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| Home > Public Information > Isaac Newton Telescope > The Optics |
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.
More information: INT optics (ING Observers' Guide).
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