| Celestron | Meade | Criterion/Bausch & Lomb | Parks |
The Schmidt-Cassegrain optical system makes use of multiple mirrors to fold a long focal length into a short tube. In order to accomplish this however, a complex corrector plate is placed as the first optical element exposed to the sky to correct for spherical abberations. The secondary mirror is placed at the center of this corrector plate and reflects the image through a hole in the primary mirror. Although this optical system is quite popular, relatively few manufacturers offer this telescope because of the difficulty of figuring the optical surfaces of the corrector plate and secondary mirror. This tends either toward very high manufacturing costs or toward somewhat compromising optical quality. The manufacturers that have entered this market have produced these telescopes in a wide variety of sizes to fit many preferences. The first significant telescope of each manufacturer has usually been the 8" (20 cm) size. While the focal length of these telescopes varies from f/6.3 to f/12.5, most are produced at f/10. Due to variations in manufacturing and quality control, many units have been made which do not give sharp images at high powers. While this is a problem for planetary observers, nearly all units give reasonably good views of deep sky objects when properly collimated.
You will sometimes find just optical tube assemblies by themselves for sale in the used market. However, these telescopes are usually manufactured and sold as a complete system on either a German equatorial mount or on a fork mount. The systems generally have a plethora of accessories available to accommodate special observing needs or astrophotography. Almost every system has a Right Ascension (RA) drive available either as a standard or optional accessory.