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  • OT Random Experiment

    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!!!
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    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.
    Last edited by Evan; 01-13-2007, 02:33 PM.
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    • #3
      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.. .

      Lenord

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      • #4
        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.
        Free software for calculating bolt circles and similar: Click Here

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        • #5
          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!

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          • #6
            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.
            Free software for calculating bolt circles and similar: Click Here

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            • #7
              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-
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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              • #8
                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.

                Free software for calculating bolt circles and similar: Click Here

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                • #9
                  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.

                  Free software for calculating bolt circles and similar: Click Here

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                  • #10
                    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
                    You never learn anything by doing it right.

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                    • #11


                      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.
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                      • #12
                        Nah!
                        Evan, at your latitude, you'd need a CO2 laser, surely!
                        Just got my head together
                        now my body's falling apart

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                        • #13
                          LASER? Go to something simple.

                          A pellet rifle would not be high-tech but would work more effectively and be easier to use against cats at the bird feeder!

                          Bill
                          Bill

                          Being ROAD KILL on the Information Super Highway and Electronically Challenged really SUCKS!!

                          Every problem can be solved through the proper application of explosives, duct tape, teflon, WD-40, or any combo of the aforementioned items.

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                          • #14
                            I was really thinking of providing the cats with chickadee flambé...
                            Free software for calculating bolt circles and similar: Click Here

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                            • #15
                              Aint gonna work Evan. Birds can see the (human) visible spectrum.
                              On the other hand, cats aint gonna see a CO2 laser
                              Just got my head together
                              now my body's falling apart

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