A decent 500mm telephoto lens is too expensive for me at the moment. Optical quality of mirror lens is considered inferior than telephoto lens. I just wanted to test the magnification power of 500mm lens for wild birds. So I bought an old Sigma 500mm f8 mirror lens lately. If the image quality is way below my expectation, I’ll use it as a telescope.
Basically, this is pretty simple. Take a telescope eyepiece, and affix it to a rear lens cap. However, if you use a normal telescope eyepiece, you’ll wind up with an inverted image. To get an erect image, you need to use an inverting eyepiece. Surprisingly, there seems to be only one such animal on the market! It’s made by Skywatcher Telescope. Their info page is here:
For many years I have used mirror lenses as spotting scopes using
monocular converters, eyepieces with prisms giving upright images
to fit various lens mounts. As has been said many times in the past the size of the central obstruction dictates contrast and also image quality. Although f/8 photographic mirror lenses take high powers very well. A typical good quality 500mm lens gives 50x and 100x with a 2x converter. Even 150x with a 3x converter for the best ones. But the contrast can be low. For double stars, the Moon and planets this is acceptable for such a small lightweight unit that doubles as a photographic lens. The central obstruction is typically 45% of the main mirror diameter. The contrast can easily be improved when used with a digital camera.
If you can find a monocular converter in the correct fitting for your lens. That is best. Otherwise if you are good at making things obtain a 8×21 roof prism binocular, about 7 pounds new, and unscrew or cut off the rear end of one barrel. This contains a 10mm eyepiece with attached roof prism. Attach this to a T2 mount correct for your lens at a correct distance and accurately centred. I am not good at making things, so perhaps somebody else can help. Best regards, David.
Canon EF Lens Spotting Scope – $315