Cleaning a reflector mirror without scratching it

[sonhouse]

Hi all. I have a very heavy 9" reflector but it is has spherical curve.
It was made for a semiconductor projection aligner which was being scrapped
and I saved the mirror from going into the trash bin. I saw an article in
one of the rags about making a frame on the back of the mirror where you
arrange a bolt to press in on the center of the mirror and it doesn't take
much bending to convert it to a parabolic curve. So I am going to try that
and see what happens. My question is this: sitting in a box for about 20 years now, it has collected a lot of dust, its really filthy. The curve is
spherical and probably as good as you can make such a mirror, I am guessing
1/20 wavelength. It was made to project an image accurate to micrometer dimensions on a silicon wafer. So what is the best way to clean it without
scratching it? I know I can blow off a lot of dust with just an air can but what about after that? Just run warm water with some detergent? I think it was aluminized, not sure of that, that is to say what the coating is. I don't think its gold, too bad about that! its been sitting around literally for 20 years. Maybe it needs recoating? How can I tell? Thanks for your help.

 

[chroot]

Whatever you do -- do NOT touch the mirror. Not with tissues, not with brushes, not with your fingers, not with "lens cleaning cloths," or anything else. If it's dirty, you will sleek it if you touch it, without question!

The best way is to give it a bath in distilled water with a very small amount of detergent like Dawn, then rinsing it numerous times with distilled water.

Alternatively, you can use collodion (my preferred mirror/lens cleaning method).

The mirror is probably not coated with anything, if it was not used for optical service. That means it's probably not going to be very good for astronomical use, but it's worth a try. Coatings, if any, can be easily seen as whitish, greenish, reddish, or purplish reflections from the mirror surface.

- Warren

 

[sonhouse]

Hi chroot, warren. Thanks for the advice. When you say give it a bath you mean just slosh the water around in the mirror kind of like the guy does when panning for gold?
Can I run the faucet water over the mirror? Would that cause scratches?
The mirror does have a coating, its VERY reflective, my guess is aluminum but after 20 years I thought it would get splotchy but last time I looked, it seemed very uniform under all that dust!

 

[chroot]

Hi chroot, warren. Thanks for the advice. When you say give it a bath you mean just slosh the water around in the mirror kind of like the guy does when panning for gold?
Can I run the faucet water over the mirror? Would that cause scratches?
The mirror does have a coating, its VERY reflective, my guess is aluminum but after 20 years I thought it would get splotchy but last time I looked, it seemed very uniform under all that dust!

I wouldn't "slosh" so much as just gently circulate the water.

You -can- rinse with tap water, but I strongly advise against it. Tap water contains chemicals which can attack the coatings. In any case, you pretty much have to do the final rinses with distilled water; tap water will leave residue that ruins images and attacks coatings.

- Warren

 

[turbo]

The best way to clean this mirror (in my opinion) is to soak it in a 50/50 solution of Isopropyl alcohol and distilled water with a little detergent mixed in. After a good long soaking, rinse with copious amounts of distilled water. If the mirror looks nice and clean after the initial treatment, you can rinse it with anhydrous Isopropyl to minimize blotching as the liquid evaporates.

To deform a mirror in the manner you describe, you will have to construct a cell that supports the edges of the mirror in a very uniform manner (a ring overlapping the edge just a bit) and you may have to experiment with the size of the pad that the deforming screw applies to the back of the mirror. Good luck - that sounds like a really fun project.

 

[Labguy]

I saw an article in one of the rags about making a frame on the back of the mirror where you arrange a bolt to press in on the center of the mirror and it doesn't take much bending to convert it to a parabolic curve.

Actually it will depend on the approximate f/ratio of the spherical mirror as to how much "bending" is needed. BUT, to form a parabola the center bolt/pad is glued to the mirror and the bolt is to pull, not push, to form the parabola. Otherwise, turbo-1 is correct about cleaning using distilled water and an isopropyl alcohol rinse.

The purple/green reflections that Chroot was mentioning are seen from coated lenses, not mirrors.