Q: What is the field dependence of the PSF? A: Observations taken during SMOV and Cycle 4 have provided some information about the field dependence of the PSF. It is clear that the PSF is visibly different away from the central field point across the largest formats. Some residual astigmatism stretches the core of the PSF a few pixels in a line away from the central field point. However, this does not affect the encircled energy within 0.1 arcsec radius by more than a few percent for any field position within the 512x512 aperture. Q: Is the structure of the PSF repeatable in the 0.1-0.2" range? A: The FOC PSF changes considerably due to the focus of the telescope. These focus changes arise due to OTA breathing and desorption. The result is that PSFs in images taken back to back can have a very different structure in the core, while still maintaining nearly the same encircled energy. These changes are most dramatic in the near-UV, around 2000 Angstroms, and decrease in severity both towards the far-UV and the visible. The FOC team is currently monitoring the focus on a regular basis and maintaining the best focus possible.
