As described in Section 5.2.2
, the detectors in the WFC3 UVIS channel are two 4096×
2051 pixel CCDs, butted together to yield a 4096×
4102 light-sensitive array with a ~31 pixel (1.2 arcsec) gap. The gap can, of course, be filled in by using appropriate telescope dithering strategies (see Section 6.11.1
and Appendix C:Dithering and Mosaicking)
. The plate scale is approximately 0.04 arcsec per pixel, providing a good compromise between adequate sampling of the PSF and a wide field of view. Geometric distortions introduced by the WFC3 optics cause the nominally square detector to map onto the sky as a rhombus, about 162 arcsec on each side.
Upon the start of on-orbit operation, the UVIS CCDs had excellent CTE. As discussed in Section 5.4.11
, the CTE has degraded significantly over the first few years of operation due to the phase of the solar cycle. Observers of faint targets in exposures with low sky backgrounds should use the post-flash mode implemented for Cycle 20 to avoid large CTE losses. See Section 6.9.2