Wide Field Camera 3 Instrument Handbook for Cycle 19 - Space ...

36 Chapter 5: WFC3 Detector Characteristics and Performance
associated with the normal Poisson distribution of incoming photons. The correction is
theoretically about 1.7 e – /photon at 200 nm, decreasing linearly to 1.0 at 340 nm.
Measurements of ground-based data, however, have indicated that the effect in the
WFC3 chips is much less, 1.07 e – /photon at 218 nm and 1.03 e – /photon at 275 nm in
broadband data (WFC3 ISR 2008-47) as well as monochromatic narrowband data
(WFC3 ISR 2010-11). The cause for this is unclear, but may be due to charge sharing
(Janesick, J.R., 2007, “Photon Transfer DM-->λ“, SPIE, Bellingham, Washington, p
45-48).
Given the low level of quantum yield measured in the WFC3 data, neither the QE
curves presented in Figure 5.2 nor the WFC3 Exposure Time Calculator (ETC)
include the effects of quantum yield. The noise distortion from multiple electrons is
not large compared to other contributions to the signal-to-noise ratio in the ultraviolet
(see Section 9.2).
5.4.3 Flat Fields
Before launch, ground-based flats were obtained for all UVIS filters at a S/N of
~200 per pixel using an external optical stimulus (WFC3 ISR 2008-12). Because the
overall illumination pattern of the ground-based flats did not precisely match the
illumination attained on-orbit from the OTA, there are errors in these ground-based
flats on large spatial scales. These errors are being measured by performing stellar
photometry on rich stellar fields that have been observed using large-scale dither
patterns during SMOV and cycle 17. In the SMOV exposures, the rms difference
between the average magnitude of a star and its magnitude in the first pointing varied
from 1.5% to 4.5%, from the long to the short wavelengths (WFC ISR 2009-19).
Reference files resulting from the completed analysis of all of the data are expected to
support photometry to ~1% accuracy over the full WFC3 UVIS field of view at
wavelengths longer than 350 nm, and to 2-3% accuracy at UV wavelengths. A
detailed description of the production of UVIS flat field reference files is given in
Section 5.4 of the WFC3 Data Handbook.
The latest information about UVIS flats can be found on the WFC3
Web site: http://www.stsci.edu/hst/wfc3/analysis/uvis_flats
Figure 5.3 shows examples of bias-corrected ground-based flats for two wide-band
filters. Both are displayed with an inverse greyscale stretch chosen to highlight
features; the vignetting in the upper-right corner is not instrument-related but an
artifact of the optical stimulus. The crosshatch features in the UV flat field (F336W)
are normal, due to the detection-layer structure in the CCDs; the level is typically