Re: ATM Change of Plans, F/2 -> F/3.2

[Aplanatic@aol.com]

Dominic,

> So you think making a precision coma 
> corrected focal reducer is easier than 
> making a decent 8th or 10th wave F/2 
> parabolic mirror? I don't think I can 
> agree with you on this one Dave,

That's not exactly what I said.  I said, "If you
design/make your own then a custom design can significantly reduce coma and
field curvature in the final system."

A two-element custom reducer for a paraboloidal mirror can help a lot with coma and field curvature, but it's not going to be perfect at large off-axis distances.  But James is trying to get good images from a small CCD camera, so I stand by what I said.

What is the fastest paraboloid that you've tried to make?  I tried for several months to figure a 13.1" f/2.35.  I put it down for a while, picked it back up and tried again for about 6 weeks or so.  Pretty intensive effort.  To make a long story short, I tried many things and decided it was too much for me.  So, I re-ground and re-figured it to a sphere and made a Schmidt corrector for it.  I simply ran out of patience.  I've also made a 10" f/3.3 parboloid and it was a world of difference.  Much easier.

I would like to hear from others on the list about their experiences with figuring mirrors in the f/2 to f/2.6 range.  Perhaps I'm making it sound -- because I'm not the most patient or skillful optician -- more difficult than it is?

James can use a commercial reducer for his application, I think, but he should look into the problems that he might have with it in terms of chromatic and off-axis aberrations.  I'm a bit worried that with an f/3.3 light cone the reducer could have substantial chromatic problems.  I don't have access to the optical prescription for commercial reducers, so it makes me a bit nervous.  On the other hand, we know that an f/3.2 paraboloid is doable in this size range, so, if the reducer causes significant chromatic or off-axis problems, the primary can still be used without it, albiet, not at the image scale that James would like.  But he might find the native f/3.2 system quite nice anyway.

Dave Rowe