Imaging with a Large Dob.

[Tiddles]

Just a quick question.

The weapon of choice for a number of imagers is a moderate aperture (80mm) refractor. If a large (16") fast (F4-F5) Dobsonian was used the length of time to capture each sub would be vastly reduced. Would this be reduced to such a level that tracking and field rotation would not be an issue.

If a sub with the refractor set-up was a couple of minutes, how quickly could this data be captured with a large fast mirror?

Thanks

Andy

[RikM]

A small APO at f/4 and 16" Dob at f/4 would need the same exposure time to reach the same image brightness, you would just have a large field of view in the APO and a smaller field of view in the Dob.

[SamAndrew]

Focal ratio is the improtant number when comparing exposure times between scopes assuming you're using the same camera.

You would need a much larger sensor with the dob to get the same field of view (not practical), and if the pixels were scaled up as well, it would then reduce the exposure time for the same result.

....I think I know what I'm talking about

[dph1nm]

Yes, if you image at the same resolution (e.g. 1" per pixel) then the dob would be need much shorter exposures. But getting it to stage where tracking isn't needed would be tricky. On the equator the sky moves about 15" per second of time, so you would need sub-second exposures. At this point you worry about being dominated by camera read noise, so you would have to do the maths to sort out what aperture you need.

I image professionally with ~2m telescopes, and 30-60sec exposures will get you deeper than several hours with a small scope.

NIgelM