From: atm{at}misterg.fsnet.co.uk (Andy Gray)
To: atm{at}shore.net
Reply-To: atm{at}misterg.fsnet.co.uk (Andy Gray)


On Thu, 14 Aug 2003 00:54:08 -0400,  Guy Brandenburg
 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

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