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.
|
Prime Focus |
Cassegrain Focus |
| Focal length (mm) |
8357 |
38130 |
| Focal ratio |
f/3.29 |
f/15 |
Field diameter (arcmin)
- no vignetting
- 50% vignetting |
40
52 |
20
22 |
| Scale (arcsec/mm) |
24.7 |
5.41 |
| Primary mirror diameter (mm) |
2540 |
2540 |
| Diameter of central obstruction (mm) |
914 |
914 |
| Effective colecting area (m^2) |
4.41 |
4.41 |
