INT Optics

The optical system of the Isaac Newton Telescope (INT) is a conventional Cassegrain with a paraboloidal primary mirror and a hyperboloidal secondary. The primary has a diameter of 2.54 meters (the original 98-inch primary mirror was replaced by a 100-inch mirror when the INT was moved to La Palma) and a focal length of 7.5 meters, giving a focal ratio of f/2.94 at the uncorrected primary focus. A three-element corrector with a flat rear surface and increased back focal distance, is used at the prime focus to give an unvignetted field of 40 arcminutes at f/3.29. On axis, 80% of the light from a point image lies within a circle of 0.3 arcseconds in diameter. The two secondary mirrors give f/15 Cassegrain and f/50 Coudé foci (although the latter was never implemented). A summary of the optical characteristics of the INT is shown below.