View Full Version : OT Random Experiment

01-13-2007, 03:23 PM
Ok, so a fleeting idea crosses my mind as I remove one of those highly polished hard discs from a dead drive. Support the disc at its circumference, then press on it at its inner diameter. Disc distorts into a -- telescope mirror!! I wonder what shape the disc gets when you do this- parabolic, or not? A second chunk of a hard drive disc is carefully cut round, and is mounted in a frame and pressurized from behind with epoxy. When the epoxy has set, there's the permanent secondary mirror!!! Celestron, look out, here I come!!!

01-13-2007, 03:30 PM
Sorry, no cigar. In order to function as a usable mirror at all the entire surface must be accurate to at least a 1/4 wave of green light. I don't mean the smoothness but the overall shape. They do look pretty though.

I am still thinking on an easy way to drive the platter motors. They are really powerful but tiny three phase motors.

01-13-2007, 08:06 PM
There was a guy that made an 8" or 10" telescope mirror by gluing a bolt to the back center of the mirror and then using a bracket/nut to pull the mirror into shape. IIRC, there was an article in S&T 4 or 5 years ago. Suppose to have had decent results with it.. .


01-13-2007, 09:14 PM
Don't take this wrong Lenord as I am not doubting you but I find it very hard to believe that anything resembling an accurate mirror could be produced that way. A point deformation introduced at the center of a plane bounded by a circle produces an inverse parabolic curve modified by the stiffness of the material, not the shape required.

I would be very interested if you could find some reference to the article.

01-13-2007, 10:34 PM
You're both right!

I also read that article in Sky & Tel. The guy supported the outer edge of the mirror on a doughnut of carpet, and epoxied a bolt to the middle. The bolt went through the piece of plywood that held the carpet and mirror and had a wingnut on the back to adjust the figure of the mirror.

But... He ground the mirror to a spherical figure first and used the bolt to tweak the mirror into shape.

On a small diameter, long focal length mirror, the difference between a spherical shape and a parabaloid is very small indeed. To try to do this from a flat surface though is another thing entirely!

I don't think this method produces a perfect parabaloidal shape but what the heck. You've only got to get within 5 or 6 millionths of an inch!

01-13-2007, 10:48 PM
Yes, that will work and spherical mirrors are easy to make. The main problem is the method is very temperature sensitive, not a desirable feature in a telescope.

Along the same lines I have heard of experiments using resistors as heaters on the mirror back to introduce carefully controlled deformation to parabolize the mirror. Again, very temperature sensitive.

01-14-2007, 02:48 AM
Aw, crap. Another brilliant stroke of genius dribbling off into the sewer. Oh well. Those hard discs make good wind chimes, anyway.

Actually, thinking about the idea more it would seem that the shape produced by mechanically pulling on the center of a disc is what Evan has described- the reverse of a parabola. In contrast, pulling a vacuum behind a solid disc would seem likely to be forming more of a parabolic shape.

As far as atronomical telescopes go, I realize it won't cut the mustard, but I'm still stuck on an old idea- that of compensating for the shape of the first vacuum formed mirror by the shape of the second mirror, which would be formed the same way, but with pressure to make a convex rather than concave surface. The quality of this 'optical system' would be very poor, but maybe it would make an ok wide field, low power viewer. Maybe not. I can just see trying to 'tune' this contraption- probably a nightmare.

I remember some time ago playing around with silvered mylar membranes (space blanket) and making a nearly 4 ft diameter burning lens that way. It would set a 2x4 on fire in seconds. In fact, one of my home made plexiglas lenses nearly set my house on fire one day. It was leaning against a wall and the sun was focusing through it. It left me a little track of baked paint as a reminder to never leave the thing where sun could hit it.

Ah, memories of burning ants and flies-writing on the siding- trying to get more power out of solar cells-

01-14-2007, 03:04 AM
A guy I met online participated in the Mythbusters solar mirror burn-the-ship contest. Mike is a NASA engineer and I suggested he use a large sheet of polished brass or copper on a sealed wooden frame and a simple vacuum pump behind it. He really liked the idea but didn't have the material to do it and so settled for a simpler multi panel design.

The vacuum approach produces a catenary curve which is very close to a parabola although not quite. It's plenty close enough for burning mirrors but unfortunately not good enough for a telescope.

Here is an unpublished pic of Mike with the Mythbusters crew. Mike is the guy in the middle with the hat.


01-14-2007, 03:13 AM
Here is a simple solar cooker I built a long time ago. It will roast hot dogs pretty well. They drip on the mirrors though.


01-14-2007, 04:37 AM
A friend of mine's dad runs a TV repair shop. Hence I am the proud owner of a ~50" rear projection TV's front fresnel lens :)

01-14-2007, 10:08 AM

I would be tempted to mount it just high enough that it would be extremely dangerous. Just because. Maybe put a bird feeder near it if I still had cats. Maybe put it low enough for the cats to wander into the focus.

Hmm. I'm not quite awake yet and I have very strange dreams.

01-14-2007, 01:48 PM
Evan, at your latitude, you'd need a CO2 laser, surely!

01-14-2007, 02:13 PM
A pellet rifle would not be high-tech but would work more effectively and be easier to use against cats at the bird feeder!


01-14-2007, 02:28 PM
I was really thinking of providing the cats with chickadee flambé...

01-14-2007, 03:30 PM
Aint gonna work Evan. Birds can see the (human) visible spectrum.
On the other hand, cats aint gonna see a CO2 laser :D

01-14-2007, 04:22 PM
A friend of mine's dad runs a TV repair shop. Hence I am the proud owner of a ~50" rear projection TV's front fresnel lens :)

I have one of these set up in a steel frame; I've taken it out to Burning Man a couple of times. It will light a 2x4 on fire in less than 1 second, and boils a 12 oz soda can of water in a few minutes. Melts lead, make glass from playa - great fun.

If you do this, be careful - it will start fires even w/ the sun at very oblique angles, so covering the lens when it's not attended is essential for fire safety.

- Bart

01-14-2007, 11:49 PM
Now I remember my phase locked spin casting table. I can set the speed to produce a very shallow concave in the casting plastic (takes less plastic, but has a longer focal length) to a quite steep concave, for which focal length is shorter than the lens length, never mind the diameter. This takes more resin, unless I pre-build a mold to near shape first. My first product was a shape which I am reasonably sure is a parabola, but which would then require a reflective coating put on to be useful as a mirror. I was going to have it coated, but it had sat around on its side for so long that the shape was distorted. Maybe I'll dig the table out and set up to make a super steep parabolic tube, and see if I can get happy using total reflection by grazing incidence to gather light instead of relying on a mirrored coating.

I'll take a wild guess here and suggest that this 'reflector tube' might be 6 inches or so internal diameter, since I have some tube of that size to make the mold from. I have no idea what kind of rpm I'll need to cause the internal fluid to form up the walls of this tube, and I also have no idea at what point of angle I would cease to get a total reflection from the non-mirrored surface of the cast. I'd be using an epoxy, probably nu-lustre 55, or envirotex to form it with, as either one will flow out to a very smooth surface. I will need to keep the speed absolutely constant, but the rotating table is set up to do just that. I will probably do a test with water to see what shape I can get, and what rpm to use. Then I'll build a mold to be roughly that shape. That way I should be able to get it done using a minumum of resin. If I'm smart, I'll line the mold with fiberglass, do the first casting, then line again with more fiberglass, then do another casting. Because the final cast would be an even thickness of resin, I should get pretty good results, and including the glass cloth will give it more rigidity.

This isn't going to be a burning lens, but it's an experiment I've always wanted to do. I have read about 'telescopes' that use the property of grazing incidence to reflect the incoming frequency (maybe it was x-rays, I don't recall) into a focal point. I believe this method was chosen because the frequency wouldn't reflect off the typical mirrored surface and could only be gathered, or amplified, by grazing it off a parabolic surface. That should work for visible light as well- but of course at some angle the light would then simply enter the material and refract through it (if it was clear) or be absorbed by it (if opaque) instead of contributing to the light gathered at the focal point.

Any thoughts on this are welcome.

Bear in mind that I'm not trying to come up with a new product, or re-invent the wheel. I love to experiment with things, and this would be just another interesting thing to try, at least for me. Who knows, maybe I'll be able to hook up my camera to it, and get some interesting pics.

01-14-2007, 11:57 PM
Don't take this wrong Lenord as I am not doubting you but I find it very hard to believe that anything resembling an accurate mirror could be produced that way. A point deformation introduced at the center of a plane bounded by a circle produces an inverse parabolic curve modified by the stiffness of the material, not the shape required.

I would be very interested if you could find some reference to the article.

Evan, take a look at a brushless motor speed control as used for electric powered model airplanes...

01-15-2007, 12:15 AM
Wow, Bill beat me to it be only a few seconds! I was going to suggest a model airplane brushless motor controller. I recall that someone at the LRK site was working on building his own controller that would sense the phases and automatically power the phases to achieve rotation in the wanted direction. No sensors, commutators, etc. I don't know what the complexity of the circuit is, but it should work for any size 3 ph motor which can generate on its own. Basically that's any permanent magnet motor.

There are several very small motors and controllers available that are brushless and sensorless these days. It would seem to me that even the very smallest of these could be used to sense, and be used as a driver for higher powered brushless 3 ph motors. Directly coupling one of the smallest of these motors to a larger (lathe or mill size) 3 ph motor could give a workable sensor/driver control to operate triacs for the larger motor.

01-15-2007, 01:11 AM

I'm not completely clear on what you would like to accomplish with a grazing incidence mirror. They are used for X-rays but the designs I have seen are a type of fresnel collector made of rings of material at the appropriate angles of incidence.

The type you are contemplating is what's known as a crab eye collector. As it produces multiple reflections of the incident light it doesn't preserve phase information and isn't image forming.

01-15-2007, 03:26 AM
Oh, crap again. Well it was an idea anyway, still would be fun to spin cast in the shop.

What I'd really like is some kind of large aperature terrestrial telescope that is compactable and lightweight for backpacking.

01-15-2007, 06:55 AM
A rotating dish with a liquid in it does form a parabaloid. There is or was a telescope, some where in canada I believe, that used mercury for it's mirror. Of course it could only look straight up. As I recall it was pretty big and they could spin it up to a relatively short focal length, which gives a wider field of view. This was before the computer controlled, segmented mirrors they use today for big telescopes.

As a matter of fact the first of the 8-meter diameter solid glass telescope mirrors were cast in a revolving mold. Before you get your hopes up too high though, These mirrors still had to spend a lot of time on the lapping machine before they were ready to use.

Still, It would be interesting to see just how that would turn out with the plastic. How would you silver it? My guess is it will make a better hot dog cooker than a telescope, but I've been wrong before. One thing, if you want to try to make a scope this way, larger f-ratios (shallower mirror) are much more forgiving of all the optics in the system. In other words, they're not so fussy to make and you can use cheap eye-pieces with them. In the days when you pretty much had to make your own telescope if you wanted one, a 6 inch dia. f/8 was 'The' place to start. Galileo's scopes were about f/40 I think, to deal with the crummy optics of the time.

01-15-2007, 10:52 AM

I'll post some pics later today to give you some ideas.

Lew Hartswick
01-15-2007, 01:22 PM
Somewhere about 50 years ago, the company I worked for was in the IR
"survelience" business, and we were doing both "replica" and spinning masters
for the imaging optics. Also things like electroless nickle plating of mirrors.
I was doing electronic design but did see the stuff going on in the optics lab.
Boy was that a long time ago. :-)

01-15-2007, 03:23 PM
Here are a couple of "travel" scopes.

This first is a back packable 8" that weighs very little and is really portable.


This second one certainly isn't back packable but is the largest scope I have seen that will easily break down to fit in a mini van. IIRC, it's a 16".


01-16-2007, 02:51 AM
Thanks for the pics, Evan. When I get more time again, I'll check into that design for a portable. I have to agree about the larger telescope, definitely not a backpacker's model!

We have a local star club here. I can see no better way to peak my interest than to attend a star party or two. One of our hiking club members is also an avid - what are they called, amateur astronomers?

Re the question about how to silver the plastic casting- basically it's done by vaporizing aluminum in a vacuum, and allowing it to migrate onto the surface. I visited a fellow who used to be the president of the observatory in Victoria, Leo I think his name was, and he had the deluxe setup for doing just this. Large stainless steel vacuum chamber, multi-stage vacuum pump, terminals inside which held the material to be vaporized- pass some juice through it until it boils off in the vacuum- lots of related equipment, like optical testers, etc. He had a couple of 24 inch diameter mirror blanks, and he talked about the problems of temperature changes affecting mirrors, especially large and thick ones. Very interesting visit, I must say. He was willing to coat my spin-cast plastic form, but it had warped from sitting crooked, and because I hadn't built in any stabilizing framework. It would have been a waste.

I may cast another form and have it aluminized, but this time I will create a proper framework, and cast a shallower form. As mentioned by Airhead, that will be more forgiving of imperfections, optically or construction wise. My spin cast table is capable of turning a 12 inch diameter form, so that could translate into a rather long telescope.

I don't expect results on par with a properly ground and figured mirror, but I will have fun playing with the technology. It'll be a ways off anyway, since I'm already up to my ears with other projects.