Mirrors

Colin Kaminski · Post by **Colin Kaminski** » Fri Jul 12, 2002 12:56 am

Here is a source for very high quality collimating mirrors.

http://www.nova-optical.com/mirror.htm

Tom B. · Post by **Tom B.** » Fri Jul 12, 2002 11:23 am

Here's another source: http://www.e-scopes.cc/ - formerly Coulter Optical.

Sizes up to 13.1 inches, f/4.5, 1/8 wave quality claimed. This is big enough (barely) for 8x10 format. This baby will set you back a mere $600. I'm using an 8" that I got from a local source for about $130 - OK for 4x5 format.

Colin Kaminski · Post by **Colin Kaminski** » Fri Jul 12, 2002 12:02 pm

I bought my 10.1 inch f 4.5 mirror from Coulter 10 years ago (I had to use it in a telescope until I could afford the rest of my holography bench ). I wish I would have waited for a 14 inch or just gotten the 8 inch. My Coulter had a small ring glued to the center to help with allignment in a telescope. It would make a perfect ring in the reference beam. I was able to remove it with acetone.

Some mirror basics I wish I had considered before I purchased my mirror:

1. Dia*f-number=the distance to your spatial filter.

2. The largest film size you can use with a given mirror must satisfy the condition SQR(L^2+W^2)<Dia.

wler · Post by **wler** » Mon Jul 15, 2002 8:50 am

I just had bought a 6''/F8 telescope mirror and
(apart from the fact that I had ordered a different one but never mind..) I wonder about getting a larger one.

Here comes the question: how important is optical quality (as lambda/10 surface etc is very costly...) ?
The idea is that one can do holograms even without such a mirror, with a diverging beam - apart from unpleasant magnification of the pseudoscopic (?) image, not much is really wrong... so this lets me wonder what can go wrong with a low quality mirror ? I guess the image's geometry would be a bit distored, but it is
slightly distorted anyway. So, is the difference between a high quality lambda/xx telescope mirror with optimal parabolic shape, and a mediocre cosmetics product at <1/10 of the price really substantial, if at all noticeable ?

Tom B. · Post by **Tom B.** » Tue Jul 16, 2002 3:15 am

For most display holograpy, I agree that using astronomical quality mirrors and lenses as collimators is overkill. For making high quality diffraction gratings and holographic optical elements, mirror quality would be much more important. It's just hard to find inexpensive "good-enough" mirrors - you can find expensive astronomical mirrors or cheap shaving/cosmetics mirrors, but nothing in between, and alas the cheap mirrors are all rear-surface reflectors.

Jean D. · Post by **Jean D.** » Tue Jul 16, 2002 5:01 am

I've read in an old book about holography that reflective layer of some back-surface mirror are protected with vernish or else.
May be it is possible to dissolve this layer with aceton or white spirit. Probably that this surface has not quality as good as real front-surface mirror but it is maybe a good way to get 'cheap' mirror.

Jean

Colin Kaminski · Post by **Colin Kaminski** » Tue Jul 16, 2002 1:53 pm

Coating mirrors is not very hard. You could coat the front surface of a mirror with a home made deposition chamber. If you go to your local astronomy club they may have one. Mine does. They do mirrors for $25 for a 8".

For that matter if you have time and patience you can grind your own 8" mirror. Actually 6 3/4" works for 4x5" film and is a lot easier to grind if you can find a blank. There was an astronomer in San Francisco who used to grind mirrors from old port holes in boats. He developed a novel mount that was easy to construct named after him "Dobsonian". Thick plate glass would work. (Since we need the bench to be temperature stabilized in order to avoid heat waves going through our beams, I would not expect the thermal expansion of plate glass to be an issue as it is in astronomy. My coltuer is made from plate glass.)

My local stained glass artist often trades me holograms for cutting up glass. He has cut many first surface mirrors as well as made ground glass for diffusers. I am not sure if he could cut a 6 3/4" circle in 1.5" glass but I would ask.

Jonathan · Post by **Jonathan** » Tue Jul 16, 2002 4:16 pm

Yes, Coulter has nice parabolic mirrors - decent quality, good prices. When I bought the 13.1" (f4.5 -->59" focal length) about 4 years ago, it was something like $600, and it handles up to 8"x10" film and plates very well.

As far as going after the last word in mirror quality, whether flat or parabolic, I would agree that it's not really necessary for display holography. Spending your money on a spatial filter and stable laser will get far more productive results.

Here's an idea for cheap flat mirrors - old SLR cameras, old photocopiers, and old... well, whatever uses FS mirrors. Recyclers, repair shops, friends upgrading, etc. are good sources. They can be cleaned up with isopropyl alcohol or acetone.

Colin Kaminski · Post by **Colin Kaminski** » Tue Jul 16, 2002 8:21 pm

If you are using holography as a measuring device then high quality optics are a must. If you are doing holographic interferometry then depending on your set up you double the error of the mirror then project it back to the object.

For art I am not sure how much it matters. Any thing close is probably OK.