I have an observing project that requires a 30 -- 40 cm (12 -- 16 inch) diameter primary and no refractive/transmissive elements (i.e. lenses).

Telescopes like this Ritchey–Chrétien has a back focus of 288mm from the rear cell. This is a typical value for the back focus for such instruments.

The problem I have is that the instrument I will be using on the telescope will foul the telescope mount if it is placed at this back focus distance.

I am looking for a purely reflective telescope where the back focus needs to be as close as possible to the rear cell.

Does anyone have any suggestions?

If I cannot find a suitable solution "off-the-shelf", I will have to look into refiguring the secondary mirror.

  • $\begingroup$ Why don't you just add spacers to move the telescope further from the mount? $\endgroup$ Commented Mar 3, 2023 at 3:56
  • $\begingroup$ Can you mention the field of view (FOV) required by "the instrument" - how far off axis you need a sharp focus? One might be able to get away with moving the secondary further from the primary (you'd have to do some ray tracing to understand the nature of the degradation). Also, have you thought about a large diagonal mirror at the exit, and mounting "the instrument" to the side? (aside: wow I never heard of an astronomical telescope with a 'highly reflective 99% dielectric coating" I wonder if it's highly reflective aluminum with a dielectric protective coating, or some broadband film stack?) $\endgroup$
    – uhoh
    Commented Mar 3, 2023 at 7:14


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