SPST129 works especially well because
- 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.
- 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:
- Excellent correction of aberrations over a 1║ diameter field of
view. Usable at 200Ám wavelength.
- About 6% distortion at edge of field.
- No cross-polarization at symmetry plane of detector.
- Includes Lyot stop and place for filters
- Secondary is large: 2330 mm X 1970 mm
- Dewar window is large: 260 mm diameter
- Dewar is large: about 2 m diameter x 3 m long, about 2 tons.
- Filters are large: about 400 mm diameter
Scaling back design:
- Dewar window size is roughly proportional to field of view.
- A large secondary can feed both large and small fields of view; a small
secondary can only feed small fields of view.
- Dewar weight is approximately proportional to volume.
- 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.