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 hour. They were characterized during ground-based testing (WFC3 ISR 2009-43
) and were hypothesized to be caused by the emission of alpha particles by radioactive isotopes in the detector (WFC3 ISR 2009-44
). The rate of appearance of snowballs has remained constant over 5 years of on-orbit observations, consistent with the hypothesis that the uranium-238 decay chain is the source (WFC3 ISR 2015-01
). Typical snowballs affect about 8 to 25 pixels, saturate 2-5 of those, and deposit about 200,000 to 500,000 electrons on the detector. The energy is deposited in the pixels instantaneously, so snowballs can be removed via up-the-ramp fitting, like cosmic rays.
As 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. (There is also some dependence on the location on the detector.) 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 7.10
. 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.
While the screening process generally eliminates the worst cases of persistence, such as those in Figure 7.10
, 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.
Measured flux rises with the number of exposures in a series of undithered exposures (WFC3 ISR 2016-11
). This can affect photometry at the level of 0.02-0.04 magnitudes. This phenomenon, likely associated with persistence, is related to the slow rise in measured flux seen in grism observations of planetary transits.
In 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 (e.g., see section 4.7 in the NICMOS Instrument Handbook
). This wavelength lies near the red end of the reddest WFC3/IR filter, F160W (WFC3 H). A variable airglow line of He I at 10830 Å can be a significant, even dominant, component of the sky background in the F105W and F110W filters and the G102 and G141 grisms. (See WFC3 ISR 2014-03
shows the observed background levels in the first few months of WFC3’s operation (green points and error bars), compared with values predicted from known instrument sensitivities and three levels of zodiacal light selectable in the Exposure Time Calculator
(ETC). Even allowing for small sample sizes and possible systematics in the observations, it is apparent that a rough estimate of the zodiacal light level may not adequately predict the observed background level of an exposure. The WFC3 team has performed extensive studies of zodiacal light and earth-shine in WFC3/IR exposures (thereby discovering the impact of the He I airglow line) to improve our understanding of the sky background and to enable us to provide practical advice to observers.
The average, low and high zodiac points show the synphot
predictions for three levels of zodiacal 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.
The zodiacal background level observed in WFC3/IR exposures has been modeled as a function of ecliptic latitude and angular distance of the target from the sun (WFC3 ISR 2014-11
). (The dependence can be formulated as a function of ecliptic latitude and angular separation of the target from the sun in ecliptic longitude, or ecliptic latitude and the angle between the target and the sun as viewed by HST. The latter coordinate system has been adopted because it has practical advantages for planning and analyzing HST observations.) The model derived for filter F160W is shown in Figure 7.12
. The Sun Angle in the figure corresponds to the keyword SUNANGLE in the HST science header. WFC3 ISR 2014-11
gives scaling factors to apply this model to other IR filters and the G141 grism, and shows that the model provides significantly better estimates of the zodiacal light than the model used by the ETC.
The highest levels of zodiacal light are avoided automatically by the HST observatory requirement that the Sun Angle be greater than 50°. For programs where a low background level is critical to the science goals, observers with a target at low ecliptic latitude may want to increase the solar avoidance angle from the default minimum of 50° to 60° or greater by excluding the range of dates when the target would be at the undesired angles, using Figure 7.12
as a guide. This can be implemented by applying BETWEEN statements to visits in the phase II proposal. The range of dates corresponding to the 50° exclusion rule can be displayed in the Visit Planner window in APT by clicking on the arrowhead that expands the schedulability plot for a visit into its components; Sun is the Sun Angle component. (Hold the cursor on a date range band to read the dates.) At low ecliptic latitudes, the Sun Angle of a target changes about 1° per day, so trimming 10 days from each end of the Sun schedulability date range increases the minimum Sun Angle to 60°. This approach provides much greater opportunities for scheduling than the use of the LOW-SKY option (Section 9.7.1
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. Selective data reduction, excluding some frames of the timing sequence in the affected exposures, can be a better option than accepting the shorter visibility period required by LOW-SKY. (See WFC3 ISR 2014-03
and Section 9.7.1
). Targets observed in the continuous viewing zone (CVZ) will always be rather close to the Earth's limb, and so can sometimes see an elevated background for a larger part of the orbit, even at shorter wavelengths where zodiacal emission ordinarily dominates. Observers have the possibility of specifying a non-standard Bright Earth Avoidance (BEA) angle, which increases the angle from the Earth's limb from 20° to 25°, but this comes at the cost of observing time and would not significantly improve most programs. Note that this is an available mode and must be specially requested through a Contact Scientist.
Investigation of unexpectedly high background levels in some WFC3/IR imaging and spectroscopic exposures led to the identification of a He I airglow line at 10,830 Angstroms as the source (WFC3 ISR 2014-03
). This line is included in the passbands of the F105W and F110W filters and both IR grisms. It is negligible in the Earth's shadow, generally strongest at low Earth limb angles outside the shadow, but sometimes strong even 40° above the Earth limb. In the worst cases, the airglow line strongly dominates the background emission. Examples of strongly affected exposures (with F105W, G102, G141) and unaffected exposures (with F125W, F160W) are illustrated in Figure 7.13
Strategies for processing exposures with frames affected by strongly variable airglow are described in WFC3 ISR 2014-03
and WFC3 ISR 2016-16
. If an flt
image has an unexpectedly high background or excessive cosmic ray flagging, the observer should check the ima
files for evidence of a variable background. The standard up-the-ramp fitting of the non-destructive reads in a WFC3/IR exposure does not work properly on images with a variable background since the variations are treated as cosmic rays. An improved flt
image (at least for unsaturated sources) can be obtained by rerunning calwf3
to turn off the ramp fitting and cosmic ray rejection. Cosmic rays can then be removed by using AstroDrizzle to combine sets of exposures. Alternatively, after running calwf3 without cosmic ray rejection, one can “flatten” the nonlinear ramps by subtracting the variable background of each read averaged over many pixels and adding back a constant for the overall average background count rate, then feed the flattened ima
files back to calwf3
for the final step of identifying the cosmic rays. Scripts and examples are provided in WFC3 ISR 2016-16
Image blemishes peculiar to the WFC3-IR detector have been described in Section 5.7.7
. Other spots of reduced sensitivity in IR images, dubbed “
, are caused by reduced reflectivity of the Channel Select Mechanism mirror. They typically have a measured half-light radius of 10-15 pixels and absorb up to 15% of the incoming light at their centers.
Blobs were first noticed in IR images shortly after WFC3 was installed on HST
. Their characteristics and the history of their occurrence are described in detail in WFC3 ISR 2010-06, WFC3 ISR 2012-15
, and WFC3 ISR 2014-21
(revised in March 2018). Pixels affected by blobs were initially identified in a single bad pixel table that was designed to be applied to data taken after June 13, 2010 (WFC3 ISR 2012-10
). This table has been superceded by a series of date-dependent bad pixel tables that track the appearance of “strong” and “medium” blobs (WFC3 ISR 2014-21
), as shown in Figure 7.14
A “blob flat field” can be used to improve stellar photometry in crowded fields. Blob flats made as described in WFC3 ISR 2014-21
are available at the WFC3 IR flats webpage
. Corrections as large as 0.1 mag have been found for blob-impacted stars in Omega Centauri, and the accuracy of the corrected photometry is comparable to the photometric accuracy for stars in blob-free regions (WFC3 ISR 2015-06
). Most observations will not be significantly affected by blobs, since blobs occupy only 1% of the detector area and their effects can be mitigated by dithering and drizzling. 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.