Being above the atmosphere, NICMOS is not forced to adopt filter bandpasses like instruments used at ground-based observatories. Instead it has filters which were designed to meet the anticipated scientific demands. Thus in practise NICMOS does not have filters matched to any of the ground-based photometric bands. Obtaining photometric calibrations for NICMOS data is discussed in Chapter 5 of the NICMOS Data Handbook.
The NICMOS team has determined that NICMOS suffers from a count rate dependent non-linearity which influences accurate photometry, especially at the shorter wavelengths. If your science requires better than 15% accuracy in photometry, apply the non-linearity correction rnlincor routine described on the non-linearity page to your reduced data.
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The NICMOS group has derived new calibration zeropoints for the NICMOS cameras, both pre- and post-NCS installation. The values presented here were determined using all calibration data taken before June 2004 and supersede all previous tables and values in the handbook. Values on this web page will always be the most up-to-date known. Due to the change in the nominal operating temperature of the detectors from ~62K before the NCS-installation to the current 77.1K, the photometric keyword values have changed. The QE has increased by 20-90% (depending on wavelength) and different sets of calibration constants have to be used for pre- and post-NCS data.
In the effort to calibrate NICMOS after NCS installation, all pre-NCS Cycle 7 calibration data were also analyzed again with the latest data reduction techniques. This has resulted in new calibration values for these data as well. The PHOTTAB photometric table that is in the calibration database and that is being used to populated image headers during on-the-fly-reduction is still an old 62K, pre-NCS table. These photometric keywords have changed for Cycle 7 data by typically 5-10% due to improved aperture corrections and these keywords are NOT APPROPRIATE for data taken at 77.1K in Cycles 11 and beyond. The new PHOTTAB tables with calibration constants were installed in the pipeline on July 29, 2004. Any observations retrieved from the archive before July 29, 2004 will have the old, incorrect photometric header items. Retrieving this data again after July 29, 2004 will fix the problem, as the on-the-fly-reduction will automatically populate the headers with the correct calibration constants. Alternatively, headers can be updated by hand using the tables listed below on this webpage.
The new NICMOS Photometric Keywords were derived from principle calibration stars P330E (a Solar analog star) and G191B2B (a white dwarf). The calibration values are the error weighted averages of the values derived for the two stars individually. We have set the error in the calibration keywords such that they reflect the uncertainty in the individual star measurement errors and the discrepancy between the two stars. We have not included the uncertainty in the absolute calibration of the reference star spectra in our error estimates. These absolute calibration uncertainties are believed to be less than 5%. This reference spectrum uncertainty will also introduce a much smaller error in colors derived from NICMOS observations in different passbands, which are thought to be less than 3%. This effect is most significant for the narrow-band filters, when the exact shape of absorption bands is not exactly modeled in our reference spectra and falls in the narrow passband. In our new calibration tables on the web we also list the wavelength dependent aperture corrections we have used to go from our fixed aperture calibrations (11.5 pixels for NIC1, 6.5 pixels for NIC2, and 5.5 pixels for NIC3) to infinite aperture calibration listed. We have derived these aperture corrections from TinyTim simulations.
We have identified variations in the photometry calibration data at the less than 1% level that are still not fully understood. The individual cameras show sensitivity variations at this level, which means that all passbands in the same camera are affected in the same way and therefore color measurements should still be very accurate over time. We also see some discrepancy between the two primary standards in NIC3 that are not fully explained.
No new photometric calibration of the polarimetry filters of NIC1 and NIC2 and the grisms in NIC3 has been performed. The calibration values for these optical elements in the calibration tables reflect the initial calibration of NICMOS performed in Cycle 7 and have only been corrected for the observed detector sensitivity gain for the post-NCS era. THESE VALUES WERE DERIVED WITH A DIFFERENT METHOLOGY AND SYSTEMATIC OFFSETS WITH THE ZEROPOINTS OF THE OTHER FILTERS OF ORDER 5-10% CAN BE EXPECTED.
Any updates to the calibration standards will be announced through this website, as well as through STAN announcements.
Photmetric Keywords and Vegamag Zeropoints for 77.1K Detectors (Appropriate for all data taken after January 1, 2000)
Photmetric Keywords and Vegamag Zeropoints for 62K Detectors (Appropriate for all data taken prior to January 1, 2000)
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This page summarizes all the information concerning NICMOS photometry including absolute calibrations of the cameras, its photometric stability and the absolute calibration of the NIC3 grisms.
Aside from Chapter 5 of the Data Handbook, other important photometry resources also provide valuable high level sources of photometry information: Differential Photometry Across the Detectors, Intrapixel Sensitivity, Photometric History of NICMOS, Absolute Spectrophotometric Standards, and Systematic Uncertainties and Special Cases. Another important photometry reference is NICMOS to Ground Based Systems Transformations. For more specific details on photometry follow the links to the documents listed below.
NICMOS Photometry Documentation
Important updates, discoveries and developments that could potentially affect NICMOS observations, calibration, or data analysis.
Frequently Asked Questions.
Q: Photometry A: ...
Status reports reflecting the current understanding of instrument characteristics, performance and calibration.
The NICMOS Instrument Handbook is the primary guide regarding the characteristics and use of the instrument. The HST Data Handbook is the primary guide for calibration, reduction and analysis of NICMOS data.
Instrument Science Reports:
ISRs are technical reports written by members of the NICMOS Group about various aspects of the instrument and data. They usually contain in-depth information about specific topics.
R. Bohlin 15 Feb 2006
R. de Jong 15 Feb 2006
E. Bergeron and M. Dickenson 06 Oct 2003
C. Xu and B. Mobasher 16 Sep 2003
R.L. Gilliland and S. Arribas 07 Jan 2003
M. Sosey 15 Jan 2002
C. Skinner 01 Dec 1995
Papers and Proceedings:
Selected NICMOS related published papers and workshop proceedings.
On-orbit Properties of the NICMOS Detectors on HST
L. Bergeron,A. Schultz,J. MacKenty,A. Storrs,W. Freudling,D. Axon,H. Bushouse,D. Calzetti,L. Colina,D. Daou,D. Gilmore,S. Holfeltz,J. Najita,K. Noll,C. Ritchie,W. Sparks,A. Suchkov
Cookbook style instructions, prescribed procedures, and helpful tips.
Examples of how to use various NICMOS-related tools, calibration and analysis techniques.
Space Telescope Analysis Newsletters:
STANs contain useful information regarding calibration and data reduction.
NICMOS STScI Analysis Newsletter 15
NICMOS standard stars
NICMOS End-of-Life monitoring
NICMOS Prime Science Observations Update
NICMOS at the January 1999 Texas AAS Meeting
NICMOS Group 01 Nov 1998 (html)
NICMOS STScI Analysis Newsletter 10
Announcement of the NICMOS Photometry Update web page
Update of NIC1 & NIC2 photometry
New Special Requirements for Proposal Processing
NICMOS Group 01 Apr 1998 (html)