A folded Cassegrain telescope saves up space and a whole world of troubles

Published 30.10.2016

A folded Cassegrain type telescope objective is nothing new, but given present advancements in optical manufacturing, a worth reconsidering.

Cutting the optical tube length in half with a flat mirror and moving the secondary to the center of primary mirror hole is not a new idea and does nothing to help manufacture and assembly. Instead making a single mirror with a binary surface, i.e. one radius at inner zone and another in outer zone, a whole world of manufacture and assembly problems can be avoided. Assembly problems would eliminate themselves, and manufacturing would get easier by making traditional manufacturing near impossible and forcing more advanced techniques.

The layour of a folded Cassegrain is exactly the same as with a regular Cassegrain, down to performance. The addition of a flat mirror does nothing to alter the ray trace, aside from the signage of some components. The layout of a folded Cassegrain is demonstrated here with a Gregorian variant: Rays from infinity hit mirror outer zone (previously primary mirror) and reflect to the flat mirror. From the flat mirror rays are reflected to the mirror inner zone (previously secondary), back to the flat mirror and from there to image plane.

The flat mirror placed instead of secondary is used twice. Now, a regular secondary of a Cassegrain telescope system has strict decentering and tilt tolerances, and is a major headache to whoever has to assemble the system, and to marketing who needs the product quickly to shelves. A flat mirror does not have decentering problems, only tilt adjustments, which are easy enough to tackle with an autocollimating microscope.

The main mirror with two (or more) zonal radii used to be unthinkable with traditional grinding and polishing techniques, but presently diamond-milling and to some extend even optical 3D printing can help here. 3D printed mirror would also be much lighter than glass.

A special design feature of the main mirror at the borderline of the two zones is proper alignment of distances so that there is no break in the surface, only discontinuity - the surface of the mirror continues with the second radius without a sharp cliff at the border. Such a transition is easy enough to program with CNC machining without a significant loss of surface, and the imaging surfaces can be designed to avoid the border completely.

Once manufacturing capabilities have been established, a folded Cassegrain is quite a simple version of the original. With only one optical surface to manufacture (high quality flat mirrors are commonly available from stock component manufacturers), adjusting becomes quite a simple procedure. A Gregorian version is suitable also to terrestrial observation, as the image remains upright.