Long throw cassegrain spherical?

Hi to some of the folks 8)

I'm curious to know whether it is relatively easy to grind/polish long FL cassegrain primaries, where I'm ASSUMING that if long enough the primary can be spherical. Or does the secondary throw a monkey wrench into this idea? I'm thinking F20 or thereabouts.

Obviously less glass removed, less work, but also there is the cored center to consider. So, will this work, or will the image suffer too much from not having the primary parabolic?

Jess Tauber

Hello:

You can make a cassegrain of any f-ratio with a spherical primary. This is the Pressman-Carmichael variant. The secondary mirror needs to be a oblate spheroid to cancel the spherical aberration of the primary mirror, and is quite difficult to figure. And the coma is horrendous, which is why no one makes these things. But you could use a f-ratio typical of cassegrain primaries, such as f/3 or f/4. There is also a solution for a Gregorian with a spherical primary mirror, and as I recall the secondary is a concave ellipse. If you want to make a standard cassegrain, but don't want to parabolize the primary, then you have the same constraint as with a Newtonian, i.e. for a 6" primary mirror the primary mirror f-ratio should be around 10. Using such a primary mirror in a cassegrain would result in a very long focal length (around f/30) and a very long tube.

When making a cassegrain, the hardest part isn't so much the parabolization of a fast primary mirror but the making of a small, convex hyperboloid, and figuring the mirrors to match. This is one reason to make a Dall-Kirkham; the secondary is spherical and easier to make with a smooth figure.

Cheers
Mike Connelley, who has made a Cassegrain but didn't do the optics himself.