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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:

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.

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.

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

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.

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"

Mirror separation 36.0"

Secondary/focus separation 46.45"

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"

Mirror separation 23.13"

Secondary/focus separation 29.00"

Primary tilt 2.9 degrees

Secondary tilt 6.78 degrees

Mark T. VandeWettering

Schiefspiegler Home

Hartmut Frommert [contact]