telescope An optical instrument used for viewing far away objects. It consists of an objective lens and eye piece *. The objective produces a real image of a distant object, (at infinity) at its focal plane. The eye piece then acts as a magnifier. The angular magnification, M of the instrument is given by,
M = b /a (t1)
where a is the angle subtended by the object at the eye, and b is the angle subtended by the final image at the eye (see fig. t1).The final image is at infinity, when the image produced by the objective is at the focal plane of the eye piece. The angular magnification at this adjustment is given by,
M¥ = - fo/fe (t2)
where fo is the focal length of the objective and fe the focal length of the eye piece. For large magnification of the instrument, the focal length of the objective should be large compared to the eye piece. The performance of the instrument also depends upon the resolving power *.
Newton made reflecting telescope consisting of a concave mirror and a small plane mirror, that reflected the rays to the eye piece as in fig. t2. Modern telescopes also employ large mirrors instead of lenses, since aberration free lenses are difficult to make. Large lenses also become distorted under its own weight.