Astronomic telescopes are easily mounted unto most dSLR's. They make pretty decent telephoto lenses since they are corrected for chromatic and spherical aberrations. A couple of the ones shown have low dispersion glass elements incorporated in the lens cell. They are connected to the camera with a $50 camera adaptor and a $25 t-ring. Some designs may also require an optical field flattener. These not only work as astronomic instruments but as terrestrial lenses as well, not as handy or compact as purpose made camera lenses but very useful none the less. The image quality is tied to the quality of the elements in the optical path. Of course they are manual focus and of fixed aperture. Below (mine) are a 1500 mm f10 catadioptric scope, a 1000 mm f5 reflector, a 900 mm f9 refractor with ED lens, and a 400 mm f5 refractor also with an ED lens. If you have a telescope, give it a try as a telephoto lens, you might be surprised at the result.