Re: [ATM] Cass mirror separation

[Mike Lockwood <melockwo@uiuc.edu>]

John,

John Sherman wrote:
> I know that the distance separating the mirrors in a Cass-type
> scope is fairly critical for high-res views, unlike in a Newt. So
> what if you have a pair of mirrors and are unsure of the designed
> separation?

Then you've got a problem.  If they are for a classical Cassegrain, 
the odds of them working together properly aren't high.  Bob May's 
later reply covered the procedure I would use if I was just doing it 
experimentally.  For a classical Cass, Moving the secondary away from 
the primary will increase the correction of the system, moving it 
closer will decrease it.

While you can generally find a separation that sort of works, the size 
of the secondary is not likely to be the right size - you'll either 
use a fraction of it (which is not so bad if the obstruction ratio is 
not high) or it will be too close and you'll lose light from the 
primary or have a narrow (or no) fully illuminated field.

> Sure, one could measure the curves and put them in OSLO
> and let it optimize. But since they're aluminized a spherometer
> won't be good to use on the convex.

You can use a spherometer on a coated secondary.  Just clean the 
mirror and spherometer contacts first, and be gentle.  The other way 
to get its radius is to put a scope together using it and then use the 
Cassegrain equations to calculate it after you have measured the 
mirror spacings and location of the focal plane.

> But is the convex a sphere?

In a Dall-Kirkham, yes, if properly made.  In a classical Cass or RC, 
it is a convex hyperbola.

> So I suppose that, since I already know the approximate separation,
> the best thing is to star test a finished scope extensively. System
> over- or under-correction should indicate a direction to move.
> Assuming both mirrors are well-made, trial and error should get me
> there eventually.

Yes - or it will tell you you simply can't get there (proper spacing 
and no spherical aberration) with the parts you have.

> Is there a better way?

Sure - make your own secondary.  :)

	Mike Lockwood

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