WFC3 TV2 Testing: Image Stability

ABSTRACT I review the image stability tests performed during the second thermal-vacuum (TV) campaign of WFC3. In the first TV campaign, significant drifts were seen in both channels in response to induced thermal changes in the instrument, but subsequent analysis indicated that the induced temperature variations were unrealistically large (compared to on-orbit conditions). In the current tests, the magnitude of the thermal variations was reduced to approximate conditions that will be seen on orbit. During those times when conditions provided a realistic simulation, the image stability was within specifications for some, but not all, of the time. Specifically, the image stability was acceptable during cold-case simulations of orbital cycling and telescope slewing, but not during hot-case simulations of these variations. During hot orbital cycling, the drift rates in the UVIS and IR channels were up to 1.3x that in the CEI specifications, while during hot orbital cycling and slewing, the UVIS drift rates were up to 2x that in the specification, and the largest IR drift rates were still at 1.3x the specification. The stability test was also run for significant periods when the thermal variations were much larger than anticipated on orbit, and during these times, the image drift rate often exceeded specifications.