent per year, respectively,while for the G230L and G185M they are -0.4 and -0.6 percent per year, respectively. The bare Al gratings exhibited sensitivity degradation on the ground; the rates of degradation were about 1.6% per year for the G225M and 4.5% per year for the G285M grating, measured during semi-annual grating efficiency tests. The trends seen in external targets appear to be wavelength-independent but grating dependent.All gratings on the FUV channel are experiencing wavelength-dependent sensitivity degradation, which is worse at longer wavelengths. Similar results are returned for the same wavelength ranges falling on different detector segments, and also made with different gratings and external targets. Below 1250 Angstroms, the sensitivity is declining at 5.6 percent per year, and from 1700--1800 Angstroms the decline is a weighted mean of 10.8 percent per year. The sensitivity changes on the NUV and FUV channels appear to be uncoupled. The sensitivity decline of the NUV bare Al gratings appears to be caused by continued growth of an oxide layer, either from additional deposition of atomic oxygen in space or migration of existing atomic oxygen in the system to the outside. Detector QE loss due to localized exposure of the FUV cross-delay line detector to UV irradiation can be ruled out by several tests which examine the sensitivity decline versus total counts in specific regions of the detector. The wavelength dependence of the FUV sensitivity degradation is the opposite sense for contamination to be an issue. The source of the FUV sensitivity decline does not appear to be due to shifts in the pulse-height distribution from gain sag, as such shifts are not observed (and would not explain the magnitude of the decrease). The loss of quantum efficiency shows the classic signature of photocathode degradation. I will present the latest results from sensitivity monitoring and discuss investigations of the observed trends.
WFC3 Image Calibration and Reduction Software
DR Howard Bushouse (STScI)
Standard WFC3 image processing consists of the calwf3 task, which removes instrumental signatures from the images, and multidrizzle, which corrects images for geometric distortion and combines dithered sets of exposures. The status and future plans for calwf3 and multidrizzle will be discussed, including the status of the various calibration reference data used by them. Tips and tricks of using the software will also be discussed, including typical types of problems to look for in your processed images and how to fix them.
A Pixel-Based Correction for Imperfect CTE in ACS's WFC
Based on a study of the trails seen behind warm pixels in long dark exposures, we have constructed an empirical model for the capture and release of charge that happens during readout of the WFC detector. The model assumes an even distribution of traps across the detector, and parametrizes the traps in terms of which electrons they impact and the profile of their release-time distribution. We adjust the parameters of our model to reduce the trailed warm pixels in the darks to delta functions, then use the same model to correct actual ACS images. We find that the model does a promising job restoring the CTE-trailed flux in real images to its original pixel, restoring photometry, astrometry, and even shape characteristics. Since any CTE-restoration process necessarily constitutes a deconvolution, it will be important to understand how it impacts noise and other issues.
The Detection and Removal of Large-scale Background Structures in NICMOS Observations
DR Eric Hsiao (LBNL)
After applying the standard corrections for well-studied NICMOS anomalies, significant large-scale spatial background variation remains. We report on the detections of fringing in the F110W filter, and a time-dependent residual amplifier glow in the calibrated NICMOS deep science images. We also describe methods developed to further correct these anomalies. Principal component analysis applied to our science images revealed that the intensity of the residual amplifier glow decays exponentially with time and resets every orbit. The images can be separated into two groups according to the glow level. A model of the background structure is derived from the algebraic manipulation of stacked science images and consists of the following components. A constant component is dominated by the residual amplifier glow as high as 20 DN at the corners and residual persistence structure at the center. A second component which scales with sky level displays clear fringe pattern with 15% variation for F110W images, but none is detected for F160W images. Using these model components to correct for the anomalies significantly improves the cosmetic appearance of NICMOS images and reduces the magnitude scatter in the photometry of faint galaxies by 20%. This work has been supported by the Office of Science, U.S. Department of Energy, through contract DE-AC02-05CH11231 and in part by NASA through grants associated with HST-GO-10496 and HST-GO-11799.
ACCESS: Mission Overview, Design and Preliminary Performance
DR Mary Elizabeth Kaiser (Johns Hopkins University)
Access, Absolute Color Calibration Experiment for Standard Stars, is a series of rocket-borne sub-orbital missions and ground-based experiments designed to enable improvements in the precision of the astrophysical flux scale through the transfer of absolute laboratory detector standards from NIST to a network of stellar standards with a calibration accuracy of 1% and a spectral resolving power of 500 across the 0.35 to 1.7 micron bandpass. The ACCESS instrument is a Dall-Kirkham Cassegrain telescope with a 15.5-inch primary mirror, a feedback controlled optical-NIR performance monitoring system, and a compact spectrograph design based on a Rowland circle mount concave grating operating as a low order (m=1-4) echelle with a Fery prism for cross dispersion. The detector is an unfiltered substrate-removed HST/WFC3 heritage HgCdTe array. To achieve the goal of a 1% absolute flux calibration requires an extensive ground calibration of the ACCESS instrument. At the core of the ground calibration is the 'artificial star' optical delivery system. The ArtStar system feeds an absolute flux calibrated collimated beam to the ACCESS instrument for the determination of the end-to-end sensitivity. The flight detector has been selected, detector electronics are being tuned and measurements undertaken to confirm and further characterize detector performance. Optical system components have begun to arrive. We will present the ACCESS design, calibration strategy, preliminary sub-system ground performance data, and instrument status. NASA sounding rocket grant NNX08AI65G and DOE DE-FG02-07ER41506 support this work.
Dither Patterns for JWST's NIRCam
DR Jay Anderson (STScI)
This poster will present the dithering strategy for JWST's NIRCam. Except in rare circumstances, such as transit studies, all exposures with NIRCam will be dithered. We have designed an extensive set of dither patterns that should address the many varied scientific objectives users may want to pursue. The patterns are flexible enough to allow emphasis on small dithers to improve sampling, large dithers to cover field or mitigate L-flat and detector defects, or an optimal combination of the two. The canned dithers provided here should ensure that both PI-science and the archive are of the highest possible quality.
On-Orbit Performance of the COS Detectors
DR David Sahnow (Johns Hopkins University)
The Cosmic Origins Spectrograph includes two microchannel plate detectors. The NUV channel uses a MAMA, similar to those used by other HST instruments, while the FUV channel uses a two-segment cross delay line (XDL) device. Since SM4, both detectors have been characterized in detail, using data collected as part of specific calibration programs and in the course of normal science observations. We will describe the on-orbit performance of both detectors since the installation of COS, and describe ongoing detector characterization work. This will include a discussion of the sensitivity, stability, dead time, dark rates (FUV), and flat fields (NUV).
WFC3 Low-Frequency Flat Field Corrections
MS Jennifer Mack (STScI)
Multiple dithered observations of the globular cluster Omega Centauri (NGC 5139) have been used to measure inflight corrections to the WFC3 UVIS and IR ground flat fields for a subset of key filters. To obtain an adequate characterization of the flat field over the detector field of view (FOV), 9 pointings were obtained for each filter using a 3x3 box dither pattern with steps of approximately 25% of the FOV. By measuring relative changes in the brightness of a star over different portions of the detector, low-frequency spatial variations in the detector response (L-flats) have been used to correct the flat fields obtained during ground testing. The broad wavelength range covered by these observations allow an interpolation of the L-flat correction for the remaining wide, medium and narrow-band filters, assuming a simple linear dependence with pivot wavelength. Initial results indicate that the required L-flat corrections are ±1.5% (standard deviation) in the IR and ±1.0% in the UVIS, and that the photometric response for a given star after applying the L-flat correction is now stable to better than 1% for any position in the field of view. Followup observations of the same field at multiple orientations will be used to verify the accuracy of the L-flat solutions and to quantify any temporal changes in the detector response while in orbit.
On-Orbit Measurement of the Point Spread Function on the WFC3 Detectors
DR Linda Dressel (STScI)
Knowledge of the point spread function (PSF) on a detector is important for scientific applications such as photometric measurements and analysis of blended sources. It is also important for ensuring that good optical alignment of the instrument is achieved and maintained. The quality of the PSF on HST instruments varies over the detector and varies with time. The focus changes throughout an orbit as the optical bench "breathes", and the average focus changes slowly over time. Locally, the detected PSF is affected by pixelation and charge diffusion or interpixel sensitivity variation. The PSF is also dependent on wavelength. All of these factors both complicate PSF analysis and make it more challenging to provide useful PSFs to observers. We will review the on-going measurement and analysis of the PSF on the WFC3 detectors.
A Redesign of MultDrizzle
We describe new developments in MultiDrizzle intended to allow users to easily and accurately align and combine HST images taken at multiple epochs, and even with different instruments. In the first part of this program, the correction of the time-dependent distortion of ACS was introduced into MultiDrizzle to provide distortion corrections good to a few hundredths of a pixel level. Now, we are undertaking a more profound change in the underlying method that MultiDrizzle (and HST) use to represent astrometry and geometric distortion. We have developed new simple extensions of the fits fomat that allow us to fully represent the ACS and WFC3 distortions in the header of the image, meaning that a calibrated image needs no other files to describe its astrometry. Finally we describe new tools we are developing to allow users to more easily align images and images with catalogues, and to use those WCS based alignments for Drizzling. Time will be left to discuss these changes with the audience and to hear to what extent these improvements will meet their needs.
COS External Spectroscopic Performance: FUV spectral and Spatial Resolution
DR Parviz Ghavamian (STScI)
During the SM4 Servicing Mission Observatory Verification (SMOV) we discovered that the on-orbit shape of the COS LSF with the HST optical telescope assembly (OTA) deviates from the profile measured in ground testing due to the appearance of broad wings. The wings are caused by mid-frequency wave-front errors (MFWFEs) that are produced by the zonal (polishing) errors on the HST primary and secondary mirrors; these errors could not be simulated during ground testing. The MFWFE effects are particularly noticeable in the FUV. While the shape of the pre-launch LSF is well described by a Gaussian in the FUV, the on-orbit LSF has up to 40% of its total power distributed in non-Gaussian wings. The power in these wings is largest at the shortest wavelengths covered by the COS medium-resolution gratings (~ 1150 A). The effect decreases with increasing wavelength but has a non-negligible effect on encircled energies even at the longest wavelengths. We have calculated optical models incorporating the MFWFE effects into the LSF for the whole spectral range covered by the FUV and NUV medium-resolution gratings. We show that for the FUV, the convolution of these model LSFs with high-resolution STIS echelle spectra yields an excellent match to the on-orbit COS spectra of the same targets. The model LSFs are available online and can be used by COS observers to assess the impact of the MFWFE broadening on their COS spectra. In ground tests it was shown that COS can spatially resolve two equally bright objects separated by 1 arcseconds in the cross-dispersion direction in the FUV. Using the FUV spectra of white dwarfs acquired during the Cycle 17 COS Spectroscopic Sensitivity Monitoring program, we show the on-orbit spatial resolution (as defined by the full-width half maximum of the spectrum along cross-dispersion) meets this specification, but in a wavelength-dependent manner. The wavelength dependence is primarily due the astigmatism introduced by the FUV gratings along cross-dispersion. The spatial resolution also depends on the central wavelength (CENWAVE) setting used, with longer CENWAVE settings yielding progressively better spatial resolution. The spatial resolution ranges from 0.5'' to 1.8'' for G130M, 0.3'' to 0.6'' for G160M and 0.4'' to 2.0'' for G140L.
The Space Telescope Imaging Spectrograph Flat Fielding
MR Sami-Matias Niemi (STScI)
The Space Telescope Imaging Spectrograph (STIS) has two types of reference files that are related to flat-fielding: L- and P-flats. A low-order flat field (L-flat) consists of a map of the large scale (> 64 pixels) variations in the sensitivity across a detector and contains the wavelength-dependent, low spatial frequency information about the uniformity of the detector. Instead, a pixel-to-pixel flat field reference file contains the high-frequency pixel-to-pixel variations and the small scale blemishes like dead and bright pixels, thinning differences, and dust motes. The main features of the P-flats are the high frequency odd-even pattern seen in the high-resolution MAMA exposures and the small scale blemishes from dead and bright pixels, dust motes, etc. seen in the charge-coupled device (CCD) images. We describe the generation of post Servicing Mission 4 STIS P-flats. Application and the effectiveness of the new P-flats is also discussed.
ACS after SM4: New Life for an Old Workhorse
DR David Golimowski (STScI)
The newly replaced CCD electronics box (CEB-R) for the Advanced Camera for Surveys (ACS) features a programmable SIDECAR ASIC manufactured by Teledyne. The CEB-R has fully restored the functionality of the ACS Wide Field Camera (WFC) and allows optimization of the WFC's imaging performance via on-orbit adjustment of CCD bias and clock voltages and serial-data transmission timing. We describe the strategy and results of the ACS Optimization Campaign during SMOV and assess the performance of ACS in the post-SM4 era. Special attention is given to the new electronic artifacts present in the WFC images, the effects of these artifacts on scientific observations, and STScI's efforts to correct or mitigate these artifacts.
HST/WFC3 In-Orbit Grism Performance
DR Harald Kuntschner (ESO/ST-ECF)
The HST Wide Field Camera 3 (WFC3) is fitted with three grisms for slitless spectroscopy. In the UVIS channel there is one grism, G280, for the near-UV to visible range (200 - 400nm; 1.4nm/pix). The IR channel has two grisms: G102 for the shorter (800-1150nm; 2.45nm/pix) and G141 for the longer (1100-1700nm; 4.65nm/pix) NIR wavelengths. Using SMOV and Cycle 17 calibration data taken on WR stars, planetary nebulae and flux standard stars, we have assessed the performance of the grisms. We have measured the field-dependent trace locations and dispersion solutions and determined the throughputs. The trace and wavelength solutions for the IR grisms were found to be linear functions, varying smoothly as a function of field of view. The UVIS grism exhibits a highly bent trace and significantly non-linear dispersion solutions. The maximum throughputs for the G102 and G141 grisms, including the telescope optics, are 42% at 1100 nm and 48% at 1450 nm, respectively. Limiting magnitudes at S/N=5 and a 1h exposure are J_AB=22.6 and H_AB=22.9 for the G102 and G141 grisms, respectively. For the IR grisms, we provide aperture corrections as a function of wavelength and determine the cross-dispersion PSF, which agrees well with design expectations and ground calibration measurements. The results are published as sensitivity and configuration files that can be used with our dedicated extraction software aXe to reduce WFC3 slitless data.
Cross-matching the Hubble Legacy Archive
DR Nathan Cole (Johns Hopkins University)
The Hubble Legacy Archive (HLA) is composed of hundreds of thousands of images taken by a number of different instruments all across the sky. A large number of these images are of overlapping fields and therefore contain many of the same objects. Through the use of a probabilistic cross-matching technique, it is possible to use these overlapping fields to determine a linkage between similar objects. This linkage can then be used to improve astrometry, as well as provide a means for connecting images taken via different instruments. We present a description of this cross-matching technique and discuss the specific choices made with regards to the first data used. We also present current results of implementing this technique to improve astrometry showing an improvement in astrometry to 0.004”. Future concerns and developments will also be discussed.
The Photometric Performance of the WFC3 UVIS and IR Cameras
DR Jason Kalirai (STScI)
We characterize the total system throughput of HST/WFC3 by imaging both hot and cool HST spectrophotometric standard stars with both the UVIS and IR cameras. For the UVIS channel, our measurements indicate that efficiencies are higher than ground tests by 15-20% at central wavelengths and 5-10% at the blue and red ends of the UVIS spectral range. For the IR channel, the instrument throughput is 10-15% higher in all filters. Over its one year lifetime, WFC3's throughput is stable in wide and medium band filters to <1%. New photometric zero points have been calculated, and updates to the exposure time calculator have been implemented.
New Bad Pixel Mask Reference Files for the Post-NCS Era
MS Elizabeth Barker (STScI)
NICMOS calibration monitoring programs regularly obtained calibration images during the post-NCS era between 2002 and 2008. Both flat-fields and darks were taken during this long base-line, which allowed us to create high signal-to-noise bad pixel mask reference files, as well as investigate any temporal dependence of the mask files. We describe here the creation of new mask files based on this extended data set and compare the new masks with the previous versions. The new masks contain more bad pixels compared to the old versions, while there are fewer pixels thought to be affected by "grot".
JWST Fine Guidance Sensor Calibration
DR Pierre Chayer (STScI)
The primary function of the Fine Guidance Sensor (FGS) is to provide continuous pointing information to the Observatory. The FGS will image two separate regions of the JWST field of view onto two independent 2k x 2k infrared focal planes arrays, which will cover a wavelength range of 0.6 to 5 microns. Once in fine guiding, the FGS will provide continuous pointing information to a precision of < 5 milli-arc seconds at an update rate of 16 Hz for J ~ 18.5 magnitude stars. In order to fulfill its functions, i.e., Identification, Acquisition, Track, and Fine Guide, the FGS will require calibration. In this paper, we present a brief overview of the FGS, summarize its calibration plan, and highlight some results from the engineering test unit cryovac test that was performed during the spring of 2010.
Red Leak Effects in Observations of Solar System Objects with ACS/SBC
DR Paul Feldman (Johns Hopkins University)
Following the failure of STIS in August 2004, attempts to obtain ultraviolet spectroscopy and photometry of solar system objects shifted to the Solar Blind Channel (SBC) of the ACS. These included spatially resolved spectroscopy using the PR130L prism of comet 9P/Tempel 1 (at the time of Deep Impact) and Europa, and imaging and photometry of the asteroid 21 Lutetia, one of the targets of ESA's Rosetta mission. Initial estimates of long wavelength ("red") contamination of the data due to impurities in the FUV MAMA detector suggested that these observations were feasible. Subsequent analyses produced better sampled, more reliable response curves (Boffi et al., TIR ACS 2008-002) that showed the long wavelength response to be much worse than expected. The analysis of differential photometry of Lutetia (Weaver et al., submitted to A&A) shows an effect that is 2.5 times larger than the published data. We also discuss the use of a PR130L spectrum of a solar analogue star 16CygB for the interpretation of the Europa emission spectrum.
Persistence in the WFC3 IR Detector
DR Knox Long (STScI)
As is the case for most if not all modern IR arrays, bright sources observed with the IR detector in WFC3 leave faint residual images in subsequent exposures. Persistence has been observed not only in dithered exposures by one observer of a single target within an orbit but also in exposures of another completely target by another observer in a subsequent orbit. The amount of image persistence in the WFC3 IR detector is a function of the degree of saturation of a pixel and time since the pixel was (over) exposed. The persistence decays roughly as a power law with time, and is typically 0.2 e/s for a pixel that was highly saturated 1000 s earlier. Here, we show examples of persistence which have been observed and characterize the effect. We also describe ways for observers to find the pixels that are likely to have been affected by persistence, and to mitigate the effects of persistence when planning observations and reducing their data.
First On-orbit Measurements of the WFC3-IR Count-rate Non-Linearity
DR Adam Riess (STScI/JHU)
Previous HgCdTe detectors on HST have suffered from a count-rate dependent non-linearity, motivating an investigation of a similar effect on the WFC3-IR detector. An initial measurement of this effect was made by comparing the photometry of star clusters observed over a wide dynamic range and at overlapping wavelengths in WFC3-IR and NICMOS and/or ACS-WFC. Utilizing a color term to account for differences in the observed bandpasses, we find a significant detection of a non-linearity in WFC3-IR photometry which is in the same direction but a few times smaller than that of NICMOS. From 235 stars in 47Tuc observed with WFC3-IR in F110W and F160W and in similar bandpasses in NICMOS Camera 2, we measure a non-linearity of WFC3-IR of 0.011+/- 0.0023 and 0.010+/-0.0025 mag per dex, respectively, over a range of 10 magnitudes (4 dex). An independent measurement utilizes 1390 stars in NGC 3603 observed with ACS-WFC F850LP and WFC3-IR F098M and yields a very similar result, 0.010 +/- 0.0033 mag/dex. The consistency of this measurement from two different comparison detectors of different technology indicates this result is robust. The impact of this non-linearity is that photometry of faint (i.e., sky dominated) sources calibrated with WFC3-IR zeropoints will appear 0.04 +/-0.01 mag too faint.
Fringing in the WFC3/UVIS detector
XX Michael H. Wong (UC Berkeley)
In late 2010, a star cluster will be observed with narrowband red filters to determine the impact of fringing (position- and wavelength-dependent patterns of brightness variation) on flight data. Ground flat fields show peak-to-trough amplitude variations of 0.5% to 16% (among 12 affected filters). Different "fringe flat fields" will be applied to the flight data to determine the best model fringing correction. The difference between models comes from two detector thickness maps, derived from two separate sets of ground test data. The disagreement between the thickness maps is consistent with an error in the monochromatic illumination wavelengths in one of the data sets, but an unexplained corner-to-corner slope across the detector remains even after correcting for the wavelength error. Flight data are needed to determine which thickness map produces the best correction for fringing.
The Astrometric Context of HST in 2010
PF William van Altena (Yale University)
For the past 20 years, the HST has occupied a unique niche in astrometry, that of high-resolution imaging and interferometry-based astrometry. As a consequence of its small field of view, HST astrometry has been primarily oriented towards the observation of carefully selected targets rather than surveys searching for new classes of objects. Ground-based astrometric, photometric and radial velocity surveys have played crucial roles in characterizing classes of objects and identifying astrophysically-interesting targets worthy of observation by the HST. The symbiotic nature of HST and ground-based astrometry is illustrated by surveys of visual and spectroscopic binaries that lead to the identification of binaries in critical parts of their orbits where one or a few HST Fine-Guidance Sensor observations of their angular separation can lead to a definitive mass determination. Photometric surveys have identified numerous Cepheid variables in the Milky Way that might be used to calibrate the cosmic distance scale, however their distances generally remain estimated only from their spectro-photometric characteristics. HST-FGS observations of carefully selected and relatively nearby Cepheids have led to a solid independent calibration of the cosmic distance scale. Ground-based observations of galactic globular clusters have determined the physical makeup of those clusters and characterized the evolutionary tracks of stars with differing masses and metallicities, but only HST imaging astrometry in conjunction with radial velocity observations have made it possible to determine the dynamical state of the clusters. The above examples highlight only a few contributions of HST astrometry to the critical calibration of astronomical objects, while in this talk I will try to place HST in the context of how we are using astrometry to understand our local universe.
The NIRSpec calibration concept
DR Guido De Marchi (European Space Agency)
NIRSpec is the main near-infrared spectrograph on board the James Webb Space Telescope, a segmented off-axis telescope in deep space. Besides the traditional 'fixed slits' for long-slit spectroscopy, NIRSpec offers both integral field unit and multi-object spectroscopy capabilities. The highest level of multiplexing is provided by the Micro-Shutter Array, a fixed grid of micro-shutters allowing up to ~ 100 objects to be observed simultaneously over a large field of view (~ 10' square) and over nearly a factor of 10 in wavelength, since NIRSpec's detector response ranges from 0.6 to 5 micron. The combination of these factors results in a number of unique challenges for an efficient calibration of the instrument. In this paper we present a high-level description of the calibration and outline some of the challenges that it entails.
NIRSpec - the JWST Multi-Object Spectrograph
DR Guido De Marchi (European Space Agency)
The Near-Infrared Spectrograph (NIRSpec), to be launched in 2014 on the James Webb Space Telescope, will be the first slit-based multi-object spectrograph used in space, and is designed to simultaneously provide spectra of faint objects in the range 0.6 - 5.0 micron at resolutions of R=100, R=1000 and R=2700. The all-reflective wide-field optics, a novel MEMS-based programmable micro-shutter array slit selection device, and large format low-noise HgCdTe detectors combine to allow simultaneous observations of >100 objects within a 3.4x3.4 arcmin^2 field. A 3x3 arcsec^2 integral field unit and five fixed slits are also available. The NIRSpec sensitivity is expected to allow detection of a continuum flux of 20 nJy (AB>28) in R~100 mode, and a line flux of 6x10-19 erg s-1cm-2 in R~1000 mode, both at S/N > 3 in 104 s. NIRSpec is being built for the European Space Agency by EADS Astrium as part of the European contribution to the JWST mission, with the micro-shutter and detector arrays being provided by NASA/GSFC.
JWST Absolute Flux Calibration Planning
DR Karl Gordon (STScI)
The planning for the absolute flux calibration of all four JWST science instruments will be described. The main goal is to provide a uniform calibration (photometric and spectrophotometric) across all instruments using a common set of calibration stars. These stars will include white dwarfs, A0V, and solar type stars. The combination of these different types of stars will serve to check for systematic biases in the calibration as well as tie the Hubble (based on white dwarfs) and Spitzer (based on A0V and solar type stars) calibrations to the JWST calibration. An initial set of calibrators has been investigated and how they map to the sensitivity ranges of the JWST instruments will be detailed. Further work will concentrate on expanding the sample to more fully map the JWST instrument sensitivities and vetting individual calibration stars.
WFC3/IR CHANNEL FILTER WEDGE
DR Elena Sabbi (STScI)
In order to verify and quantify if any of the WFC3/IR imaging filter presents variations in its thickness (filter wedge) we observed the young and bright star cluster NGC 1850 consecutively through all the WFC3/IR filters without moving the telescope. By comparing the stellar coordinates in an exposure with the respect of another we found a weak relation between shift and wavelength that may be ascribed to the fact that the WFC3/IR channel is not exactly in the focal plane of the telescope.
Wavelength Calibration of the Cosmic Origins Spectrograph
XX Cristina Oliveira (STScI)
The Cosmic Origins Spectrograph (COS) was installed during the most recent servicing mission of the Hubble Space Telescope. The FUV channel, providing more than 10x the FUV throughput of STIS, covers the wavelength range from 1150 to 1800 A with the medium resolution gratings (G130M/G160M) and from ~900 to 2400 A with the low resolution grating (G140L). The medium resolution gratings in the NUV channel (G185M/G225M/G285M) cover ~1700 to 3200 A, while the low resolution grating (G230L) covers 1700 to 3200 A. We describe the wavelength calibration of the FUV and NUV channels, which uses a combination of ground and on-orbit data obtained during SMOV. We consider the effect of drift of the Optics Select Mechanisms (OSM) on the wavelength scales and describe how the onboard wavecal lamps allow us to correct for the OSM drift. We report the accuracy achieved with the current wavelength calibration strategy and summarize our cycle 17 wavelength calibration program.
WFC3 UVIS Linearity Near and Beyond Saturation
DR Ronald Gilliland (STScI)
The linear behavior of the WFC3 CCDs near and beyond saturation will be characterized through comparison of counts in back-to-back short and long exposures on rich star fields where pairs of simple aperture sum photometry can be readily compared. If near perfect linearity exists, then the ratio of counts after summing over all pixels that have been bled into past saturation for the long exposure will simply reflect the relative exposure time. ACS and STIS both showed excellent linearity when using a gain keeping saturation on-chip. The default gain of WFC3 keeps saturation safely on-chip, however minor deviations from linearity at the ~10% level exist for stars that are more than 2 magnitudes brighter than saturation. This behavior is different on the two CCDs and will be characterized. A rough calibration sufficient to maintain photometric accuracy to 1-2% even for heavily saturated stars will be given. A map of the count level at which saturation sets in for point sources (CCD full well depth) will also be shown.
Post-SM4 Flux Calibration of the STIS Echelle Modes
MS Azalee Bostroem (STScI)
Like all STIS spectroscopic modes, STIS echelle modes show a wavelength dependent decline in detector sensitivity with time. The echelle sensitivity is further affected by a time dependent shift in the blaze function. To better correct the effects of the echelle sensitivity loss and the blaze function changes, we derive new baselines for the echelle sensitivities from post HST Servicing Mission 4 observations of the standard star G191B2B. We present how these baseline sensitivities compare to pre-failure trends. Specifically, we highlight where the new results differ from expectations, discuss anomalous results found in E140H monitoring observations, and review plans for future monitoring and calibration of these modes.
The Legacy of the Hubble Space Telescope Spectrographs
PF J. Christopher Howk (Univ. of Notre Dame)
The spectrographs of the Hubble Space Telescope have each offered order of magnitude improvements over their predecessors, including those space-based instruments before HST. They have provided a wealth of information on the local and distant universe, opening up new fields of study that were not possible prior to the launch of HST. They have, of course, come with their own set of design and use challenges. I will discuss the legacy of HST spectroscopy and look forward to the types of spectroscopy we might wish to see in future space missions.
Anomalies and Artifacts of the WFC3 UVIS and IR Detectors: An Overview
MR Michael Dulude (STScI)
The WFC3 Quicklook project was designed to provide basic visual inspection of every UVIS and IR image acquired by WFC3. Since first light, a number of anomalies or artifacts have been found. These include trails from passing satellites, light leaks, scattered light artifacts, and a handful of detector idiosyncrasies. In this poster, we present a "rogue’s gallery" detailing the various types of anomalies found so far, their likely causes and possible remedies.
Trend of Dark Rates of the COS and STIS MAMA Detectors
DR Wei Zheng (Johns Hopkins University)
The dark rate of the STIS NUV MAMA detector was about an order of magnitude higher after the SM4 repair than before, with an initial rate of ~0.01 count/pixel/sec. Measurements over the past year show a dual-component exponential decline with e-folding timescale of ~40 and 400 days. The most recent measurements show a rate of 3.6E-3 count/pixel/sec, which is ~3 times the historical average value. The COS NUV detector dark rate started at a very low value of 6E-5 count/pixel/sec, and has displayed a steady increase, approaching the ground-tested level of 3.7E-4 count/pixel/sec. Still, the rate of COS NUV detector is considerably lower than that of its STIS counterpart. The rates for both detectors are sensitive to detector temperature, and the rate fluctuations can be fit with an empirical model. We will discuss the measurements and trends of the COS and STIS MAMA detector dark rates and their implications for data analysis.
WFC3 UVIS Linearity Low to Near Saturation
MR Abhijith Rajan (Space Telescope Science Institute)
We present measurement results of the linearity of the WFC3 UVIS detectors by sampling over the response curve from very low counts up to near saturation. We use data collected from the last thermal vacuum test (TV3) before SM4 and from the Cycle 17 calibration proposal 11925. TV3 data tested the response at high count levels; the in-orbit program obtained exposures of NGC 1850 to verify the linearity response and to characterize the linearity at very low counts between a few hundred and a few thousand electrons. Our results indicate that the CCDs are linear to < 1% between a few thousand and ~65,000 electrons.
WFC3 IR Detector Behavior
MR Bryan Hilbert (STScI)
Using data taken during Servicing Mission Observatory Verification (SMOV4) and Cycle 17, we have characterized many aspects of the on-orbit behavior of the IR Channel. We find the dark current in the IR channel to be 0.043 - 0.050 e-/s/pixel. The correlated double sampling (CDS) read noise in RAPID sequences is 20-22 electrons, similar to that measured in ground testing. The effective noise measured in an image created from 16 reads of a SPARS200 ramp is 11.6 - 12.7 electrons. We have also recently finished the creation of a bad pixel mask for the IR detector, which contains a list of pixels with non-nominal behavior that should be ignored in WFC3/IR data analyses. Current analysis projects also include an investigation into the pixels' non-linearity behavior, as well as monitoring the rate and characteristics of "snowballs" which appear in IR channel data.
Science Capabilities of the Cosmic Origins Spectrograph
PF James Green (University of Colorado)
I will highlight the observational capabilities of COS, and the increase in sensitivity it provides in various observing situations. The scientific return of the instrument in its first year of operation will be quantified and presented, along with selected observations from the first year of COS data.
Internal Monitoring of WFC3/UVIS Charge Transfer Efficiency
XX Vera Kozhurina-Platais (STScI)
We summarize the results of one year of Charge Transfer Efficiency (CTE) monitoring of the WFC3 UVIS in-flight detector, based on data acquired during the monthly internal Extended Pixel Edge Response (EPER) test. We present an algorithm for CTE assessment and fit a power-law functional dependency of CTE on signal level. We find that at each signal level, CTE declines linearly over time and CTE losses are worst at the lowest signal levels. We compare our in-flight results to similar pre-flight baseline data based on radiation tests performed on a flight-like device.
Multi- Wavelength Geometric Distortion Solution for WFC3/UVIS and IR.
XX Vera Kozhurina-Platais (STScI)
The standard astrometric catalog based on ACS/WFC observations of Omega Cen field has been used to examine the geometric distortion of WFC3/UVIS and IR as function of wavelength. The multiple observations of this field taken with the large dither patterns and large range of the HST roll-angles were exposed through 10 pass-band UVIS filters and 5 IR filters. A 4th order polynomial model was used to derive the geometric distortion coefficients relative to the distortion - free coordinates of our astrometric field in the UVIS and IR channels. As a result: 1) the geometric distortion can be successfully corrected with 2-5 mas precision level in UVIS and IR channels respectively; 2) the non-perpendicularity of coordinates axes (skew) can be used to assess the scale change from filter to filter, time and HST roll-angle; 3) the coefficients of the geometric distortion in the UVIS and IR channels are used in STScI Multidrizzle software in order to correct WFC3 images for distortion.
WFC3 IR "Blobs", IR Sky Flats and the measured IR background levels
DR Nor Pirzkal (STScI)
The near infrared background, as seen using the WFC3 IR channel, varies by several orders of magnitude. Ever since WFC3 was installed on HST, we have been monitoring and examining incoming observations of sparse fields. This monitoring has enabled us to identify the WFC3 IR "Blobs" (Pirzkal et al., ISR 2010-06). It has also allowed us,for some of the broad band filters, to assemble IR Sky Flats. We have also been able to measure the observed IR background levels in many of the WFC3 IR filters. Here, we show how we have used WFC3 observations to construct the Sky Flats as well as how we detected and created maps of IR "Blobs". We finally show how the measurements of the IR background vary and how they correlate with HST pointing.
Calibration of B,V,R,I Photometry for the Wide Field Channel of the HST Advanced Camera for Surveys
DR Richard Shaw (NOAO)
We present new observations of two Galactic globular clusters, PAL4 and PAL14, using the Wide-Field Channel of the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope, and reanalyze archival data from a third, NGC2419. We matched our photometry of hundreds of stars in these fields from the ACS images to existing, ground-based photometry of faint sequences which were calibrated on the standard B,V,R,I system of Landolt. These stars are significantly fainter than those generally used for HST calibration purposes, and therefore are much better matched to supporting precision photometry of ACS science targets. We were able to derive substantially more accurate photometric transformation coefficients for several commonly used ACS filters compared to those published by Sirianni, et al. (2005), owing to the use of a factor of several more calibration stars which span a greater range of color. We find that the inferred transformations from each cluster individually do not vary significantly from the average, except for a small offset of the photometric zeropoint in the F850LP filter. Our results show that the published prescriptions for the time-dependent correction of CCD charge-transfer efficiency appear to work very well over the ~3.5 yr interval that spans our observations of PAL4 and PAL14 and the archived images of NGC2419.
Cosmic Origins Spectrograph Commissioning: Overview of COS Servicing Mission Observatory Verification
DR Charles (Tony) Keyes (Space Telescope Science Institute)
The Cosmic Origins Spectrograph (COS) was installed on the Hubble Space Telescope (HST) during Servicing Mission 4 (SM4) in May 2009. COS is designed and optimized to obtain spectra of faint objects at moderate spectral resolution (R>16,000) with two channels: FUV, covering wavelengths from 1150 to 1450 Å; and NUV, covering 1700 – 3200 Å. Two low resolution gratings (R>1500) cover the <900 – 2050 Å (FUV) and 1650 – 3200 Å (NUV) wavelength regions. An imaging capability is also available on the NUV channel. As part of the HST Servicing Mission Observatory Verification (SMOV) program, an extensive period of checkout, fine-tuning, and preliminary characterization began after the installation of COS. The COS SMOV program was a cooperative effort between the Space Telescope Science Institute and the Instrument Definition Team based at the University of Colorado. Nearly 2800 COS exposures in 34 separate observing programs were obtained during the course of SMOV. Early activities included an initial instrument functional checkout, high-voltage turn-on and initial characterization of the detectors, NUV and FUV channel focus and alignment, wavelength calibration, and target acquisition verification and assessment. Once this initial period was completed, science-related calibrations and verifications were performed in order to prepare the instrument for normal operations. These activities included flux calibration, detector flat field characterization, spectroscopic performance verification, high S/N operation, and thermal and structural stability measurements. We present the design, execution and results of the SMOV program, including the interrelationships and dependencies between the various tasks, and how the pre-launch plan was adjusted in real-time due to changing conditions.
Cross Calibration of Space Infrared Telescopes
DR Sean Carey (Spitzer Science Center / Caltech)
I will discuss the cross calibration efforts between the various instruments aboard the Spitzer Space Telescope and to previous missions such as MSX and ISO, and the current mid-infrared observatories, WISE and AKARI. Current cross-calibration accuracy between the Spitzer instruments is of order 5%. While cross-calibration is conceptually simple, the details can be interesting. Complications such as the impact of errors in linearity solutions in measurements of common calibrators and differences in photometric truth used to calibrate individual instruments will be explored. Efforts to prepare cross-calibration to JWST will be described. The challenges of bootstrapping to extremely sensitive instruments (MIRI, NIRCAM and NIRSpec) and longer wavelengths (Herschel PACS and SPIRE) will be explored.
Calibration of HST's Fine Guidance Sensors
XX Edmund Nelan (STScI)
HST's three Fine Guidance Sensors are critical for the fine pointing and stabilization of the observatory. The ability of the FGS to measure the position of a guide star to ~1 milli-arcsecond at 40 Hz also empowers it to serve as an astrometric and high speed photometric science instrument. In addition, the interferometric design of the FGS enables it to resolve binary star systems with angular separations down to approximately 12 mas, well below the diffraction limit of HST. Over the years the FGSs have been calibrated as needed to support their role as guiders. FGS1r has been calibrated to support the FGS science program, including a re-calibration of its geometric distortion in late March 2010. In SM4 FGS2 was replaced by the refurbished FGS2r2. This new FGS was calibrated for guide duty after its on-orbit commissioning. This talk will outline the calibration history and status of HST's three FGSs.
Updated Status and Performance of the Space Telescope Imaging Spectrograph's Charged-Coupled Device
MR Michael Wolfe (STScI)
A description is provided of the overall performance of the STIS CCD after HST Servicing Mission 4 (SM4) and during Cycle 17 calibrations. Most aspects of CCD performance are found to be consistent with extrapolations of the trends seen prior to the failure of STIS in August 2004. The gain values for 1 and 4 have not changed since the Servicing Mission Orbital Verification after SM4 and the Read Noise through Amp D, as determined from unbinned bias images taken during Cycle 17 calibrations, has for the most part remained constant. As is expected due to the on-orbit radiation environment the dark current is perpetually increasing. Additionally, the slope is still present in the dark current along CCD columns, resulting in a dark current near the detector readout amplifier that is about 32% lower than the mean over the whole detector. The spurious charge has increased as well and has a slope that results in a smaller value at the top of the chip (near the readout amplifier) when compared to the center of the chip. The spurious charge is approximately 38% lower at the top of the chip (row 900) than it is in the center of the chip (row 512) for gain = 1. The same behavior for the gain = 4 spurious charge is also present but the percentage difference between the top of the chip and the center is 6.5%. The charge transfer inefficiency (CTI) has also continued to increase as well, although the increased dark current will significantly moderate the actual CTI losses for typical observations. The currently measured CTI values are in excellent agreement with a simple extrapolation of trends seen during Cycle 17 calibrations. Additionally, the number of hot pixels has continued to increase despite the monthly annealing of hot pixels. Although the gain and read noise have stayed essentially the same, the number of hot pixels, dark current, spurious charge, and CTI have continued to increase. The current sensitivity trends of all CCD modes will also be presented and it will be shown that these trends have essentially remained the same.
JWST Pipeline: Discussion of Data Reduction
XX Robert Jedrzejewski (STScI)
The JWST Mission will, like HST, have a dedicated Calibration Pipeline that will be used to calibrate all science data from the observatory. Rather than follow the HST model of creating separate calibration pipeline programs for each Science Instrument, we will instead make use of the considerable simplification of separating the detector calibration from the instrument-specific calibration. Since JWST will only have two types of detectors, much of the detector calibration code will be common from instrument to instrument. Similarly, the instrument-specific behaviors can be classified as imaging, spectroscopic and coronagraphic, and we anticipate that, even though the architecture of the instruments may show significant variation, many of the primitive operations will be applicable to more than one instrument or mode and will allow re-using code. These simplifications will enable us to create a flexible, powerful pipeline that can be cleanly separated into independent calibration modules that can be evaluated, tested and improved straightforwardly over the life of the project.
Detecting Cosmic Rays in Infrared Data
MI Rachel Anderson (STScI)
Cosmic rays are a known problem in infrared astronomy, causing both loss of data and data accuracy. The problem becomes even more extreme when considering data from a high radiation environment such as in orbit around Earth, or outside the Earth's magnetic field altogether, unprotected, as the James Webb Space Telescope will be. To find the best method to correct for this disturbance we develop three cosmic ray detection methods, apply them to simulated multiaccum ramps with varying combinations of slope, number of frames, number of cosmic rays, and cosmic ray frame number and strength, and then we compare the results. The methods discussed include a 2-point difference method, a deviation from the fit method, and a y-intercept method.
UBVRI-ZY and ugriz calibrated zero-points from 20 calspec standards covering a wide range of color.
DR Andrew Pickles (Las Cumbres Observatory Global Telescope. LCOGT)
We present UBVRI-ZY and ugriz magnitudes for a library of digital spectra, calibrated on 20 calspec standards with standard photometry and covering a wide range of color, from DA white dwarfs through G/K stars to VB8 (M7V). The zero point dispersion is 0.027, 0.020, 0.008, 0.014 and 0.016 mag respectively in UBVRI, where the UmBmVcRcIc filter profiles minimize these dispersions. We find zero points for u=u'=0.03 +/- 0.03 mag, and zero points for griz and g'r'i'z' of 0.0 +/-0.02 mag. With these zero-points we find Vc(Vega) = 0.020 +/- 0.008 mag from an average of 17 calspec standards, and colors which are close to zero. We can derive VL(Vega) = 0.013 +/- 0.02 mag using the Landolt/Cohen VL filter profile and 17 calspec standards, or VL(Vega) = 0.023 +/- 0.02 mag using only DA WD standards, but these latter values should be corrected for color.
An example reduction of WFC3/IR slitless data
DR Martin Kuemmel (Space Telescope - European Coordinating Facility)
The G102 and G141 grisms in the WFC3 IR channel together cover the wavelength range 0.8-1.6 micron with resolutions of R~210 and R~130, respectively. In this contribution we present an example reduction of G102 and G141 slitless data taken during the WFC3 Early Release Science programme. All core tasks are done by the spectroscopic extraction software aXe, which was specifically designed to handle data from the HST slitless modes. The reduction scheme includes a sky background subtraction with a master sky image to achieve a homogeneous, flat background for the spectral extraction and the most recent in-orbit calibrations. Moreover we present a new method for co-adding data from individual slitless images that, similar to MultiDrizzle in direct imaging, allows the rejection of deviating pixels from e.g. cosmic ray hits. As results we show the spectra of some emission line galaxies down to m(F140)_AB=24.5, demonstrating the remarkable efficiency and capability of the WFC3 NIR grisms.
WFPC2 Filters after 16 Years on Orbit
MS Pey Lian Lim (Space Telescope Science Institute)
Wide Field Planetary Camera 2 (WFPC2) was installed on Hubble Space Telescope (HST) in December 1993 during Servicing Mission 1 by the crew of Shuttle Mission STS-61. WFPC2 replaced Wide Field Planetary Camera 1 (WFPC1), providing improved UV performance, more advanced detectors, better contamination control, and its own corrective optics. After 16 years of exceptional service, WFPC2 was retired in May 2009 during Servicing Mission 4, when it was removed from HST in order to allow for the installation of Wide Field Camera 3 (WFC3). WFPC2 was carried back to Earth in the shuttle bay by the crew of Shuttle Mission STS-125. In a joint investigation by Goddard Space Flight Center (GSFC) and Space Telescope Science Institute (STScI), the Selectable Optical Filter Assembly (SOFA) of WFPC2 was extracted and the filter wheels removed and examined for any on-orbit changes. The filters were inspected, photographed and scanned with a spectrophotometer at GSFC. The data have been analyzed at STScI with a view towards understanding how prolonged exposure to the HST space environment affected the filters and what the resultant impacts are to WFPC2 calibrations. In this presentation, we will summarize our results from these post-SM4 laboratory studies, including a comparison of pre- to post-mission filter throughput measurements, evaluations of the UV filter red leaks, and assessment of the condition of the filter coatings.
Slitless Spectroscopy with HST Instruments
DR Jeremy Walsh (Space Telescope European Co-ordinating Facility)
Slitless spectroscopy is generally regarded as a niche, perhaps 'difficult', astronomical observation technique. However the low background from space and the high spatial resolution offered by HST instruments has enabled it to become a powerful survey tool, with some applications to single object work. The ST-ECF has been involved with slitless spectroscopy from NICMOS, through ACS to WFC3. The techniques and software have also been developed for the bulk extraction of NICMOS and ACS slitless spectra ingested into the Hubble Legacy Archive. Slitless spectroscopy has been selected for the spectroscopic channel of the Euclid Dark Energy mission currently under study for an ESA Cosmic Vision M-Class mission. The critical choices made in the development of calibration techniques and extraction software for HST slitless spectra are reviewed, with particular emphasis on ACS and WFC3. Possible future developments are outlined.
Ironing out the Wrinkles in STIS
DR Thomas Ayres (University of Colorado (CASA))
On the J.J. Drake "Ladder of Cosmic Sexiness," the calibration rung falls even below the lowly ones of "Stars" and "The Sun." Nevertheless, we cognoscenti of such things know full well that without good calibrations, there will be little signal and much noise. Our knowledge of remote objects, environments, and events thus is only as good as our understanding of our instruments. A case in point is the solar system's premier high resolution ultraviolet spectrograph in space: STIS. The second generation HST instrument has unmatched spectral resolution, up to 114,000 in its echelle H modes, and full coverage of the key FUV and NUV wavelength regions with relatively few settings. For the initial seven years of its operations, 1997-2004, STIS was a workhorse in studies of stars, planets, interstellar matter, and even extragalactic sightlines. Now, a repaired STIS is teamed up with super high sensitivity, but lower resolution, partner COS to open new windows into the broad range of cosmic environments and processes that can be probed by ultraviolet spectroscopy. (Putting the "physics" into "astrophysics.") The present report describes an effort to push STIS beyond its already remarkable precision by carefully analyzing the way in which the echellegrams are processed, particularly with respect to wavelength assignments. The method is to run the CALSTIS pipeline on deep exposures of the Pt/Cr-Ne calibration lamps, treating them as if they were science images, and compare the resulting sharp-line spectra with laboratory measurements to map out any persistent deviations. Large scale coherent systematics, if present, can be modeled and compensated, either at the level of the pipeline dispersion constants, or as a post facto distortion correction. (The latter approach was followed in the so-called StarCAT project, which cataloged STIS echelle spectra of 545 objects classified as "stars.") Unfortunately, while there are excellent laboratory measurements of the STIS lamps below 1800 A, the situation at longer wavelengths is not as good, thanks mainly to the presence of chromium in the STIS sources, which has a rich, though poorly cataloged, spectrum at the longer wavelengths. This speed bump was circumvented by utilizing STIS as its own laboratory spectrometer, bootstrapping from the existing ground measurements of Pt and Ne lines. The large number of newly characterized reference wavelengths, especially in the NUV, coupled with the higher line densities achieved in coadded deep wavecal spectra, allowed some remarkable new insight into the dispersion properties of the instrument, even to the level of obtaining a simultaneous dispersion solution over all of the up to 26 independent settings of each of the four echelle modes. It is astonishing that this works as well as it does. Part of the reason is that the nonlinear terms in the dispersion model apparently are dominated by a quadrupolar geometrical distortion in the MAMA cameras, which is separable from the dominantly linear dispersion properties. The current set of dispersion constants in the calibration library could be significantly improved by exploiting the deep wavecal material and the new "laboratory" wavelengths to recalibrate; and the processing could be made more robust by performing a geometrical distortion correction prior to wavelength assignment. This work has resulted, as well, in lots of pretty pictures (depending on how one views the artistic merits of wavecal spectra).
DR Anatoly Suchkov (JHU)
The ACS/WFC detector consists of two CCDs, each of which is read out through two amplifiers. While reading each quadrant of the detector, the electronic crosstalk between the amplifiers induces faint, typically negative, mirror-symmetric ghost images on the other three quadrants. The effect is strongest for high-signal offending (source) pixels. Analysis of crosstalk before Servicing Mission 4 (SM4) showed that its impact on ACS/WFC science is not significant and can be ignored in most science applications. After SM4, crosstalk due to low-signal offenders is much weaker than before SM4 and does not produce ghosts similar to those seen in pre-SM4 images. For high-signal offending pixels, there are substantial differences between the gain=1 eˉ/DN and gain=2 eˉ/DN cases. For the default gain setting of 2, the crosstalk is similar to what it was before the SM4, up to 5–8 eˉ per pixel on the same CCD. For gain=1, the crosstalk is ~100 eˉ per pixel for saturated offending pixels on the same CCD, which is more than an order of magnitude above the pre-SM4 level. The crosstalk from saturated pixels is ~20–30 eˉ per pixel on the other CCD, which is also much higher than it was before SM4.
NICMOS Coronagraphy: Recalibration and the NICMOS Legacy Archive PSF Library
DR Glenn Schneider (Steward Observatory, University of Arizona)
NICMOS coronagraphy, with well-matched template Point Spread Function (PSF) subtraction, probes the closest environments of occulted targets with the highest imaging sensitivity in intrinsically high contrast fields at the smallest radial distances afforded, uniquely, by HST. NICMOS PSF-subtracted coronagraphy has been invoked in a wide variety of HST programs with science themes as divergent as detecting and characterizing disks of circumstellar material in neo-natal stellar environments, to studying faint nebulosity associated with luminous active galaxies, to searching for planetary-mass companions in extrasolar planetary systems recently born and in the "stellar graveyard." The investment in HST time in the execution of these and other NICMOS coronagraphic programs, has met with mixed returns. Stunning successes importantly advancing their fields highlight more frequent, and unfortunately somewhat common, failures arising from highly compromised technically-achievable performance due to the lack of suitable template PSFs required to produce high-fidelity, photometrically robust, high contrast coronagraphic images. To remedy this situation are undertaking a rigorous, homogeneous, and complete recalibration and analysis of the full archival set of raw NICMOS coronagraphic images (through HST Cycle 15) previously obtained and residing in the MAST to create a Legacy library of template PSFs enabling the recovery of the large body of science otherwise lost. This PSF library and enhanced data recalibration processes, along with generically applicable analysis software we have created, will: (1) critically augment the needs of observational programs reliant on high fidelity PSF subtractions, (2) increase survey yields and improve photometric efficacy, (3) reduce the observing time (HST orbit allocations) otherwise required for near-contemporaneous reference PSF observations, and (4) greatly enrich the yet-unrealized potential of the many NICMOS coronagraphic observations already acquired from the broad spectrum of science programs previously executed. Here we discuss the recalibration mythologies, data products, and analysis tools that we have developed under HST AR program 11279 and are delivering to STScI/MAST.
Calibrators for the in-orbit spectrophotometric calibration of the Medium Resolution Spectrograph of MIRI onboard the JWST.
DR Theodore Nakos (University of Ghent)
We present a list of 32 stellar sources selected for the in-orbit calibration/characterisation of the Relative Spectral Response Function (RSRF) of the Medium Resolution Spectrometer (MRS) of the Mid-InfraRed Instrument (MIRI) onboard the James Webb Space Telescope (JWST). As these sources will be used for spectrophotometric calibration, it is of key importance that they span a wide range of spectral types. The different selection criteria used for the list compilation and the analysis steps of the obtained data are described. The first results of the source modelling are also presented.
WFC3 UVIS Detectors
DR Sylvia Baggett (STSCI)
Installed on the Hubble Space Telescope (HST) in May 2009, the Wide Field Camera 3 (WFC3) is a panchromatic camera possessing both a UVIS and IR channel. The UVIS channel contains a pair of 4096x2051 pixel e2v Technologies CCDs which cover a 160x160 arcsec field of view at 0.04 arcsec/pixel. A set of 62 filters (including the quads) plus an ultraviolet grism provide coverage of the full spectral range, from 200 to 1000nm. On-orbit performance has been excellent: read noise is ~3e-, dark current is now ~3 e-/pix/hr, and linearity is better than 1% up to 65K-70K e-, depending upon amp. The monthly anneal procedures have been effective, repairing a large fraction of the hot pixels that develop over time, the charge transfer efficiency is evolving as expected, and the 'bowtie' monitor program has successfully prevented any hysteresis issues. This poster will summarize the detector performance over the first year of on-orbit operations.
Performance and Calibration of Wide Field Camera 3
DR John MacKenty (Space Telescope Science Institute)
The Wide Field Camera 3 was installed in HST in May 2009 during SM4. Designed to provide a factor of 10 or better increase in the near-ultraviolet and near-infrared imaging capabilities of the observatory, WFC3 has surpassed these expectations by large margins and is operating well. This talk will provide an overview of the instrument's design, its in-flight performance, and our strategy and priorities for its calibration.
On-Orbit Performance of COS Target Acquisitions
DR Steven Penton (University of Colorado)
We present an analysis of the on-orbit performance of the COS target acquisition (TA) procedures, both imaging and spectroscopic. In particular, we will discuss the impact of the Cycle 17 FGS-to-COS science instrument aperture file (SIAF) adjustments and their impact on Cycle 18 COS TAs. Many observers will be able to skip the time consuming ACQ/SEARCH procedure and proceed directly to the imaging (ACQ/IMAGE) or spectroscopic (ACQ/PEAKXD+ACQ/PEAD) peakup stages. Observers requiring the best photometric accuracies will be advised as to the best TA strategies for NUV and FUV observations.
Calibration of the HST NICMOS F110W Using High Redshift Red-Sequence Galaxies
DR Pascal Ripoche (LBL)
We present a new method to measure photometry zero-points using high redshift red-sequence galaxies. This method allows us to measure the HST NICMOS F110W zero-point close to the sky level and, thus, avoid calibration errors due to the poorly constrained HgCdTe non-linearity at these faint flux levels. This is the level at which almost all of the highest redshift supernovae observations are obtained with NICMOS camera. We combined HST optical and VLT near-infrared observations of distant red-sequence galaxies to constraint their spectral energy distribution and thus derived NICMOS calibration from the observed fluxes with NIC2 F110W. Using 23 red-sequence galaxies in three distant galaxy clusters, we determine the absolute NICMOS F110W zero point to 2.5% accuracy. This result shows that the non-linearity is over-corrected using the standard STScI pipeline leading to a significant zero-point offset at the sky level. This work has been supported by the Office of Science, U.S. Department of Energy, through contract DE-AC02-05CH11231 and in part by NASA through grants associated with HST-GO-10496 and HST-GO-11799.
A New Technique for Measuring Absolute Proper Motions with HST: Using Background Galaxies as Positional References
DR S. Tony Sohn (STScI)
Over the past few years, it has been demonstrated that HST is well suited for high accuracy astrometric sciences. For example, HST's imagers have been used to measure relative proper motions of stars for deriving the internal dynamics of globular clusters. Absolute proper motions have also been measured in several studies but so far, they relied on a single reference object, a quasi-stellar object (QSO) within the field. In this talk, we present a new way of measuring proper motions of stars. Instead of relying on background QSOs, we developed a technique that uses numerous background galaxies as positional references. We discuss this technique in detail and present preliminary results for one test case, proper motion of M31. Our new method can in principle be applied to any existing multi-epoch HST data and opens up a possibility to measure accurate transverse motions for galaxies out to a few Mpc.
Measurement of the Count-Rate Non-linearity in WFC3-IR Detectors
DR Robert Hill (NASA/GSFC)
The Detector Characterization Laboratory at NASA/GSFC has investigated the count-rate non-linearity (reciprocity failure) characteristics of 1.7μm cut-off HgCdTe devices that are very similar to the WFC3-IR detector. The reciprocity failure follows a power law behavior over the range of fluxes tested (0.1-10^4 photons/second). The slope of the power law varies among detectors, ranging from ~0.3-1%/dex at 1.1μm, which is much smaller than the ~6%/dex effect observed with the HST NICMOS 2.5μm cut-off detectors. Reciprocity failure is nevertheless an important effect in the calibration of WFC3 data, as well as in other applications in which there is a large difference in flux between the photometric standards and the scientific sources of interest. Furthermore, the variation among detectors demonstrates that demonstrates that a measurement made with the WFC3-IR detector is necessary in order to provide a proper calibration of the WFC3-IR data.
Post-SM4 ACS/WFC Bias Striping: Characterization And Mitigation
DR Norman Grogin (STScI)
All images taken with the restored ACS Wide Field Camera (WFC) exhibit a modest additional noise component in the bias level that is constant along rows of the two WFC CCDs (spanning both amplifier quadrants on each CCD). We believe this to be caused by low-frequency (1 mHz to 1 Hz) 1/f-noise on the reference voltage from the new Application-Specific Integrated Circuit (ASIC) that governs the offset of the pixel signal subsequent to its correlated double-sampling. The striping amplitude is roughly Gaussian with sigma=0.9-, as compared with the underlying WFC read noise of 4.2e- (gain=2). The OPUS pipeline superbias and superdark images, averaged from dozens of individual biases and darks, show very little residual striping and thus do not exacerbate the striping in the pipeline-processed science images. The impact to most science programs will be insignificant; however we recognize that the highly correlated nature of this striping may complicate analyses at very low surface brightness, particularly for images where the background counts are also low. In addition to our detailed characterizations of the striping and its evolution during Cycle 17, we present results from our testing of stripe-removal codes to correct a broad variety of science images. We have determined that the WFC overscan is a poor estimator of the stripe amplitude, requiring the stripes to be fit across the imaging area where care is needed to avoid removing source flux as well. STScI is making the stripe-removal code available to the community as stand-alone software to correct flat-fielded ACS/WFC images (FLTs) prior to drizzling.
Monitoring of the Wavelength Calibration Lamps for the Hubble Space Telescope
DR Ilaria Pascucci (Space Telescope Science Institute)
The Space Telescope Imaging Spectrograph (STIS) and the Cosmic Origins Spectrograph (COS) are the two optical-UV spectrographs on board the Hubble Space Telescope. To determine the wavelength scale for individual science observations, internal arc lamp spectra accompany most observations of external targets. Here we present a detailed analysis of the changes in the COS and STIS internal lamp fluxes and spectra over time, and also compare our results to pre-launch ground testing, and to laboratory accelerated aging testing of similar lamps. Most of the analysis presented here focuses on the behaviour of the lamps in the far-UV (FUV). We find that the STIS LINE lamp has faded by a factor of about 15 in the very short FUV wavelengths (1150-1200 Angstrom) over the 13-year period on which STIS was in space, a much steeper fading than predicted from accelerated aging tests in the laboratory. We also find that all STIS lamps have faded during the period in which the spectrograph was not operational (2004-2009) thus pointing to on-orbit conditions as an additional and important cause of lamp fading. We report that the COS P1 lamp output appears to decline with usage with a similar slope as the LINE and HITM1 lamps on STIS. Finally, we recommend switching from the LINE to the HITM2 lamp for a more efficient wavelength calibration of the STIS settings covering the very short FUV wavelengths.
An Overview of Detectors
DR Bernard Rauscher (NASA Goddard Space Flight Center)
Removing the instrument signature of the detectors is an important part of calibrating any imager or spectrograph. When doing this, it helps to understand how the detectors work, and the kinds of artifacts that can manifest themselves. This talk will provide an overview of the hybrid HgCdTe and Si:As detectors that will be used by the James Webb Space Telescope, with an emphasis on calibration.
Recovery of very low signal-to-noise transiting lightcurves using self-coherence
MR Ingo Waldmann (University College London)
The detection and characterisation of telluric planets from space and ground facilities is one of the main goals of future exoplanet research, thus the importance of being able to fully recover and understand data in low signal-to-noise (SNR) conditions is crucial. Methods such as ’self-coherence’ allow for the retrieval of signals buried far below the noise level. In particular, at SNRs < 1 the noise may suppress high-order coefficients in the eclipse signal’s Fourier series. As a consequence the shape of the retrieved transit-curve becomes inherently linked with noise, as well as the overall morphology of the timeseries. Understanding the details of this process is key to fully recovering the shape and depth of the original signal. In this paper we have used Monte-Carlo simulations of very low SNR data, to investigate morphological changes of the original eclipse at low-SNRs. Furthermore, we demonstrate the full recovery of the eclipse’ transit-depth down to SNRs of 0.15 per spectral channel using calibration techniques described here. A clear understanding of methods such as self-coherence is essential for probing ever weaker atmospheric signatures such as the ones observed for extrasolar planets.
The NICMOS Legacy Archival Calibration Project
DR Anton Koekemoer (STScI)
The legacy recalibration and reprocessing of the entire archive of NICMOS data, consisting of more than 100,000 observations obtained since its installation on HST in 1997, has led to the production of a final archive that includes all the calibration and processing improvements that have been obtained as a result of our improved knowledge of the instrument performance during this time. This archive is unique in being the only set of HST observations in the 1 - 2 micron regime for over a decade until the installation of WFC3 in 2009, and provides the foundation for a wide range of follow-up science. The infrared detectors are susceptible to a wide range of calibration issues that are specific to infrared detectors, including persistence, temperature and bias-dependent effects, and an extensive program of calibration and software development has been carried out to correct these. These have been applied in a full recalibration and reprocessing of the entire archive of NICMOS data, including also MultiDrizzle combination and release in the HLA, resulting in a dataset with dramatically improved scientific value that will serve as a unique and extensive archival legacy in its own right, in addition to benefiting future infrared instruments and science programs.
NEW SCIENTIFIC CAPABILITIES OF THE HST WFC3
PF Robert O'Connell (University of Virginia)
I will describe some of the new scientific capabilities enabled by the extended wavelength coverage (0.2-1.7um), large suite of specialized filters, and improved "discovery efficiency" of HST's Wide Field Camera 3. Some examples from Cycle 17 programs include the star formation histories of nearby disk and early-type galaxies (from both resolved sources and integrated light), discovery of multi-age pre-main-sequence populations in massive Local Group star clusters, the evolutionary state of galaxies at intermediate redshifts (z ~ 1-4), and identification of candidates for the highest redshift galaxies at z ~ 8-9 in the Hubble Ultra Deep Field.
DR John Mather (NASA GSFC)
The James Webb Space Telescope is now in the construction phase, having passed its Mission Critical Design Review in April 2010. I will review the key scientific objectives that were used to set the engineering parameters, and I will describe the observatory concept and the characteristics that are important to calibration. The JWST's optical system is adjustable after launch, so its optical performance is time-dependent. The optical train can be perturbed by temperature changes and gradients and by vibrations. In addition, the wide range of instrument capabilities from imaging to coronography, from tunable filter imaging to integral field and multislit spectroscopy, offer many chances for error and for cross-calibration to discover and compensate for such errors. Obtaining accuracy and precision beyond the basic requirements will be full of interesting challenges.
The JWST Integrated Science Instrument Module: Status and Test Plans
DR Matthew Greenhouse (NASA GSFC)
The Integrated Science Instrument Module (ISIM) of the James Webb Space Telescope (JWST) is discussed from a systems perspective with emphasis on development status and element-level I&T plans. The ISIM is one of three elements that comprise the JWST space vehicle and is the science instrument payload of the JWST. The science instruments, their supporting systems, and plans for element-level testing ahead of ISIM delivery to the Observatory will be discussed.
Application of the SIDECAR ASIC as the detector controller for ACS and the JWST near-IR instruments
DR Markus Loose (Markury Scientific)
The SIDECAR ASIC is a fully integrated controller for high-performance optical and infrared detectors. It combines all functions on a single microchip, including output signal amplification and A/D conversion, bias voltage generation, clock generation, and housekeeping telemetry. The SIDECAR ASIC has been implemented into the CCD electronics box for ACS (during the SM4 repair of the instrument), and it is built into all of the JWST near-infrared instruments. The presentation will give a brief overview of the core capabilities and features of the SIDECAR ASIC, and then focus on the performance and operational aspects relevant to HST/ACS and JWST. Challenges with respect to 1/f noise on the bias voltages and overall strategies for further noise reduction will be discussed. In particular with respect to JWST, flexible data acquisition modes combined with elaborate post-processing have shown to provide significant noise improvements over the baseline approach. In this context, the full in-system programmability (even on-orbit) provides a valuable tool to adapt the operation and configuration of the SIDECAR ASIC to changing conditions.