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Refractor and Reflector OTAs
There are two main types of telescopes: refractors and reflectors. The OTA of a refractor telescope contains a system of lenses used to collect light, condense it into a single point of concentration, and magnify it so that it can be seen by the human eye. The OTA of a reflector telescope contains mirrors, one curved and one flat, instead of lenses. These mirrors can reflect light without the aberrations sometimes experienced by refractor telescopes, but the images reflected are not always as crisp.
Other Components of the OTA
Besides containing the primary mirrors or lenses, the OTA must also contain the appropriate mechanical pieces to hold these optics in place. These often include a focuser and tube rings. The focuser holds the eyepiece in place and adjusts the image's focus, and the tube rings, also known as mounting rings, fasten the OTA to the mounting. Occasionally the OTA is also equipped with a finder, a smaller telescope of lower magnification that can be attached to the larger telescope to assist in detecting objects to be viewed.
Appearance of the OTA
The appearance of an OTA varies depending on the type of telescope it supports. The OTA of a refractor telescope is long and narrow, resembling a traditional spy glass. The OTA of a Newtonian reflector telescope is shorter but still narrow, and the OTA of a Schmidt-Cassegrain reflector is the most compact of all, round and short like a waste basket.
Optical Telescope Assembly
Space telescopes such as the Hubble must also have an OTA, but on a space telescope this acronym stands for "optical telescope assembly." However, the basic make up of the OTA remains the same. The Hubble Space Telescope's OTA uses mirrors like a reflector telescope, reflecting the captured light and directing it toward the instruments that transmit the image back to Earth.