On Sun, 27 Nov 2005 20:58:48 -0500, Tom Rauschenbach
wrote:

That makes a great deal of sense to me. Especially in light of statements
that seem to imply that long exposures aren't really needed for CCD data
collection. Although I have to wonder why there aren't discussions
about readout noise/image. What are the costs and benefits of many short
exposure images over fewer long exposure images ? Of course tracking
issues are an obvious one.

There are many discussions about noise issues, just not a lot on this
forum. Readout noise is the primary noise source when you are doing any
kind of video astronomy, either with a webcam or a video camera and
frame grabber. It is primarily readout noise that determines your S/N.
Because of the short exposures, dark current noise is insignificant.

There is only one fundamental benefit of short exposures- the ability to
capture images during brief moments of atmospheric stability. A
collection of many images can be graded for quality and the bad ones
discarded. This is functionally equivalent to high order adaptive
optics. Readout noise makes for a stiff noise penalty, but with hundreds
or thousands of images the noise is substantially reduced. But the
technique is only useful for very bright objects- the Sun, Moon, and a
few planets.

Once you start imaging DSOs, you need long exposures- many minutes is
usually required to maximize S/N.

_________________________________________________

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