Re: [ATM] fringe testing a convex and concave surface together.

[Mike Lockwood <melockwo@uiuc.edu>]

Mark, Dave,

Mark Holm wrote:
> Not personal experience here, but reading things in S&T and the 
> late ATM Journal.  Isn't the preferred method to use collimated 
> light?  A fellow writing in ATMJ (the very last issue, if I 
> remember correctly) mounted a Newtonian upside down and used a beam
> splitter at the eyepiece end so the scope acted as both a 
> collimator and decollimator.  He was also using the water flat 
> method.  As I recall the S&T article from even longer ago, the 
> author suggested using a Fresnel lens with a simple beam splitter 
> to collimate.  I don't know if he viewed at a distance or used the 
> Fresnel lens as a decollimator as well.  Evidently, the optical 
> perfection of the collimator/decollimator can be pretty awful 
> without messing up the test.  The only hitch I remember is that he 
> recommended a fairly high f-ratio Fresnel lens.  Most of the ones 
> on the surplus market are out of overhead projectors, and have 
> fairly low f-ratio.

Collimated light is nice, but it requires extra equipment.

For the water flat method, the thickness of the water is on the order
of thousandths of wavelengths, so there is significant error in the 
test results due to perspective/parallax if the light is uncollimated. 
  I have used a Fresnel lens for the water test (testing a flat 
submerged in water) and it works.  Yes, a high F-ratio is necessary.

For contact interference, the air gap is typically a few wavelengths
thick, so the perspective/parallax error is much smaller.  A 
collimated light is nice, but it's easier just to get farther away to 
view when you need serious accuracy.

Dave wrote:
> The amount of perspective-induced curvature in a contact Fizeau 
> interferometer is proportional to the spacing between the surfaces.
>  If you are brave enough to directly contact the surfaces then
> there is usually no need for a fancy viewing setup.

In my testing, with fairly clean surfaces, I notice minor changes in 
the fringes as I get farther away (going from 2 feet to 8 feet, for
example), especially when doing critical testing of flats that almost
match.

> If you try this, never slide the surfaces over each other.  Bad 
> things will eventually happen.

Yes!

And when separating the pieces, lift up gently on the top piece and
wait patiently as air gets back into the gap.  Watch the fringes
increase in number until there are so many that they disappear.
Eventually the bottom piece will "detach" and plop gently back onto
the spot it had been sitting in, which for my test setup, is on black 
velvet.  The separation process is LOTS of fun to do with two 
full-thickness 10" flats!

	Mike Lockwood

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