eg. 800mm lens on a Canon 1D 1.3x crop dSLR (1040mm in 35mm terms) or 600mm on a APS-C dSLR with 1.6x crop (960mm in 35mm terms)
500mm f/4 IS EF lens with TC1.4x gives 1120mm on APS-C dSLR and 910mm on APS-H
eg. Nikkor 500mm f/4.0 lens on a Olympus dSLR = 1000mm f/4.0
eg. Olympus 300mm f/2.8 ZD with 2x TC = 1200mm f/5.6
this will give an angle of view of about 2deg x 1.4deg (thus the moon would occupy only 37% of image height)
unguided exposures need to be less than 0.8secs to avoid significant star trails which would limit one to
the moon and perhaps Jupiter
on a cheap HEQ5 equatorial mount you should be able to get good 10sec sub-exposures which may be pushing it for a star clusters and nebulae (the latter if you have the IR blocking filter removed).
you really need a high quality mount for this such as a Losmandy
a Tak 180ED astrograph which is a 500mm f/2.8 could be used without autoguiding on a Losmandy mount for 1min exposures, slower apertures will require longer exposures and the hastles of autoguiding.
for example, the Andromeda galaxy imaged with a total 50min at f/8 at 1600ISO, could be imaged at 12min total at f/4, and 12x1min is MUCH easier than 10x5min, and if you use the Canon 1D mark III, its improved noise would allow 3200ISO and thus maybe 9X45sec at f/4 using a 1.4x tele-extender with the Tak 180ED to give 1120mm effective focal length at f/4.
targets 0.5-2deg in size:
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northern hemisphere:
equatorial:
southern hemisphere: