A telephoto mirror lens is very similar to a telescope. The image coming through the lens is bounced off of mirrors before going to the image. This allows the lens to be much shorter than it would need to be without the mirrors. The lens has no diaphragm and is normally used with adjusting the shutter speed to reduce light. Many of these have small filters that are attached to the rear of the lens. Neutral density filters are used to reduce light when the shutter speed does not reduce enough. Close-focus tends to be much, much closer than with a standard lens. Hand holding is difficult as with any lens, but will not require a very strong tripod. A monopod is the normal choice.