Re: [ATM] Stripping to re-figure?

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

To me, the main reason for stripping the coating and not polishing it 
off is to not affect the figure, especially in the case where the 
mirror shows no astigmatism and has a lot of correction in it.  Adding 
the correction back can take quite a bit of time if the mirror is 
fast, and polishing the coating off will take longer if the mirror is 
seriously aspherical.

Rick M wrote:
> the reason for the
> not full thickness lap was because i read the coating
> will plug up the lap so after the removal i poured
> more pitch on top and have the lap all ready to go
> now.

I've never experienced a coating "plugging up" a lap.  In my 
experience the Aluminum just floated away in the slurry, and I washed 
it off.  The lap was unaffected.  If the mirror is quite bad or has a 
turned edge, just polish away and the coating will be gone fairly quickly.

It is VERY illustrative to watch where the coating is removed as you 
polish it away.  It will show you EXACTLY where material is being 
removed.  Try some different strokes (MOT, TOT) and see what the 
effect is!

Caustic solutions will etch the glass if left on the glass too long. 
That's a risk I'm personally not willing to take, not being a chemist.

Ferric Chloride is a commonly used chemical for etching PC boards. 
It's easy to get, and it won't hurt you if you get a few drops on your 
hands.  It should, however, be disposed of properly.  Overall, it is 
an effective, safe way to strip the coating off a mirror.  Put a paper 
towel on the surface of the mirror, pour on only barely enough to 
saturate the paper towel, and wait.  A single bottle from Radio Shack 
will strip a LOT of mirrors.

Keep in mind that after stripping some coating residue may remain, so 
it's a good idea to polish all parts of the mirror surface at least a 
little bit to make sure it's gone, and the surface is smooth and clean.

	Mike Lockwood

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