This webpage contains information on the foci characteristics of the WHT.
Foci at the WHT
There are three types of foci at the WHT telescope, the Prime, the Cassegrain and the Nasmyth foci. Instruments can be mounted at the corrected f/2.81 prime focus, f/11 cassegrain focus, or either of two f/11 Nasmyth foci.
Optical characteristics of WHT foci
|
Focal Station |
Prime (without corrector) |
Prime (with corrector) |
Cassegrain |
Nasmyth (without image derotator) |
Nasmyth (with image derotator) |
Nasmyth (with infrared derotator) |
|
Focal length (mm) |
10440 |
11739 |
45738 |
45738 |
46419 |
45738 |
|
Focal ratio |
F/2.498 |
F/2.81 |
F/10.94 |
F/10.94 |
F/11.11 |
F/10.94 |
|
Field diameter (arcmin) (no vignetting) (50%vignetting) |
|
40 60 |
15 19 |
7 23 |
2.5 5 |
2.9 |
|
Scale (arcsec/mm) |
19.76 |
17.55 |
4.51 |
4.51 |
4.44 |
4.51 |
| Scale (micron/arcsec) | 50.6 | 57.0 | 221.7 | 221.7 | 225.2 | 221.7 |
|
Diameter of central obstruction (1) (mm) |
19.76 |
17.55 |
4.51 |
4.51 |
4.44 |
4.51 |
|
Focus/mirror shift (2) |
|
|
20 |
20 |
21 |
20 |
1) Theoretical obscuration (baffles + vanes) = 9.083%
2) Movement of focus position for unit movement of secondary mirror