.title Optical Telescope Assembly .synonyms optical telescope telescope assembly .description The central component of the Hubble Space Telescope, consisting of a 94 inch (2.4 m) reflecting Cassegrain telescope and supporting structures. .contents The Optical Telescope Assembly .~ (fig. 5)~ will mount a 94-inch (2.4 m) reflecting cassegrain-type telescope, consisting of a paraboloidal primary mirror and a hyperboloidal secondary mirror. The optical effect is the same as that of a telephoto lens. Light beams enter the open end of the telescope and strike the .~ primary mirror~ at the rear. The parabolic shape of the primary mirror causes the beams to converge upon the .~ secondary mirror~. The secondary mirror's hyperbolic shape aligns the beams in parallel and bounces them back through a hole in the center of the primary mirror. The .~ scientific instruments~ are positioned behind the primary mirror where they can be moved into the path of the reflected light. The .~ primary mirror~ is made of ultra-low expansion titanium silicate glass with an aluminum-magnesium fluoride coating. To reduce weight, the front and back plates are fused to a honeycomb core. The mirror will be heated during operation to 70 degrees F to minimize variations from its original accuracy. This heat will come from electrical strip heaters, powered by the .~ solar arrays~, which will radiate to the back of the mirror. The .~ orientation~ and .~ stabilization control~ system of the telescope can point with an accuracy of 0.01 arc second and can hold onto a target for extended periods within 0.007 arc-second. This angle would translate into a distance slightly larger than the diameter of a dime if the telescope were 500 miles away. Such accuracy is necessary in order to study very faint objects, some of which require exposures of 10 hours or more. The telescope can be turned and changed from one direction to another at a rate of 90 degrees in 20 minutes.