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Like most modern research telescopes, the SOFIA telescope uses a mirror to concentrate and focus the incoming light. When it comes to large telescopes, mirror-based systems (called "reflectors") have proven to be much more practical than lens-based telescopes (called "refractors") because they are much easier and less expensive to build and use.
SOFIA’s primary mirror, located near the bottom of the telescope, is 2.7 meters (almost 9 feet) across. The front surface, which is highly polished and then coated with Aluminum to ensure maximum reflectivity, is deeply concave (dished inward). Incoming light rays bounce off the curved surface and are all deflected inward at the same time they are reflected back up toward the front of the telescope.
Before the light reaches the telescope’s front end, however, it is intercepted by a small secondary mirror (about .4 meters across), which sends the light back down toward the center of the main mirror. About a meter above the center of the main mirror, a third mirror sends the light out through the side of the telescope, down a long tube which projects through the main aircraft bulkhead into the interior of the SOFIA aircraft. There, at the telescope’s focal point, the light will be recorded and analyzed by one of several different instruments.
Astronomers tend to compare telescopes based on the diameter of their primary mirrors. SOFIA’s telescope is usually referred to as a 2.5-meter meter telescope, rather than 2.7 meters, because the optical design requires that only about 90% of the mirror’s reflecting surface (called the "effective aperture") can be used at any one time. Although SOFIA’s telescope is only considered "medium-sized" compared to normal ground-based research observatories, by placing it on an aircraft, it is the largest telescope that can observe the Mid- and Far-Infrared, that is otherwise inaccessible from the ground.
Under an international agreement between the United States and the German governments, the SOFIA telescope is being supplied by the German Aerospace Center (DLR). DLR is supplying the telescope and additional operation support in return for a portion of SOFIA's valuable observing time.
SOFIA Telescope Assembly
SOFIA Telescope Mirror
The telescope's primary mirror blank was cut from a blank of Zerodur (TM) developed by Schott Glaswerke in Mainz, Germany (near Frankfurt). Zerodur was selected because it is a unique glass-ceramic material with a zero thermal expansion.
The mirror blank started out measuring over 3 meters in diameter and weighting 3,800 kg. Through the lightweighting process, the mirror blank became the SOFIA telescope mirror of 2.7 meter with a svelte weight of 880 kg (1,940 lbs).