FAQ on FOC PSFs and Camera Focus

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.