South Pole Telescope Design
8 December 2003

South Pole Telescope Design

SPST129 works especially well because

1. It is an essentially simple design: Gregorian focus, followed by a two-element focal reducer.  The only complications are that there is a field mirror at the Gregorian focus for control of field curvature and a "fold" mirror inside the focal reducer in order to place the focal plane at a convenient position.
2. The mirrors in the focal reducer bend the beam the "right way" to minimize aberrations.

The result is a design which meets all science requirements:

1. Excellent correction of aberrations over a 1º diameter field of view.  Usable at 200µm wavelength.
2. About 6% distortion at edge of field.
3. No cross-polarization at symmetry plane of detector.
4. Includes Lyot stop and place for filters

Disadvantages:

1. Secondary is large: 2330 mm X 1970 mm
2. Dewar window is large: 260 mm diameter
3. Dewar is large: about 2 m diameter x 3 m long, about 2 tons.
4. Filters are large: about 400 mm diameter

Scaling back design:

1. Dewar window size is roughly proportional to field of view.
2. A large secondary can feed both large and small fields of view; a small secondary can only feed small fields of view.
3. Dewar weight is approximately proportional to volume.
4. Shaped secondary is specific to design; for example, in SPST129B, it is not possible to correct the distortions introduced by the SPST129 secondary mirror with just a single additional optic.