I've acceptably guided my C11 @ F/6.3 with a setup like that... For my setup now, that would be the next best option to my OAG I think... Being that I image at 480mm F/L, a 200mm guide scope is plenty....
Its quite a neat, elegant setup, and for most people, easy to do.. Nearly everyone has a finder lying around..
I made mine a little differently.. I ground down a spare 2" to T-thread adapter about 0.5mm in overall diameter, using a rubber mallet I banged the 2" adapter into the rear of the finder.. Obviously, this makes it a dedicated guider from then on, not possible to revert it back to a normal finder scope...
I did have to remove the locking ring from the objective focusing thread, and then screw the objective the whole way on until it was tight, this gave me very good focus, and guiding results were great.. I've done this a couple of times in this way.. I will say that a 9x50 finder will need a small amount of the tube removed to reach focus, 7mm from memory....
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