Schiefspiegler

The first telescope I made myself was a Schiefspiegler, an off-axis design invented by Anton Kutter The Schiefspiegler.pdf. It is an ideal telescope for planetary observations, as it has no central obstruction and gives diffraction limited images with good seeing provided. I choose an aperture of 110 mm and focal length 2200 mm, allowing to image the moon:

and sun on a 24x36mm SLR (at the time 50 years ago an EDIXA, later a PENTAX).
The beginnings, 1960 -1965

I thought I might try to model it with an optical design software, to see if it works as designed.

Optical design with BEAM4 http://www.stellarsoftware.com

In my notes I found some data on the original design, which was adapted to get the required focal length. I used these data as starting values and modified them in the optimization of the design. The exact data I used I could not find any more after 50 years since I built the Schiefspiegler.

After optimization the following design resulted:

Focus (Zvx4) was adjusted automatically, the angles (pitch 2, 3, 4) manually.

The rays were generated with the circular ray generator:

with the resulting multi spot diagram:

Througout the field the spots show some coma. This was corrected with an aspheric deformation of the secondary (made by Lichtenknecker) which was not modelled, since the deformation parameters are not known. Experience showed that with correct adjustment of the angles, diffraction limited images free of coma and astigmatism could be obtained over the full field of the camera.
The field of view covers +/- 0.006 x 0.008 rad, +/- 20.6’ x 27.5’, +/- 14 x 18.5 mm (SLR Portrait, half of the field is shown because of mirror symmetry). The values above the spot diagrams are horizontal and vertical input angle in radians and rms spot size in mm (0.008 – 0.015). Notice that at F/20 the diffraction limited spot size is around 20 microns, so the full image is well corrected.
Another possibility is the 2-D plot of the rms PSF (≅ sqrt(2)* HWHM), calculated with more rays (3 circles, 39 rays) and for a square field of similar size (+/-0.007 rad or
+/- 24′)

Conclusion

The optical design software allows with minimal effort to check the design of the Schiefspiegler and study the influence of different design parameters. It was verified that it should give diffraction limited images with correct alignment. With a new aluminization of the mirrors a few years ago, the instrument still works the same as 50 years ago.