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After you have done a few images you will get a feel for this.
If they are too short in total exposure you won't be able to get the noise completely under control.
You almost basically can't expose too long except for the opportunity cost of missing out on imaging other targets.
Generally speaking and as a very rough figure most images require about 3-8 hours. Galaxies longer.
There are things that speed that up. Wider aperture. Faster optics. More sensitive cameras like the latest CMOS cameras that are very sensitive with low noise. They for example require 50% less time to get the same type of image that a typical CCD would require.
Of course there are practical considerations that limit exposure time. Weather, work and willingness. Then gear problems that prevent a proper image from being taken.
Greg.
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