The Bower 500mm f/8.0 Manual Focus Lens is a catadioptric mirror T-mount lens

When combined with the T-mount to fit your camera system, this mirror lens mounts to camera and works on both "full frame" cameras as well as the "APS-C" style digital SLR cameras.  T-Mounts are available for all the digital SLR systems, as well as for C-mount systems.  However, in every case the fixed f/8 aperture is not communicated to the camera body.  As such, you can use the lens in "M" manual exposure mode or "A" aperture-priority mode only.

Despite this lens' long 500mm focal length, it has a compact 3 x 3.5" size and weighs under 1 pound, making it a bit easier to handle than normal 500mm refractor lenses (it should be used with a tripod or monopod in any case).  It also has a much different rendering of out of focus areas than a normal lens; highlights will be doughnut-shaped rings with more saturation and contrast on the edge of the ring than on the center.

This lens is an ideal candidate for astrophotography use when mounted to a Vixen GP2 Photo Guider (or other motorized equatorial telescope mount).