From: Guy Brandenburg 
To:  atm{at}shore.net
Reply-To: Guy Brandenburg 


I have a question about testing aluminized flats.

It is relatively simple to test a piece of purportedly flat glass that is
NOT aluminized, using a monochromatic light source and a reference flat. I,
and others, have built devices that produce monochromatic light, and you
can see the diffraction lines quite well. In my case, I took an ordinary
fluorescent light bulb - the type that produces 1 sharp green line at 546.1
nm, not two right next to each other - and used some tissue paper as a
diffuser, and then got some theatrical lighting gels to filter out all the
other lines. I have a spectrometer from Project S*T*A*R that I used to see
how well the gels filter out the other lines, and the filtering works quite
well as far as I can tell.

My setup is very much like the one described in the LeCleires' new book on
making telescopes.

(When you go to buy theatrical lighting gels at a theatrical supply store,
the sample color thingies come in a little book, and each color has a
little graph which is alleged to show what percent of the light gets
through at every wavelength of the more-or-less visible spectrum! That's
part of how I decided which colors of gels to use. I also discovered that
there are 2 common types of fluorescent tubes, as I mentioned - those with
2 sharp green emission lines, and those with just 1 sharp green emission
lines.)

But 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?

--
Guy Brandenburg
Washington DC, USA
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html

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