CCD optical imagers are the workhorses of nearly all telescopes.
On large apertures, these imagers are invariably mosaics of
several CCDs in order to provide a large field of view (FOV),
and astronomy has been enriched by their exciting discoveries.
However, such mosaics are not without their drawbacks. Each chip
may have slight differences in the shape of its DQE curve,
resulting in substantially different color terms from chip to
chip. In their study of stars in nearby galaxies, Massey et al.
(2006, AJ, 131, 2478) argue that for 1% photometry one really
had to treat the 8 chips in the KPNO/CTIO Mosaic Cameras as
separate detectors. Because of the gaps between chips, dithering
is needed to fill in the gaps. For stellar photometry that then
means that one needs to do 8 (chips) x 5 (dithers) = 40x more
work than with a single chip and a single pointing. Worse, such
instruments greatly compromise the astronomer's ability to do
very faint surface brightness studies: the sky brightness is
constantly changing with