A Gregorian Telescope

The Gregorian design has been drawing my interest for some time now and since the quick and dirty RFT I made was never used, I decided to turn the 150mm mirror into a fast primary. The Gregorian that is based on it, is sort of a test-case for gaining experience before starting a larger version.


The design parameters are as follows:

  • Primary: 150mm, RoC=900mm (F/3), k=-0.99
  • Secondary: 37mm, RoC=170mm, k=-0.62
  • System: f=3600mm, FoV=25mm (0.4°)
  • Mirror separation: 546mm, BFL=766mm
The intention is to also be able to make photos using my EOS 450d camera, especially of solar system objects. The EOS sensor has a diameter of about 25mm. The field of a Gregorian is curved, but in this design it is diffraction limited up to a width of 20mm. The design is aplanatic, which means that only astigamtism and higher order abberations are present. When using eyepieces the field curvature is in the same direction of that of the eyepiece, so the visual performance potentially is very good. In any case, the magnification quickly increases (180x with a 20mm eyepiece) so the actually used field will be only a fraction from the design maximum.

For this system, ATMOS calculates the spot diagram around the paraxial focus as follows:

Spot diagram

As can already be seen here the field is curved outward, i.e. best focus towards the edge of the field is at larger BFL values. The remaining astigmatism limits the diffraction limited flat focal field to about 0.34°. Because the system allows quite some defocus, a larger diffraction limited flat field can be achieved at the cost of slightly more paraxial image deformation. The table below gives the Strehl values at several (BFL, Field angle) points:


Concluding, defocus of up to 0.5mm stretches the diffraction limited flat field up to the desired 0.4°, which is probably sufficient for photographic use.

Building the scope

More details about the construction can be found here:

The drawing below presents an overview of how the design worked out.
You can click on the drawing below to zoom in on an area of interest: Gregorian drawing