The secret in the way a telescope works is that it gathers more light than the eye can. More light means you can see fainter stuff, and more magnification means you can see more detail.

The secret in the way a camera works hooked up to a telescope is that it takes the light gathered by the scope and adds it up over time in a long exposure. This is the magic of long-exposure deep-sky astrophotography - it allows us to photograph things that are hard to see visually even with a telescope.

There are many different kinds of telescope designs. The three most popular are refractors, reflectors and catadioptrics. Refractors use a lens. Reflectors use a mirror. Catadioptrics use a combination of a lens and mirrors.

Refractors

Refracting Telescope

Refractors can be very high quality instruments, but are also expensive compared to other designs of the same size aperture. Refractors usually require very little maintenance and can be excellent for use in astrophotography.

Achromatic refractors are basic models with a doublet lens. An achromat usually has some blue or purple color around bright stars. Achromats are the least expensive type of refractor.

Apochromatic refractors use more expensive glass, in a doublet or triplet lens configuration, and have better color correction. Apochromats usually make good photographic telescopes.

Some inexpensive refractors made with ED glass are labeled apochromats and have little color visually, but will have some blue / purple fringing in photographs. These are really semi-apochromats. They have better color correction than an achromat, but are not true apochromats.

Refractors usually require an additional optical device for astrophotography called a field flattener. Most refractors have a curved focal plane and a field flattener is required to bring the stars away from the center of the field into focus.

The blue / purple fringing found in some refractors can be improved with the use of a minus violet filter.

Reflectors

Reflecting Telescope

Reflectors are a bit more complicated mechanically than refractors, and require more maintenance. They usually need to be collimated (optically aligned) each time they are used. They can offer excellent optical performance for the price. Because of the way light interacts with mirrors, Newtonian telescopes are perfectly apochromatic and do not suffer from any false color.

Many Newtonian reflector telescopes are difficult to use for astrophotography because they are designed primarily for visual use. You may not be able to bring your camera to focus because the focuser will not rack in far enough. This is because a visual scope is designed to be used with eyepieces and not a camera. If you can bring your camera to focus, the sensor may not be fully illuminated. This is because the secondary mirror is not large enough for photography since it was only designed to fully illuminate the field of an eyepiece. Vignetting results from a sensor that is not fully illuminated. Vignetting means a darkening at the corners of the field where the sensor does not receive enough light.

Some Newtonian telescopes require an additional optical device called a coma corrector for astrophotography. It corrects for the optical aberration of coma in the corners of the photographic field.

Catadioptrics

Schmidt-Cassegrain Telescope

A Schmidt-Cassegrain is the most common form of the catadioptric design. It is a compromise design which combines relatively large apertures and long focal lengths in a very compact optical tube. The tube is closed like a refractor, so it doesn't require as much maintenance, but a large mirror in a closed tube can sometimes take a long time to stabilize with the ambient temperature and reach its best performance. Schmidt-Cassegrains also have a thin refractive corrector optic at the front of the tube which can be very prone to dew.

Schmidt-Cassegrains are not an optimum design for astrophotography because of their slow focal ratios, long focal lengths, large size and weight, and optical aberrations at the corners of the field. Telescope focal ratios will be discussed in the next section. Some people try to use a focal reducer to make the focal length shorter and the f/ratio faster, but usually this results in excessive vignetting. Although not ideal for astrophotography, they can be used for it if you already own one.

Kinds of Telescopes - The Bottom Line There are three basics telescope designs - refractors, reflectors and catadioptrics. Refractors are all-lens systems; reflectors use mirrors; and catadioptrics use a combination of both.