What setup for beginner astrophotography

[sgazer]

What sort of exposure time am I looking at?

I found most of my imaging was done with either 3 or 5min exposures, with about 16 stacked (reduces noise by factor of 4). This was with an f5 scope. At f10, you would need 4x longer, so 12-20mins which is quite difficult to achieve, even with a guided setup and the noise increase a lot for DSLRs above about 10mins. Plus there's more chance for something to go wrong; tracking error, plane, cloud, satellite, etc etc, with long exposures.

[ollypenrice]

Presuming you use the same camera, then a longer focal length needs a longer exposure time to get the same brightness and noise-level. However, you also get a bigger image (i.e. the target is spread across more pixels) for the extra time you spend. Of course, if we pin the aperture at 80 as per th thread title then, yes, the focal length is what determines the F ratio.

You can "bin" the images you take, which will reduce the noise level and the size of the image and thereby allow you to take shorter exposures while retaining picture quality.

This is only true if we pin the aperture at 80mm, which is fair enough given the thread title. Just to clarify, though, the focal length in isolation has no effect on exposure time but does, as you say, determine the image scale (how 'zoomed in' you are.) Brightness on the sensor (which determines exposure time) is determined by the focal ratio - and also by the sampling rate and the camera sensitivity, as you say, though you cannot bin DSLR or OSC so it boils down to F ratio in reality.

If using a DSLR the fast F ratio is even more important than if using a CCD because DSLRs are time-limited by thermal noise. Those DSLR images which approach CCD quality are invariably taken in very fast systems. However, nothing in imaging compares with the plug and play simplicity of a refractor and there you will struggle to get down to F5 without spending rather a lot!

Olly