|Space Telescope Science Institute|
|WFC3 Instrument Handbook|
Like the UVIS channel, the IR channel uses 16-bit Analog to Digital Converters (ADCs), providing a digital output signal in the range between 0 and 65,535 data numbers (DNs). The default gain setting for the IR channel is 2.5 electrons/DN and is the only one offered to observers. Note that this setting results in a measured gain of ~2.4 electrons/DN; see Table 5.1.The default gain is compatible with the full-well saturation level of the IR channel, which is about 78,000 electrons (~33,000 DN at the default gain), and with the detector readout noise, of order 20-22 electrons per correlated double sampling.Cosmic rays affect the image quality. On-orbit measurements indicate that cosmic ray events occur at a rate of 11 ± 1.5 CR/s for WFC3/IR. The use of MULTIACCUM mode makes it possible to filter out cosmic rays because it provides a series of intermediate non-destructive reads. The intermediate reads are used by the WFC3 data pipeline to identify cosmic ray hits, similar to the use of CR-SPLITs in CCD observations. AstroDrizzle also checks for and attempts to remove cosmic rays from the “drizzled” images when there are multiple (usually dithered) exposures of the same field. (See the DrizzlePac documentation.)Passages through the South Atlantic Anomaly (SAA) cause the highest number of cosmic ray hits. When the HST is within the predefined SAA exclusion zone, IR observations are not normally taken and the detector is set to auto-flush mode to minimize the effects of SAA passage on instrument performance. Unlike NICMOS, where the detector electronics had to be switched off during SAA passage, it is possible to perform time-critical observations in the SAA with WFC3/IR.Snowballs (named for their fuzzy appearance) are transient extended sources that appear in IR exposures at the rate of about one per half hour. They may be caused by radioactivity in the detector or bonding material (WFC3 ISR 2009-44). A snowball affects between 11 and 34 pixels and contains between 200,000 and 900,000 e-. (See WFC3 ISR 2009-43.) The energy is deposited in the pixels instantaneously, so snowballs can be removed via up-the-ramp fitting, like cosmic rays.7.9.3 On-Orbit DegradationUnlike the CCDs, minimal cosmic-ray damage to the IR detectors was anticipated, and essentially none has been observed. During ground testing using a particle accelerator, the WFC3/IR arrays were subjected to radiation doses much higher than expected in their entire orbital lifetime, without sustaining significant long-term damage or measurable degradation in QE. Searches for the development of new bad pixels are conducted as part of the regular calibration program; the number is growing slowly if at all (WFC3 ISR 2010-13, WFC3 ISR 2012-10). The mean dark current remained unchanged in the first three years of in-flight operations (WFC3 ISR 2012-11). (See Section 5.7.2)In-flight dark reference files made from SMOV and cycle 17 data were delivered in March 2010 (full array) and October 2010 (subarrays) to replace the ground-based reference files. Subarray darks made from non-banded data taken in cycle 18 (see WFC3 ISR 2010-16 and Section 7.4.4) were delivered in February 2011. Full array darks with better signal-to-noise, made using data from cycles 17 through 20, were delivered in October 2013. Summaries of WFC3 reference files are displayed at:7.9.4 Image PersistenceAs discussed in Section 5.7.9, the WFC3/IR detector exhibits image persistence, particularly during and following observations of targets that saturate the detector by more than the pixel full well depth. The amount of image persistence depends primarily on the brightness of the amount of charge accumulated by a pixel in an exposure, and secondarily on the length of time in an exposure. The basic reason for this behavior is that the traps involved in persistence have finite trapping times, as well as finite release times.Image persistence has been observed both within a set of dithered exposures in a single orbit and in observations where the target observed in a previous orbit was particularly bright. Two examples of persistence are shown in Figure 7.8. The left panel shows an image obtained with WFC3/IR in parallel to a COS exposure from program 11519, Visit 0V. The bright diffuse object in the center of the field is the persistence after-image of the nearby Sb galaxy NGC 2841 observed two hours earlier in program 11360 visit R1. The right panel shows an image obtained of the gamma ray burst GRB090423 as part of program 11189 visit H2, which followed observations of globular cluster 47 Tuc (program 11677 visit 19) and a stellar field in Orion (program 11548 visit AJ). The dither patterns used in these sets of observations are clearly visible in the image.Figure 7.8: Persistence seen in WFC3/IR exposures following exposures of a bright nearby galaxy (left) and following dithered exposures of two fields of bright stars (right).Such obvious examples of persistence from previous visits are fairly rare in the HST data. Using information in the Phase II proposals, STScI scientists attempt to identify visits that are likely to cause this much persistence. STScI planners inhibit WFC3/IR observations for several orbits after observations from these "bad actors", long enough for the afterglow images to fade. Observers can help in preventing persistence from affecting future observations with appropriate observing strategies, and, in cases where persistence cannot be avoided, by flagging the problem with written comments in the phase II proposal and notifying the Contact Scientist or Program Coordinator.While the screening process generally eliminates the worst cases of persistence, such as those in Figure 7.8, the process is not perfect and so observers need to be aware of the possibility of persistence in their images. Indeed, in light of the fact that WFC3/IR exposures constitute a significant fraction of the overall HST observing program and in order to maintain HST scheduling efficiency, only a small fraction of the visits can be declared to be "bad actors". This means that many IR images have some pixels that are affected by persistence. In the vast majority of cases, the number of pixels that are affected by persistence due to exposures from earlier visits is small compared to the number of pixels that should be flagged for other reasons. The tools and procedures needed to identify pixels affected by persistence and to mitigate its affect are described in the WFC3 Data Handbook. Observers are advised to take advantages of these tools to check whether the ability to extract science from their data has been adversely affected by persistence. It is the responsibility of observers to check their data and request a repeat of a visit that has been compromised by persistence by filing a HOPR within the prescribed period.While avoiding persistence created by other programs is largely outside of the control of an individual investigator, evaluating whether persistence within a visit will affect the quality of their data is the responsibility of the observer. Specifically, observers need to consider whether persistence within a single visit will affect their ability to extract science from their data. They also need to consider whether there will be a problem with persistence if two visits from their program are scheduled back to back.In carrying out this evaluation, it is important to remember that fairly short exposures (100 sec) of relatively bright (18th mag) stars observed through the broad-band filters saturate pixels in the IRarray and so there will often be “self-induced” persistence within visits. Furthermore, the amount of persistence decays inversely with time. So, as shown in Figure 5.26 in Section 5.7.9, a pixel that has been saturated to an effective fluence of ~100,000 electrons will show an afterglow of about 0.3 electrons per sec at 1000 seconds, but ~2 electrons per sec at 200 seconds. (We do not currently have very good estimates of persistence at shorter times.)In most cases, persistence will not turn out to be a serious problem, particularly if the objects that have the potential to cause persistence are spatially separated from the area of the image from which science is obtained, and/or if the number of affected pixels is small, and/or if dither patterns are small. In these cases, the procedures described in the WFC3 Data Handbook should be sufficient to address any issues associated with internal persistence in the images that will be obtained.However, in some cases, persistence will be an issue, especially if the science target is faint, but the field is filled with considerably brighter objects. In these cases, one should avoid the tendency to set the exposure time for the fainter science target, ignoring the problem that will be caused by the rest of the field. One should be careful in situations where dither offsets are much larger than the PSF or where mosaics are being constructed within a visit. Observers should take care that bright or saturated sources within a field are not dithered across the same parts of the detector that record the highest science priority areas of the field. Planning observations of dense fields, globular clusters, or very bright star formation regions may be complicated in instances where precise photometry or very large dynamic range are required.There are a variety of strategies one can adopt to minimize the effects of persistence. In many cases, the best approach is to take more shorter exposures to limit the numbers of pixels affected. If that is not effective, there may be trade-offs between the number of dithers and the amount of persistence one is willing to tolerate.Various tools to estimate which regions of an image are likely to cause persistence and additional strategies to mitigate its effects are described in Appendix D:Bright-Object Constraints and Image Persistence.
7.9.5 The IR BackgroundIn space, the dominant sources of background radiation are zodiacal light and earthshine at shorter IR wavelengths, and the telescope thermal emission at longer wavelengths. For HST, the sum of these two background components has a minimum at about 1600 nm (roughly lying in the H band).The surface brightness of zodiacal light is a function of the ecliptic latitude and longitude, and reaches a minimum ~120 degrees from the sun, i.e., near, but not at, the ecliptic poles increasing again to 180 degrees (anti-solar direction).Figure 7.9 shows the observed background levels in the first few months of WFC3’s operation (green), compared with values predicted from known instrument sensitivities and expected levels of zodiacal light. Additional analysis of the IR background is underway, as described by Pirzkal at the 2010 STScI Calibration Workshop.Recent analysis of archival WFC3/IR grism and broad-band filter observations suggests that the IR channel is susceptible to a strong airglow line of He I at 10830 Å, perhaps related to auroral activity and seen even at high angles above the bright earth limb (>40 degrees). This line can result in significantly elevated and variable background levels in the IR grisms and filters sensitive to it (G102, G141, F105W, and F110W) at the level of an additional ~1 electron/s/pixel or more above the nominal zodiacal background (see Figure 7.9). The WFC3 instrument team is currently evaluating scheduling options for minimizing the contribution of this feature to the observed background count rate. Observers wishing to minimize the background in reference exposures for the G102 grism can consider using the narrower F098M filter in place of F105W or F110W.Figure 7.9: Infrared background levels for WFC3. The average, low and high zodiac points show the synphot predictions for three levels of zodicial light and no earth-shine. The observations selected include calibration and GO images of empty or sparsely populated fields, giving an accurate representation of early WFC3 observations but not covering all background conditions. Observational means (green points), ranges (error bars), and exposure counts are plotted.For pointings very close to the bright Earth limb, the zodiacal background may be exceeded by earth-shine. The brightness of the earth-shine falls very rapidly with increasing angle from the Earth’s limb (due to the effectiveness of the HST baffles), and for most observations only a few minutes at the beginning and end of the target visibility period are significantly affected. The major exception to this behavior is a target in the continuous viewing zone (CVZ). Such targets will always be rather close to the Earth's limb, and so can sometimes see an elevated background for part of the orbit, even at shorter wavelengths where zodiacal emission ordinarily dominates.For targets faint enough that the background level is expected to be much brighter than the target, the observer has two options when crafting the Phase II proposal:
(1) specify a non-standard Bright Earth Avoidance (BEA) angle, which increases the angle from the Earth's limb from 20° to 25° (note that this is an available mode and must be specially requested through a Contact Scientist), or (2) specify the LOW-SKY option, which restricts observations to targets more than 40° away from the Earth's limb and restricts scheduling to times when the zodiacal background is no greater than 30% above the minimum achievable level. The second option decreases the available observing (visibility) time during each orbit and implies scheduling constraints.7.9.6 BlobsImage blemishes peculiar to the WFC3-IR detector have been described in Section 5.7.7. Other spots of reduced sensitivity in IR images, dubbed “blobs”, are caused by reduced reflectivity of the Channel Select Mechanism mirror. Blobs were first noticed in IR images shortly after WFC3 was installed on HST. Their number increased to ~25 in early 2010, stabilized, then began to increase again in the latter part of 2011. They have a measured half-light radius of 10-15 pixels and absorb up to 15% of the incoming light at their centers. They are described in detail in WFC3 ISR 2010-06. Those appearing through cycle 19 are flagged in the IR bad pixel table (WFC3 ISR 2012-10). Future updates to the bad pixel table are planned. A more detailed analysis of the history of the appearance of new blobs and of the color of the blobs is given in WFC3 ISR 2012-15. Appropriate dither strategies to mitigate the effects of blobs and other artifacts in IR images are described in WFC3 ISR 2010-09. See Section C.2 for the specifications of the WFC3-IR-DITHER-BLOB pattern implemented in APT.7.9.7 Optical AnomaliesAnomalous features are found in some IR detector images. For example, the optical system may cause stray light from sources outside the detector FOV to be scattered into images. Scattered earthlight can greatly increase the background on part of the detector when HST is pointing near the bright earth limb. (See Section 6.10 in the WFC3 Data Handbook.)