The properties of the image produced by a telescope are similar, whether formed by lenses or mirrors. The real image produced is inverted; i.e., top and bottom are reversed, as are left and right. In a terrestrial refracting telescope used to view objects on the earth, an additional lens is used to invert the image a second time, so that objects appear as they do when viewed with the unaided eye; in an astronomical telescope, image inversion is unimportant and no lens is added to invert the image a second time. The angular size of an object as seen from the position of the telescope may be expressed in degrees or in radians (1 radian equals about 57°). The angle in radians determined by the object is given by the ratio of the object's diameter to its distance from the telescope. The size of the object's image is the product of this and the focal length of the image-forming lens or mirror. For example, the angular size of the moon's diameter is about 1/2°, or roughly 1/100 radian; a telescope with a focal length of 60 in. (152 cm) would produce an image of the moon 0.6 in. (1.52 cm) in diameter. The brightness of the image depends on the total light gathered and hence is proportional to the area of the objective or the square of the diameter of the telescope.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.