Faint Object Spectrograph and Camera
EFOSC2, being a Focal Reducer, distorts the PSF to a considerable extent
and this distortion varies across the CCD. We determined the PSF shape
variation by determining the PSF ellipticities in acquisition (20 second
exposure) images of dense stellar fields under good seeing (< 0.75 arcsec)
conditions and then averaged the values as a function of position on the
CCD. The resulting PSF map is plotted below.
The vectors indicate the orientation and magnitude of the PSF ellipse
at that location.
It must be noted that this represents the minimum distortion that one
should expect, which is "intrinsic" to the instrument/CCD combination.
The image quality on any single image could be worse due to improper focus,
telescope tracking effects, telescope optics distortion (due to stresses
in primary and secondary mirrors etc).