My interest invites the question "Why a 20 degree field?". If it is for guiding a telescope to an object it should be possible to do this just as accurately with no optical aid. I have the Vixen SG's and the view is good but I wouldn't immediately think of one half as a finder.
The advantages of using a small, ultra-wide finderscope instead of something like a Telrad are:
- it shows many more stars, which is an advantage, especially in light polluted skies
- it is actually SIGNIFICANTLY smaller than a Telrad...
- because of the magnification, it can show a lot of large, dim deep-sky objects directly, such as M33, etc., even in less than perfect skies.
- it is significantly more precise than the Telrad, since the resolution is much, much higher, even just at 3x, because you can see many more stars and the perceived distances between guidestars are much larger.
- a second hand 50mm camera lens is so cheap, you can build a ~4x25 finderscope with AWESOME field correction CHEAPER than what a new Telrad or a new 6x30 finderscope costs, provided you already have a decent wide-field eyepiece. You don't have to dedicate the eyepiece to the finderscope, though, as you can just preset the focus and just insert the eyepiece into the finder, when you need it and then use the eyepiece on the main telescope, when neccesary.
- it doesn't require electricity...
People have critizised small finderscopes for decades, and not without reason, as most of them have been much too dim, but the problem with small finderscopes is often not that they are small, but that their magnification is much too high for their aperture, leading to a dim view with a much too restricted field of view. A small finderscope with low magnification and correspondingly large exit pupil and bright views is a whole different animal entirely. Couple this with a modern 80° eyepiece and a flat-field objective and you're in for a VERY different experience.