Focal Length -This is the measurable length between the main mirror and the focal point or eypiece. Common lengths range from about 400-3500mm. The longer the focal length, the higher the magnifying power the telescope produces with a given eyepiece. They keep a guide star stationary during long exposures to avoid star trailing. There are many different styles and setups that can be used for 35 mm or CCD applications.
Some systems allow for guiding and photographing through the optical tube at the same time.
Reflector Telescope - Reflectors use a large, heavy, concave mirror instead of a lens to gather and focus the light. Refractor Telescope -have long, relatively thin tubes with an objective lens up front that collects and focuses the light. Straight-through light collection provides less image error and usually supplies better viewing. The best most telescopes can achieve is about 1 arc second due to Dawe's limits. This is not a really important characteristic of most telescope sytems and becomes better with larger scopes.