On 14 Jul 2007 16:36:08 +0100 (BST), Thomas Womack
wrote:

When you increase the ISO setting, the amount by which an amplifier in
the CCD chip in the camera multiplies the signal before digitising it
is increased. So each photon hitting the camera generates a larger
fraction of a count in the output from the CCD.

Astronomical CCDs often have only the equivalent of a 'very high' ISO
setting.

Actually, astronomical cameras have the equivalent of a very low ISO
setting. They hardly apply any gain at all. This is in contrast to
digital cameras which apply a high gain when the ISO setting is large.
My Canon 300D is closest in performance to my ST8 when the former is
operating at ISO 200.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com