I prefer a right angle finder for axis-symmetric telescopes such as SCTs and refractors.
I recently ordered an Evoguide 50mm ED mainly for AP guiding, on sale of course, from Astronomics, who provided the following caveat with regards to its use as a finder, to wit: "... Visually there will be some compromises. Due to the 20mm of backfocus, or 60mm if you remove the 40mm extension, you would have to use it like a finderscope straight through. There will not be enough room for a diagonal...."
Nonetheless, I thought I'd try using it as a right angle finder anyway, and here is my report. I decided to "donate" the Celestron 1.25" 90 deg diagonal that came with my Celestron Edge 8, much preferring the use of a Baader Clicklock and 2" diagnonal/eyepieces instead with this latter instrument. Please note that, while this converts the Evoguide to a right angle finder, it is NOT a RACI finder. RACI diagonals generally use Amici roof prisms and have longer light paths though the diagonal than just a plain star diagonal gives. My inverted image finder thus yields the same field orientation as my Edge 8 and my Stellarvue 125mm refractor (I alternate using both). The shorter optical path length of the plain diagonal is important because, when I installed the Celestron diagonal on the Evoguide, there was just enough focal length to focus properly: any longer optical path would not have worked. Even so, I found it necessary to remove the extender on the Evoguide to provide more optical path length through the diagonal, and this was just enough.
I decided to also "donate" a 1.25" Besser 20mm eyepiece of 70 deg field of view to my new "finder." The result is very sharp (for a finder) images, and I am quite happy with the result. Below are listed the "specs" of my new finder. The only number I might have wished to be different is the rather short eye relief of 12mm. This turns out to be adequate for me but something like 18mm would have been nicer. However, the Besser eyepiece is best-in-class (inexpensive, that is) for optical quality, so I accept the compromise. A longer focal length Plossl would have worked too but ... the magnification would have been less for about the same field of view. BTW, the Besser barrel cannot be unscrewed to allow for easy installation of a human hair reticle. Instead, I will use a small dot from a magic marker directly on the aperture lens, probably best if it is not quite centered.
Skywatcher Evoguide 50ED APO/Bresser70deg20mm"
Objective Diam. (mm) 50
Focal Length (") 9.4
Focal Ratio 4.8
Exit Pupil Diam. (mm) 4.2
Finder FOV (deg) 5.83
Weight (oz, w/ eyepiece) 38
Eyepiece Eye Relief (mm) 12
Eyepiece FOV (deg) 70
Eyepiece* F.L. (mm) 20
I realize my setup is more expensive than the usual 50mm finder but now I also have an excellent guide scope too.
Happy observing always,