Re: ATM testing aluminized flats

[atm@misterg.fsnet.co.uk (Andy Gray)]

On Thu, 14 Aug 2003 00:54:08 -0400,  Guy Brandenburg
<gfbranden@earthlink.net> wrote:

> when I try testing a known 1/20 lambda reference flat on an 
>ALUMINIZED and unknown but purported 1/8 lambda flat, I don't see any 
>fringes at all, either with my setup or a friend's setup. We understand 
>that the reflection from the aluminized surface may be overwhelming the 
>diffraction lines, but --- .......
>other than removing the aluminum coating, what should we do?

In a different life, I used to test solid aluminium mirrors
in an Twyman Green interferometer. To even up the
reflectivity of the un coated glass flat and the Al, we used
a nylon mesh screen in front of the mirror (this was like a
fine bug screen). We were using a laser source with a 10cm+
coherence length. Whether your set up will tolerate the
additional path length for a screen, I don't know - I would
guess you only get a <<0.5mm coherence length from a
fluorescent tube. 

Having said all that, I have seen *interference* fringes
between an Al mirror and a glass flat under fluorescent
lamp. They do have to be in very good contact, though - much
more "intimate" than glass / glass, and not something that
you'd do if you were worried about marking the coating.

HTH,

Andy.

Andy Gray, N. Wales, UK