The pixel scale varies across the focal plane with an average 0.265″ / pixel. Significant distortion is present on the 4 detectors in the corner, which might be more adequate for guiding or aperture photometry.
The 8 central CCDs are completely illuminated while the 10 peripheral CCDs have a variable amount of vigneting which can be as high as 60% at the outside borders.
To quickly focus the camera, the control system shifts the CCD charges at various focus positions delivering a focus image as follows:
The focusing algorithm estimates the focus position using all the stars in the focal plane (between 20 to 400 stars depending on the galactic latitude) and providing information of the focus and flatness of the focal plane mosaic:
Typical focus value is around 82.3 mm.
Central detectors deliver a low ellipticity and FWHM PSF (depending on seeing conditions).
Below you can find a sample image of a 100s exposure using the r filter.