INT - The Optics
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Isaac Newton Telescope

The Optics

The optical system of the INT is a conventional Cassegrain with a paraboloidal primary mirror and a hyperboloidal secondary. The primary has a diameter of 2.54-m (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 7.475-m, giving a focal ratio of f/2.94 at the uncorrected primary focus. It weighs 4,361 kg and it is made of Zerodur and has a negligible coefficient of expansion (10-7 K-1 approximately). On axis, 80% of the light from a point image lies within a circle of 0.3 arc sec in diameter. 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 arc min at a scale of 24.68 arc sec/mm. The images are calculated to be smaller than 0.5 arc sec diameter everywhere over the unvignetted field for incident wavelengths in the range 3650 Å to 10140 Å. The two secondary mirrors give f/15 Cassegrain and f/50 Coudé foci (although the latter was never implemented on La Palma). The scale at the Cassegrain focus is 5.41 arc sec/mm and the unvignetted field is 20 arc min.
Optical characteristics of INT foci
Prime Focus Cassegrain Focus
Focal Length (mm) 8357 38130
Focal ratio f/3.29 f/15
Field diameter (arcmin)
- no vignetting
- 50% vignetting


Scale (arcsec/mm) 24.7 5.41

More information: INT optics (ING Observers' Guide).

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Last modified: 11 January 2013