Designs with dichroics in cubes in converging beam, inside camera.

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.

cubes11.zmx

1. 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.
2. ADC is moved to region of collimated beam.
3. Total length, from guider flange to focus, is 1.6 meters.
4. The first, simple lens is 204 mm diameter; all other lenses are under 120 mm diameter.
5. 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.
6. 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