Gregorian Cassagrain Design Guidelines

Hello all,

Well I have had an 8 inch f/3 mirror from Surplus Shed now for quite a while (Spherical, Coated, with the center hole). I was thinking I'd build a demonstrator stand for it that put a white screen at it's prime focus and have students look thru the hole in the mirror to see the image formed.

After much thought and some education, I was wondering about the old Gregorian design that uses a concave secondary mirror to get the light cone out of the hole so that an eyepiece would come to focus. Anybody got suggestions for the range of secondary sizes and f# that would work ?

Thanks in advance,

P.S. I'm not looking for high contrast, just looking for a demo setup. I'd even try a convex secondary like the SCT and Mak designs use if I could get something cheap.

Ed, a two-mirror system with spherical primary needs very strongly aspherized convex secondary (oblate ellipsoid) in a Cassegarin-like arrangement (known as Pressmann-Camichel).
Unless you want to punish yourself, better stay away from it. Besides, it has nearly ten times the coma of a comparable classical Cassegrain.

The only difference with Gregorian arrangement is much easier to make concave secondary. For accessible focus, you'd need secondary magnification of 3.3 (for an f/10 system) or more, and 50%+ c.obstruction. In this case, a f/2+ secondary would be relatively mild hyperbola (-1.2 conic). Please note that I said it's much easier to make than Pressmann-Carmichel's secondary, not that it's easy It could probably be pulled down to -1 conic(parabola) by combined increase in c.obstruction and decrease in sec. magnification, but it still would be a challenge.

This system would be corrected for spherical aberration, but would have ~16 times stronger coma than classical arrangement (w/parabolic primary). It would perform at a level of an f/4 parabola coma-wise, but with stronger field curvature. Astigmatism would be also 3-4 times stronger, but of no consequence considering the size of coma.

It would probably be easier to get one of those short-tube 6" catadioptric Newtonians from Celestron and (I think) Skywatcher. They should have ~f/3 spherical primary and a corrector lens in front of diagonal that corrects spherical aberration of the primary and also, being a negative lens, doubles the focal length. This lens would work with any f/3 sphere, assuming it is placed at nearly identical distance in front of the focal plane. These ota's go pretty cheaply, and if you sell other parts, you could get this corrector lens for next to nothing.

If you go this route, just make sure the primary is ~f/3.