Re: [ATM] convex secondary

[Mike Lockwood <melockwo@uiuc.edu>]

Jonathan,

Jonathan Lavoie wrote:
> Jonathan Lavoie wrote:
> Around F2.9
> 
> According to Richard the K is almost -5. Didn't do the calculation 
> myself... still learning to use the software MODAS.

So that's an F/2.9 mirror with 5 times the correction of a parabola. 
That's a lot of correction.

> Bad news about my idea of shortening the 16" RoC to use it in a Hindle 
> test then grind it back to it's good focal length: I forgot that it's 
> current RoC is twice the F length so if I want to use it as a Hindle 
> there is a lot of glass that will be removed. So this idea becomes 
> impraticable.

I thought that might be a problem once I saw that the secondary was 
F/2.9 - the Hindle sphere ROC has to match the secondary ROC, I 
believe, and that's one FAST 16" mirror.

> I heard of another test for convex but can't remember where. The idea 
> was to test from the back through a liquid of similar refraction index 
> that is in contact with the back surface, and just do standard foucault 
> measurement. Anyone heard of it?

I've heard of using Glycerin, because it matches the index of 
refraction of Pyrex.

Seems like if you polished the front and back of the tool and the 
secondary and then put the two together with glycerin between (flat 
surface to flat surface, so that the curved surfaces were facing out) 
then you could test through the tool and mirror with minimal 
refraction.  This is either the test I'm remembering, or it's close to 
this.  I don't know how much of a problem testing through the Pyrex 
would be.

Here's a nice link to a 10" RC project:
   http://www.trivalleystargazers.org/gert/Astro_en.htm

Scroll down to the "Techniques" section and there are links to making 
the tube, primary mirror, and secondary mirror.  This site has a LOT 
of pictures and takes a while to load, but it's worth it.  Very 
interesting stuff.

	Mike Lockwood

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