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

[Mike Lockwood <melockwo@uiuc.edu>]

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

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