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). The channel is 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 be filled in by using appropriate telescope dithering strategies (see Section 6.11.1 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 in 2009, the UVIS CCDs had excellent CTE. As discussed in Section 5.4.11, the on-orbit radiation environment damages the CCDs over time and causes degraded CTE (charge transfer efficiency); in addition, the CTE degraded somewhat more than expected over the first few years of operation due to the solar minimum at the time. Observers with exposures with low sky backgrounds should use the post-flash mode (implemented in 2012) to avoid large CTE losses (Section 6.9.2).