-- BEGIN included message
- To: owner-atm@shore.net
- Subject: BOUNCE atm@shore.net: Non-member submission from ["Tom Warm" <Tom_Warm@mmacmail.jccbi.gov>]
- From: owner-atm@shore.net
- Date: Tue, 18 Feb 1997 13:33:49 -0500 (EST)
>From atm-owner Tue Feb 18 13:33:37 1997 Received: from mmac2.jccbi.gov (mmac2.jccbi.gov [162.58.8.4]) by relay1.shore.net (8.8.3/8.8.3) with SMTP id NAA23038 for <atm@shore.net>; Tue, 18 Feb 1997 13:33:35 -0500 (EST) Received: from mmacmail.jccbi.gov by mmac2.jccbi.gov (5.0/SMI-SVR4) id AA10842; Tue, 18 Feb 1997 12:32:32 +0600 Received: from ccMail by mmacmail.jccbi.gov (SMTPLINK V2.11 PreRelease 4) id AA856298058; Tue, 18 Feb 97 12:27:04 CST Date: Tue, 18 Feb 97 12:27:04 CST From: "Tom Warm" <Tom_Warm@mmacmail.jccbi.gov> Encoding: 85 Text Message-Id: <9701188562.AA856298058@mmacmail.jccbi.gov> To: atm@shore.net Subject: ATM "Frozen" liquid mirrors ATM "Frozen" liquid mirrors: I tried making spincast epoxy "mirrors" as described in the SciAm article. You may be interested in my experience. I bought a gallon of epoxy used to repair car fenders for about $40 at Autozone. The hardener comes with it. I bought a cake cover about 12" in diameter and 6" high for a mold. It was made of a flexible plastic, like tupperware. I was lucky, because the epoxy didn't stick to it. On my 1st try I had no idea how much epoxy to use, so I mixed up a pint of epoxy in the cake cover, removed the spindle from my turntable, and put the cake cover on the turntable at 78rpm. That's when I found out that I should have mixed it in another container, and poured it into the cake cover. It was impossible to center the cake cover exactly, even with the ruler-on-the-side-as-it-spins method. I got it the best I could, and let it go, but it was clearly slightly eccentric. It really stunk up the house with a sickening acrid smell. By the next day it had hardened. The result vaguely resembled a paraboloid shape, but I had not used enough epoxy. There was an ugly 2" jagged hole in the middle, and several big ripples in the epoxy. I figured that the ripples were caused by air currents. Then I discovered that the cake cover had a tiny bump in its exact center, left over from the injection molding process used to make the cake cover. I centered the bump on the spindle hole, mixed up another pint of epoxy in a used gallon ice cream carton, and poured it in on top of the 1st spincasting. I covered the cakecover with saranwrap, held on with a rubberband. This did 2 things. It prevented the air currents, and it held in the smell (but I had moved the turntable to the garage, anyway). This time it worked great! The next day I turned the cake cover upside down, and my "mirror" dropped out. The 2nd pint had encased the 1st pint. It was opaque, dark green with a seemingly paraboloid shape. The mirror surface was smooth to the touch, but had a non-reflective, flat finish. The outside (the part touching the cakecover) had a glass-like finish, proving that epoxy will hold a shiny finish. I have not tried to polish the epoxy. The SciAm article had recommended making the epoxy cure as slowly as possible. I experimented with the amount of hardener to use, using various percentages of the amount recommended on the epoxy can - 10%, 25%, and 50%. I found that the more hardener, the more it shrank. 50% left a 3/8" gap all the way around. 25% left a 1/8" gap. The gap for 10% was hardly noticeable, but had to be there, because the mirror dropped out easily. I don't know what shape they shrank into, but it probably wasn't a perfect paraboloid. The SciAm article gave focal lengths for 33 rpm (about 44cm, I think), and 45 rpm (20cm), but not for 78 rpm. Assuming a linear relationship between rpm and focal length, I estimated it ought to be about 7 cm. I smeared some oil on the surface, pointed it at the sun, and with a small disk of paper stuck on the end of a pencil, I found the approximate focal point. It did in fact seem to be about 7 cm, making it about an f/0.25 !!. I thought maybe I could pour the epoxy into a real telescope mirror, and get a convex paraboloid, which I could then use as a mold to pour molten glass onto to make a concave mirror. But I didn't know if epoxy stuck to glass. So on one of my experiments I mixed the epoxy in a large Pyrex measuring cup. The epoxy stuck to the Pyrex tightly, ruining the measuring cup. I guess I won't making a mirror mold this way! One time I tried using a square tupperware container. Have you ever seen a square paraboloid? You could make an off-axis Shiefspiegler type mirror this way by putting a plank on the turntable, and placing the mold on the plank away from the center (counterbalanced of course). You will have to remove the turntable arm to do this. It is interesting to note that no matter how far out you put the mold, the focal point will always be directly above the turntable spindle hole. I have not been able to determine the coefficient of expansion of epoxy due to heat. Does anyone know? I also have not found an easy way to adjust the turntable to other speeds. Any ideas? Hope this helps. I would be interested to hear others' experiences. If you want, you can e-mail me at Tom_WARM@MMACMAIL.JCCBI.gov Tom WARM
-- END included message