I thought for a point source (star) and a plate scale (focal length) matched so
that
all the starlight falls in one pixel the exposure goes with the inverse of the
area or D^2
and for an extended object (sky, moon etc.) the exposure goes with 1/F#^2

Dan

"Wade A. Hilmo" wrote:

Hello,

To determine exposure time, focal ratio is the important number. An F/5
scope will always require longer exposure time than an F/4, regardless of
the aperture. It is expected that the F4 scope will require less exposure
time than the F7.5 scope.

Focal length determines image scale. Given the same camera, the same
object, and the same sky conditions, a 12" F4 will require the exact same
exposure time as an 8" F4, but 8" will have a wider field due to it's
shorter focal length.

I hope that this helps,
-Wade

"Stephen Paul" wrote in message
...
Picking up a related conversation we left off a while ago (or at least I
did), I'd like to discuss the subject header a bit further. In the
previous
conversation, I wasn't sure why it was suggested that the more important
parameter in imaging was the focal length, not the focal ratio.

Experiments I've been conducting, show that imaging with an 80ED at F7.5
(600mm fl) requires longer exposures than imaging with a 200mm F4
reflector
(800mm fl).

Given the same camera, the same object, and the same sky conditions, which
is going to require less exposure time, a 12" F5 or an 8" F4?

What is the relationship?

Thanks,
Stephen Paul