Designs with dichroics in cubes in converging beam, inside
Designs can be found here.
One cannot simply take the design of a focal reducer and drop
in two big cubes of dispersive material and expect to get a good result.
The design must be significantly modified.
- This design images a 6 arcmin × 4.5 arcmin field onto a 40
mm × 30 mm part of a single CCD. Image is nearly diffraction-limited
over this rectangular area.
- ADC is moved to region of collimated beam.
- Total length, from guider flange to focus, is 1.6 meters.
- The first, simple lens is 204 mm diameter; all other lenses
are under 120 mm diameter.
- The cubes are 121 mm and 85 mm (basically 5 inch and 4
inch). They are made from fused silica. No attempt has been made to make them smaller in this design—they
could be made 25% smaller.
- The cone of rays from each field at the dichroics has an opening half-angle
of 8°. Angles of incidence at the dichroic beamsplitter therefore vary from 37° to 53° for all field
positions. The deviations as a function of field position of these
angles for the edge of the cone are typically 0.3°. Chief ray for each field position has
an angle of incidence of 45.0° ± 0.1°.
Images are near diffraction-limited