WFC3 Near-IR Channel: PSF and Plate Scale Study

This report focuses on the PSF properties and plate scale optimization for the Near-IR channel of WFC3. We discuss how various effects lead to the final observed PSF in NICMOS Camera 2 and suggest apodization of the cold stop as a possible way to improve contrast for the Near-IR channel of WFC3. We derive a model for the pixel response of the NICMOS arrays and use this model to produce simulated images which are then employed to evaluate the WFC3 photometric accuracy as a function of pixel scale. We find that images with a pixel size of 0.15’’ are shallower than those with a pixel size of 0.10’’ by about 0.3 mag. This can be compensated by longer exposure times still maintaing a 45 percent discovery efficiency advantage for the larger pixel size. The difference between the two pixel sizes in the presence of crowding is much smaller than the deviation of both from the ideal case of infinite resolution.