Re: [ATM] A two way null test for convex hyperboloids

[Mauritz Andersson <mauritz.andersson@gmail.com>]

James,
Thanks for the questions, I'll try to answer.

You wrote:
> That's a pretty nifty null, but how would you fabricate it?
> 
> Assuming
> 
> #1 You found some Quartz
Not necessary, any glass homogenous enough can be used. See my other
posting with a BK7 prescription.

> #2 and you ground it convex on both sides to just the right RoCs
Yes, that is necessary!

> #3 and in doing so you created your 20mm center thickness,
It's not very sensitive to that, you can be off by at least 2 mm.

> #4 and found yourself in the fortuitous position of not having any wedge to
> contend with
Yes, wedge has to be kept under control. It is not too bad though it
seems (I tried it in oslo)
since it mainly introduces a coma deviation. A small coma error on a
conic surface means that the optical center of the surface is
displaced. If small enough it can be compensated in collimating the
final Cassegrain telescope.

> #5 and while polishing both sides out, you didn't change the ROC of either side
Yes, has to be kept under control with a spherometer.

> #6 and finally, after polishing both sides out, both sides ended up spherical.
This is not strictly necessary, you only need a smooth surface to start with.

> How do you proceed from here?
Here comes the iterative testing and polishing on each side to rescue.
Test surface 2 by reflective Foucault test (*). Polish surface 2
closer to seeing a null.
Test surface 1 by reflective Foucault test. Polish surface 1 closer to
seeing a null.
Test surface 2 by reflective Foucault test. Polish surface 2 closer to
seeing a null.
...
When seeing a null from both sides, you have converged to the desired
conic surfaces.

*Ligth path: source, refract surf 1, reflect surf 2, refract surf 1,
back to knifedge positioned close to the source.

> You have to manipulate two convex surfaces to known Conics, which means you have
> to test both sides individually by some other means.
Well thats the point, you do not need another test. We have two
surfaces and we test them in two different ways, a Foucault test for
each orientation of the lens. If you see a null in both directions,
you have the desired conic surface.

> On a positive note, if you achieved the desired prescription, this null test
> would make for a nice Sanity check.
> 
> Take Care,
> James Lerch
> http://lerch.no-ip.com/atm (My telescope construction,testing, and coating site)

Very best regards,
Mauritz
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