This table compares two 4.25 inch schiefspiegler designs, one by Oscar Knab and one by Dave Groski.
Dave's is a bit higher focal ratio, in order to reduce the residual aberrations.
| Knab | Groski | ||
| Primary Mirror Diameter | 4.25″ | 4.25″ | |
| Secondary Mirror Diameter | 2.20″ | 2.17″ | |
| Radius of Curvature | 127.5″ | 137.795″ | ( both primary and secondary) |
| Conic section | Sphere | Sphere | (both primary and secondary) |
| Focal length | 111″ | 114.83″ | |
| e | 36″ | 41.34″ | (separation of primary to secondary) |
| e′ (p′) | 46.45″ | 45.93″ | (distance from secondary to focal plane) |
| φ1 | -2.79° | -2.4471° | (tilt of primary) |
| φ2 | +6.41° | +6.11° | (tilt of secondary) |
| Δ′ | 8″ | 9.38″ | (distance from center of primary to Optical axis of secondary) |
| Δ | 3.25″ | 3.52″ | (distance from center of secondary to Optical axis of primary) |
| Γ | +4.4° | +3.9° | (image plane tilt) |
| Symbols match to Knab's drawing in Sky and Telescope, October 1961, page 233 and to Kutter's Figure 2, except that Knab denotes the distance from secondary to focal plane as e′, while Kutter denotes it as p′. | |||
Note: Dave Groski and I have both analyzed Knab's design with Oslo.edu and with Winspot. We both find that the value given for e′ (p′) appears to be short. It appears that the correct value is about 49.1 inch. Dave speculates that Knab may have subtracted off the path length of a star diagonal, and mistakenly retained the shorter value in his diagram, even though the diagram does not include a star diagonal. Also, as noted in Bulletin A, and in the Knab article, the focal plane is tilted by about 4.4 degrees with respect to the line between the center of the focal plane and the center of the secondary mirror. The edge of the focal plane nearest the primary mirror is closer to the secondary mirror than the farther edge. Knab and Kutter suggest that the tilt be ignored, claiming that the eye can compensate for the focusing difference. If you want to include the angle, an easy way to do it would be to use a 2 inch focuser and a custom 2 inch to 1.25 inch reducer. The 1.25 inch hole in the reducer would be bored at the 4.4 degree angle so that the eyepiece focal plane would then be parallel to the telescope focal surface. Focuser travel needs to remain parallel to the line between the center of the focal surface and the center of the secondary mirror.
My Oslo (.len) and Winspot (.sp) files for both the Knab and Groski designs, can be downloaded here.
Copyright © 2006 Mark D. Holm
Mark Holm
February 12, 2006
