Optics-Talk-With-MikeL

Fred,

1) matching the corrector to the mirror is the expensive part ?

The quality optical glass and the labor involved to grind, polish and figure it are the expensive parts.

If the mirror is parabolic (and not spherical), then no corrector plate is needed. This is a classical cassegrain design. Just add a custom made hyperbolic secondary.

If the mirror is spherical, then the mirror can either be refigured (not too expensive) or a corrector plate can be made (expensive). If it is left spherical the corrector plate compensates for it, and that is a schmidt-Cassegrain design.

For commercial scopes, they make crappy correctors very quickly and cheaply and this is cheaper than correcting the primary. For custom telescopes, fixing the primary is far cheaper.

> The mirror is already a finished item so I thought to adapt to it.

It could still be refigured, and probably should be tested, depending on where it came from.

> 2) You're not forced to use the long effective F ratio if you accept a > larger secondary -- is this correct ? > ------ I'd be most interested in a wide field light bucket.

If you want wide field, I would not make a Cassegrain. Use the primary alone and make a Newtonian. If it is spherical I can figure it to the require parabolic figure.

> So what if one did a classical cassegrain accepting a large > secondary ? possible / practical or not ?

The minimum F/ratio you're going to get is F/10, and that's going to require a huge secondary, around 8" by my calculations, and more if you need a lot of back focus. That's still a 200" focal-length instrument.

Is this for visual use or imaging?

If you really want a wide-field Cassegrain, I'd start with a much faster primary.

Mike L.

Conclusions

For a wide field scope its best to to a Newtonian scope