From: "Peter" 
To: "Guy Brandenburg" ,

Reply-To: "Peter" 


Guy,

I have tested aluminized secondary mirrors many times with a reference flat
with absolutely no problem. The fringes are clearly visible even in a well
lit room. Try using a true monochromatic source. Try the test with the flat
against a piece of window glass to determine if the fringes are visible
then switch to the aluminized mirror. It could also be that the two flats
are too far apart so try placing the reference on top of the other.

Peter

----- Original Message -----
From: Guy Brandenburg 
To: 
Sent: Thursday, August 14, 2003 12:54 AM Subject: ATM testing aluminized flats


>
> 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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