6.3 UVIS Channel Characteristics

In concept and functionality, as well as in many design details, the WFC3 UVIS channel is patterned after the ACS/WFC channel. The UVIS channel contains an optical train providing focus and alignment adjustments as well as a correction for the OTA spherical aberration, a filter-selection mechanism, a shutter mechanism, and a CCD detector assembly (which uses the same camera-head design as ACS/WFC). These are supported by a thermal-control subsystem and also by control and data-handling electronics subsystems.

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 6.12.1 and Appendix C). 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.