Re: Rayleigh Water Test (Was: Re: [ATM] 30" Flats)

["matt" <electro_optic@bellsouth.net>]

-----Original Message-----
From: Mike Lockwood <melockwo@uiuc.edu>
To: ATMList <atm@atmlist.net>
Date: Thursday, February 17, 2005 12:30 PM
Subject: Re: Rayleigh Water Test (Was: Re: [ATM] 30" Flats)


>Hi,
>
>matt wrote:
>> A very viscous liquid, something like honey or resin in viscosity, rather
>> than water or oil , coupled with vibration insulation methods used for
>> optical benches is what I had in mind . Has anybody tried something like
>> this and are vibrations still a problem in such a setup ?
>>  It appears much simpler , far less costly and less time consuming to
jury
>> rig such a liquid test than to fabricate or purchase large flats . For an
>> amateur trying to make very few but large instruments (in the 20" to over
>> 30" range) , the cost (in time and money) of all auxiliary equipment
becomes
>> quickly prohibitive .
>
>I think there may be problems relating to the thickness of the liquid
>layer and the coherence length of light in the liquid.
>
>For the water test, I believe a thin layer of liquid is required
>(1/16") for the best contrast.  The idea is to match the amount of
>light reflected from the top of the liquid surface to the amount
>reflected by the glass surface back through the liquid, so that
>fringes form that are easy to see.
>
>If anyone can provide the reflectivities of polished pyrex and fresh
>water, this will help.  (I think fresh water is about 4%.  If polished
>Pyrex is less, then the thinner the water layer, the better.)
>
>It seems like the more viscous liquids would typically be less
>transmissive, and reduce the light intensity as it passes through.
>So, the layer thickness would have to be adjusted to get decent
>contrast, which may be impossible if the layer needs to be thinner
>than 1/16".  Also, the coherence of the wavefronts may be affected
>more by such a liquid (and certainly by making it thicker), thereby
>decreasing contrast.
>
>I think these are the main issues that are in play.  I don't claim to
>have all the facts exactly right.
>
>Again, I think Ed Jones covered this in the S&T article a while back.
>  I've been unable to find a copy to read.
>
> Mike Lockwood
>


I was mostly interested if the test was performed on a finished and
functional telescope, with an already coated mirror, and the viscous liquid
had a very reflective surface, no transparency, and layer thickness would be
adjusted just to get maximum flatness ( not for transmissivity to match
uncoated glass) . You know, a simple double pass autocollimation
interferometric test with a small interferometer in the eyepiece holder, the
scope pointing  at the "flat" , and nothing else . Cheap, but would it work,
that's the million dollar question. I might have to try.

thanks,
matt tudor

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