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Firstly, any reputable retailer will know whether their product has
sufficient back focus for prime focus work. Many manufacturers/agents/retailers don't mention the subject in their
adverts, or on their websites.
Any scope defined as an astrograph will by definition be o.k, but may not
be able to focus for visual work, so if buying one of those you would need to check that.
You could take a DSLR with you to the shop, and see if the desired scope will
focus on an object hundreds of metres away, and still have some focuser movement left.
raymo
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