What Don is suggesting is to reverse the shoe on the scope and the reverse the finder in the bracket. That way the finder is inserted from the top and the tab on the bracket prevents the finder from slipping and falling, potentially damaging the mirror.
As a matter of fact, within the last year, my GSO finder slipped out the dovetail shoe in my 120mm Eon, hit the concrete floor and broke off the plastic finder eyepiece. If I had had the finder mounted the way Don had suggested, it couldn't have slipped out.
I have done that in the past but pretty much stopped doing it because the finder moves around a bit.
I was able to jury rig an eyepiece for the GSO finder but it's a 25 mm so the exit pupil is large ad the magnification quite low. I estimate it at 6x-7x, pretty low for a 50mm finder.
I still don't understand why the manufacturers ever started putting a notch in the back. Even if they were originally designed for refractors, I don't understand how having a notch in the back provides any benefit in a refractor.
Reversing the finder in the bracket so you could slide the bracket into the shoe from the front would better protect the mirror but it doesn't seem like the finder would fit very well in the bracket backwards. I haven't tried it though and I will take Don's word for it that it works.
My plan is to replace the GSO with a Stellervue that I have on hand but haven't gotten around to setting it up. The Stellarvue bracket is one of those 6 screw brackets like that Agena you linked, and it looks like there are machine screw holes in the bottom on both ends. So if I can find a machine screw with the right threads, I could put a screw on the front end of the bottom of the bracket. That would let me load the finder into the shoe from the front and have a screw sticking down to act as a stop to protect the mirror. It would also give me a much better quality finder. The GSO finders are serviceable, and work well for the price but the Stellervue finder seems to be of much higher optical quality. And I have some 1.25 nebula filters I want to try out with the finder to see what I can see in a small widefield finder as a companion to the bug dob.
Edited by Ihtegla Sar, 27 July 2021 - 12:33 AM.