Re: OWLA observations and rouge

[John Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Tue, 12 Mar 1996 09:07:48 -0600
>From: Dan Wenz <djwenz@xxxxxxxxx>
>Subject: Re: OWLA observations and rouge

>On Tue, 12 Mar 1996, Patrick Boeckstijns wrote:
>> A few months back a few people mentionned "rouge" to polish lenses.
>> Were can you get it ? How do you use it ? Is it suitable to remove scratches
>> from a lenscoating ? Can it remove the damaged coating all togeather ? Can
>> lenses be recoated ? 

>I've used rouge about 1000 years ago to polish a telescope mirror. The 
>material supposedly caused the Pyrex surface to flow ever so slightly, to 
>cover the (very) small scratches remaining from fine-grinding. 

Unlikely. A few months ago I got in a debate over the flow characteristics of
glass, and looked up several references. Glass is nominally a "supercooled
liquid" (some would debate the exact wording), but at room temperature it's
so stiff (~1000000000000000000 times as viscous as molten glass, which in
turn is ~100 times as viscous as water), that in practice you're not going
to see any noticeable effect in any plausible time frame. As a test, some
glass samples were placed under a bending stress of ~100000 psi for 26 years,
with no permanent deformation. (Above numbers from memory.)

Rouge is an extremely fine abrasive, I believe in a supporting matrix.
The usual method of polishing is to progressively replace the fine scratches
with even smaller scratches, until the surface is as smooth as you want.
On an optical surface, that would imply the remaining scratches in the final
stage of polishing need to be much smaller than a wavelength of light.

>I would 
>think that on soft glass faster polishing would take place. The polishing 
>was done on a pitch lap, which also flowed to conform to the mirror's surface.

See above. The pitch lap is also ground.

>I would hesitate to use any kind of polishing compound on a (soft) lens, 
>especially a coated one, for fear of removing the coating and/or very 
>slightly changing the lens' figure. 

I agree about the coating. Sometimes breathing on the lens and carefully
rubbing with lens paper seems to improve a damaged coating, but I make no 
guarantees (and also I don't understand why it might help - it just 
appears to).

A heavy-duty polishing could change the figure of the lens. But I don't know
the relative dimensions of material removed in the various stages of grinding
and polishing - it might be possible to improve the surface of a bare lens
by very fine polishing, without significantly changing the focal length etc.

John R.


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