Re: [ATM] convex secondary

[Mike Lockwood <melockwo@uiuc.edu>]

Jonathan,

Jonathan Lavoie wrote:
> Why wouldn't the secondary be 4", I thought the only difference between 
> Cassgrain and RC was the figure of both primary and secondary mirrors?

If you build both at the same focal ratio, then that's true - the 
optics have the same physical dimensions.

I assume you know the differences in the optical design and what each 
design is good for, but I'll mention them for the benefit of others.

Optically, the difference between the two is spot size and shape over 
the field of view.  For a given F/ratio, (I believe I am right when I 
say that) an RC has better (rounder) star images over the field as a 
whole (no coma, great for wide field imaging), but a classical Cass 
will have a smaller spot size at the center of the field (better for 
planetary viewing).  If I'm not right, I trust someone will correct me!

I assumed that if you made an RC then it would be made with a lower 
focal ratio than a Classical Cassegrain, and therefore the secondary 
mirror would be larger and closer to the primary.

> I know anything about interference testing, does it need special 
> equipment? How accurate is it?

You need a monochromatic light source.  It is as accurate as your test 
plate.

> About grinding and polishing twice the primary, it does not scare me 
> because there is not much glass to remove each time and the first 
> polishing does not need to remove 100% of tiny pits at the edge. If this 
> method of null testing is doable and has good fiability I think I would 
> go for this technique.

If that's what's involved, then that technique sounds promising.  I 
just wanted to make sure you knew about alternatives.

	Mike Lockwood

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