Archived from: http://telescopemaking.org/schief.html
The Kutter Schiefspiegler
A simple tilted component telescope for the amateur
A Simple Schiefspiegler, drawings by Oscar Knab
The Schiefspiegler is a tilted component telescope invented by Anton Kutter.
One may think of it as an off axis segment of a Cassegrain, but rather than
having the fast primary (typically f/2 to f/4) and high secondary magnification
(typically 3 to 4 times) of the normal Cassegrain, the Schiefspiegler has
a long focal ratio for the primary (typically around f/15) and a smaller
secondary magnification (typically 1.6x).
Schiefspieglers are very long focus instruments. The instrument listed
above is a 4.25 f/27 instrument. In the times of 20" f/4 instruments, one
might wonder why this instrument would be desireable. A number of reasons
leap to mind:
So, what do you need to know to build a Schiefspiegler of your very own?
A quick trip to your local library should net some articles that have been
published in Sky & Telescope. Articles appeared in the Gleanings for
the ATM by Oscar Knab and Anton Kutter, describing the basic
Schiefspiegler and the tri-schiefspiegler that came later. Oscar Knab
also had a short article on building a 3" f/20 Schiefspiegler in
Telescope Making #3.
I strongly recommend that you read these articles, they are very useful.
- The Schiefspiegler is perfectly color free.
Being an entirely reflecting design, the Schiefspiegler has no color
aberrations like refractors.
- The Schiefspiegler has no central obstruction.
The debate goes on about just how bad the obstruction in Newtonians actually
are, but the Schiefspiegler has no obstructions, which makes it very useful
for viewing low contrast objects such as the planets and double stars.
- The Schiefspiegler is (relatively) simple to build.
The two spherical mirrors have the same radius of curvature, which allows
you to grind them against one another. After polishing the concave one to
an accurate sphere (for which the Foucault test is very sensitive), you can
interference test the secondary against the primary to ensure its figure.
- A Schiefspiegler is inexpensive to construct
Two 4.25" blanks of Pyrex (or quartz or Sitall or whatever) are pretty
inexpensive, much cheaper than the necessary glass for a refractor.
Above I have reproduced the diagram for the 4.25" Schiefspiegler. If you
click on the image above, you will get a version of the diagram digitized
at 300dpi. It contains most of the critical dimensions, and will be quite
useful in understanding the basic mechanical layout. Below is the
design for the smaller three inch f/20 instrument, also by Oscar Knab.
- An Improved 4 1/4 inch Unobstructed Oblique Reflector.
Sky and Telescope Gleanings for the ATM, October, 1961
- Making a 3 inch Schiefspiegler.
Telescope Making #1, Fall 1978
A Smaller Schiefspiegler, drawing by Oscar Knab
Designs for Two Small Schiefspieglers|
Measurements for 4.25" Kutter Schiefspiegler
|Primary mirror aperature||4.25"|
|Secondary mirror aperature||2.20"|
|Radius of curvature||127.5"|
|Effective focal length||111.0"|
|Primary tilt||2.76 degrees|
|Secondary tilt||6.41 degrees||
Measurements for 3" Kutter Schiefspiegler
|Primary mirror aperature||3.00"|
|Secondary mirror aperature||1.44"|
|Radius of curvature||81.5"|
|Effective focal length||71.0"|
|Primary tilt||2.9 degrees|
|Secondary tilt||6.78 degrees|
This page Copyright 2001, Mark T. VandeWettering.
Permission is granted to reproduce and distribute these files for
non-profit, personal use.
Mark T. VandeWettering