Re: [ATM] convex secondary

[Mike Lockwood <melockwo@uiuc.edu>]

Jonathan,

Jonathan Lavoie wrote:
> Thanks Mike for this testing alternative.
> Can you describe how the interference test is done?

Fringe testing lets you visually compare the shapes of two polished, 
nearly-mating glass surfaces.

You put two clean, polished glass surfaces in close contact.  Only a 
thin layer of air remains between them.  Monochromatic light is used 
to illuminate the two pieces from above.  The light is reflected by 
the two polished surfaces that are in close contact.  These two 
reflected wavefronts constructively and destructively interfere, 
forming "fringes".  That's the quick and dirty summary.

If you're interested, I (or others) can suggest links to better 
descriptions.

> I guess the test plate has a concave shape identical to the convex wanted?

For the easiest testing, that's right.

> Where do you put the monochromatic light source and how can we get such 
> light?
> What do you look at when testing the mirror?

Here's a link to my testing setup:

http://bi-staff.beckman.uiuc.edu/~melockwo/int_tester/int_tester.html

There are many simpler ways to do this, and my setup is modeled after 
those depicted in various books.  I built this tester to be easy to 
use in the long run (more than one mirror).  So far it's made four 
mirrors - two Cassegrain secondaries and two flats.

The new page about flat testing (described in a message to the list 
earlier this morning) has lots of photos taken with this test setup, 
but with a new green light source.

Here's another very simple setup - just a laser pointer:
   http://tlepage.home.mindspring.com/fringes.html

	Mike Lockwood

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