ATM Protecting coated mirror.

[Andrew Grisdale <roadwarrior@techieguy.com>]

Hello all,

First of all, to introduce myself, I am a semi-newbie, I have read up on ATM a few times 
in the past but have not taken the full plunge until recently. Most of my interest at the 
moment is in mounts and drives for the commercial scopes I have (recent 60mm Alt/Az 
"Trust" brand, but identical to 4 other name brand 60x700mm's, and a couple of 60's/70's 
Japanese 40mm) and my SLR camera, however, an optics question...

I stripped down a Polaroid camera ("one step" Land camera) and found a front surface 
mirror within, the coating looks fairly nice and I would like to cut a small secondary 
out of it for a 4 inch newtonian. This isn't going to be a wonderful performer, it is 
kind of an experiment before I commit any of my very sparse funds to real optics, it will 
have for now an F10 primary, back surface silvered, that was sold as a magnifying vanity 
mirror, I spent about an hour in the store picking one out :-) anyway, that folly aside, 
I thought I could build this as an experiment then maybe upgrade the primary to a hand 
figured 4 1/2 silvered with silvering solution, then make a proper secondary .....
...

So the question is, how can I protect the coating on this polaroid mirror so I can cut it 
to an ellipse? 

The glass is fairly thin so probably I will be doing it with cuts, nibbles and grinding 
rather than by the biscuit cutter method

My first thought was to use candle wax, but I don't know how much trouble I would have in 
removing it, I was thinking to heat off some, then wash with kerosene (parafin) then 
rinse, but I have no idea how effective that might be.

For a buck at a flea market, the camera seemed well worth the stripping for a couple of 
reasonable quality lenses, a 366/1 gear train, the fs mirror, a couple of metal gears, 
and some machined rods that I like the look of for using in a Crayford style focusser. 
The next one I find though I am going to give consideration to converting to a paper 
negative astro cam.

Thanking you in advance for any help you can give me.

Andrew G.

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