When the default gain for the UVIS detector is used, photometric information well beyond saturation can be recovered for relatively isolated sources in unbinned exposures (where the CCD full well is the limiting factor), but not in binned exposures (where the ADC is the limiting factor). This is discussed in detail in
Section 5.4.5 and
Section 5.4.6.
The cosmic-ray fluxes for WFC3 UVIS are comparable to the levels seen in ACS, STIS CCD, and WFPC2. As with these previous
HST instruments, typical WFC3 imaging observations need to be cr-split or dithered to obtain adequate cosmic-ray removal (see
Section 5.4.10). Dithering can also mitigate bad pixel effects, and can be used to sample the point spread function; it is recommended for the vast majority of observations.
No significant image-persistence effects following over-exposure were observed in instrument-level ground test or on-orbit data using the UVIS CCDs, as expected for back-illuminated devices.
Shutter-induced vibration, or shutter jitter, affects only very short exposures. Shutter jitter causes slight blurring in image data and is not to be confused with exposure time deviation. Exposure time deviation is discussed in
Section 6.7.1, and the UVIS shutter mechanism is described in
Section 2.3.3.
The image quality analysis carried out during the third thermal-vacuum campaign revealed that vibrations of the UVIS shutter caused changes in the width and in the central pixel flux of point sources in short exposures. The analysis, and a schematic of the UVIS shutter mechanism, are given in
WFC3 ISR 2008-44.
To test on-orbit performance, observations of the calibration standard star GD153 were acquired during SMOV. These confirmed that only the shortest exposures (0.5–3.0 sec) were significantly affected by the shutter vibrations. Furthermore, only side B of the shutter blade degrades the PSF. There is no noticeable degradation in exposures taken with side A of the shutter blade. Because the shutter blade always rotates in one direction, a series of short exposures show an odd/even effect, with exposures acquired with the shutter side B being blurred and shutter side A exposures being unaffected.
For the shortest exposures (0.5 s), shutter vibrations produce a side-B PSF that is ~15% broader than the side-A PSF. At 1.0 sec, the difference in FWHM between the two shutter sides drops to 7%, and it is less then 1% for 10 sec exposure times. For images taken with side A shutter operation, no significant PSF width changes were seen between 0.5 s and 350 s exposures. No flux is lost due to shutter jitter, so the effect can be mitigated in point-source photometry by using big enough aperture (r > 5 pixel) to measure the flux. The analysis of the shutter jitter effect using SMOV data is fully described in
WFC3 ISR 2009-20.
The outer window is contaminated, seemingly by a mineral residue introduced during acceptance testing of WFC3. These contamination features have been dubbed “droplets” due to their appearance at the time of discovery. In external flat-field images, these features have a strength of approximately ±0.5%. The droplets cause changes in PSF profile, such that flux in the core is redistributed to the near wings. In large-aperture (10 pixel radius) photometry of point sources stepped across a strong window feature, the feature does not significantly increase the photometric scatter. For small-aperture (3 pixel radius) photometry of point sources stepped across a strong window feature, the photometric scatter increases from ~0.5% to ~1%. Quadrant A has the lowest density of features. There are approximately 50, 129, 108, and 179 droplets in quadrants A, B, C, and D, respectively.
The best strategy for mitigating the flat-field features is an appropriate dither pattern. Although there are positions within a flat-field feature that cause systematic errors at the level of a few percent in point source photometry, other positions separated by 20 to 40 pixels show much smaller errors, suggesting that dithers on this scale would be sufficient for most photometric programs. To ensure a point source does not hit a particular feature twice requires larger dithers of approximately 100 pixels, which is the typical diameter of these features.
WFC3 ISR 2008-10 describes the characterization of the droplets and their photometric effects based on ground testing, and
WFC3 ISR 2009-27 reports that about 30% of droplet positions have shifted by about 1 pixel after launch, but have been stable since then.
