Re: [ATM] Making flats

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Vladimir Galogaza wrote:
> I have changed thread from "Malacara ...............for archiving/searching 
> sake..

Thank you!

> Bob Mariott was kind to dig URL for Tom Waineo's article about making and 
> testing flats.  Thanks Bob.

You mean Bill Marriott.  :)

> As I understood Tom, RC  testing method requires acurate sphere.
> " A flat can be tested at a angle to a accurate spherical
> with no astigmatism - Waineo"
> ABC method requires nothing like this. Three blanks are cheeper than any 
> acurate reference element.

Yes, but it is easy to accurately test a slow sphere, and a capable 
ATM can make their own reference sphere.  It can be smaller than the 
flat, depending on the angle of the RC (Ritchey-Common) setup.  It's 
not at all hard to test the sphere to 1/50th wave if you are careful. 
  My 10" F/12.5 reference sphere tested well beyond that.

Often spherical mirrors of smaller size are available surplus, already 
polished, though it may take some searching to find one over F/10.

> About making vs testing.
> What has testing method (like RC) to do with making method (like ABC)?
> What is real advantage of "RC method" over ABC for MAKING  (!) flats?
> Isn't it possible to make flat by ABC and test it by RC.

Yes it is possible, but RC has some advantages:

RC requires only one flat blank to be ground and polished, so it is 
cheaper if you don't want to buy all of the blanks.

For the RC test you don't have to build an interference testing 
booth/setup.  You can use your existing Foucault tester, but you will 
probably want to add a pinhole and eyepiece holder to check for 
convexity/concavity.

For RC, there is no chance of scratching the flat because the flat is 
not touching another flat.  With contact interference this can happen 
quite easily, especially if you are inexperienced.

RC can also be used to easily test COATED flats.  This is difficult 
(but possible) with the contact interference method.  I have checked 
coated elliptical flats by putting a flat on top of them, but the 
coating better not have any corroded spots in it or that will cause a 
"bump" on the surface and you won't get the air gap small enough to 
see useful fringes.  The fringes also have very low contrast, so 
photographing them is often necessary to examine them.

If, for the RC test, the flat is set up to reflect at a very shallow 
angle, as Bill mentioned, this will provide a more sensitive test for 
subtle zones than contact interference (just how closely can you find 
the center of that fringe? :o ).  The test for overall convexity or 
concavity of the flat surface is done using a pinhole and checking the 
return for astigmatism.

	Mike Lockwood

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