Easy Parabolic Mirror from a Trash Can Lid

Parabolic reflectors for solar applications are nice stuff, and making your own is a great project in itself. One of the easiest ways we have seen is that of [GREENPOWERSCIENCE], who uses nothing more than a trash can lid, mylar film, and tape. You need a way to make a partial vacuum though.

The idea is so simple that it´s almost like cheating. Cut a circle of mylar slightly larger than the lid, and tape it all around, taking care of stretching the mylar in the process. After you´re done with this, you end up with a nice flat mirror. Here´s where the vacuum is needed to force the film into parabolic shape. Extract the air from a little hole in the lid that was previously drilled, and tape it to prevent the loss of the vacuum. The atmospheric pressure on the mylar film will take care of the job, and magically you get a nearly-parabolic reflector ready for work.

In this other video, you can see the reflector in action burning stuff. One obvious problem with this technique is the loss of the vacuum after some time, about an hour according to the author. Here´s another way to make a more durable mirror also with mylar as the reflecting element, however the quality of the resulting mirror is not as good.

 

34 thoughts on “Easy Parabolic Mirror from a Trash Can Lid

  1. Coat the back of the mirror with epoxy or some other self-hardening material. Assemble as mentioned in the article. Tune the focus as desired and then wait for it to harden. I’m sure there will be some spring-back that will require trial and error.

    1. Lay a sheet of glass fiber onto the back of the mylar. Soak the glass fiber with epoxy resin. Lay the can lid over the glassed mylar then bring up the edges and secure them.

      Apply vaccum and keep it on until the epoxy cures. Hopefully it will retain the shape and bond to the mylar film. Should probably test the epoxy for bonding on a sample of the film and if it doesn’t stick, try polyester resin or some other catalyzed resin that sets slow enough to allow for assembly and vacuuming.

      1. Instead of pumping air out and laying the fiber glass on the back of the mirror, one could also pump air in (e.g. with a simple bicycle pump) and lay the fiber glass on the outside of the mirror. Once cured, remove the trash can lid.

        1. Leave the pump attached with a barbed fitting and some vinyl tubing. I think this would make a fun mirror for the kids. Maybe a proximity sensor to inflate/deflate as they move.

          I love this whole concept.

    2. If you can’t apply vacuum to the back, fill the whole thing with epoxy and have some overflow holes around the top. Then clamp a SECOND mylar (or whatever) treated lid on top, and apply positive pressure until you see eopxy oozing out those holes.
      If worried about pressure leakage, fill the upper container with lead shot and sand.

  2. I remember seeing something similar with glasses a while back. Fluid was syringed in between two thin sheets of plastic mounted in place of the glass lenses. The focus could then be adjusted.
    The best parabolic lens I read about a while back was a large round bath of mercury that was spun, letting physics do the rest.

  3. The glasses will only be convex and good for farsighted only and of limited range. Then they break the knob off so they can’t be readjusted.
    If you make a partial vacuum in the lid and mylar, you will make a barometric device and also thermometer. In other words it will never be stable. To make regulated, suck at the center so the film blocks the inlet to suction source. The film becomes a giant diaphragm regulator. Adjust depth by moving tube in and out.

    1. Even if you heatgunned the dish until you needed iven mitts and used a sealant round the edge, I think the porosity of the mylar would have it lose shape in a couple of weeks. If not exactly that then outgassing of the lid surface inside from adsorbed air would be a factor too.

  4. Great for solar furnaces, not so much for telescopes. A telescope mirror must be paraboloidal to within a few millionths of an inch if it is to produce decent images at other than low power. If the edge of the trash can lid is not accurately planar the mirror will be astigmatic. Likewise if the mylar has trivial variations in thickness. Expansion/ contraction with temperature changes, is likely to have a rather large effect.

    RIght now I’m polishing a 14″ mirror. Solid quartz, 3/4″ thick…and even at that it’s considered “thin” by telescope-making standards and will require a specialized 18-point support system to get decent images.

  5. You end up with a catenary curve this way, like the sag in power lines, and not quite a parabolic curve. It will still make a 2×4 ignite on a sunny day, but you’re wasting your time with reflecting telescopes. – Been there,…

  6. You end up with a catenary curve this way, like the sag in power lines, and not quite parabolic. It will still get a 2×4 to ignite on a sunny day, but you’re wasting your time with reflecting telescopes. – Been there,…

  7. Look around for someone throwing out an old DirectTV dish. It’s about the same size, includes a wall mount and a place to attach a feed antenna, and you need not klutz with creating a vacuum.

    To get one, you might spread the word that’d you’d be happy to remove one from someone’s rooftop for free. Apparently, satellite TV companies consider that antenna the homeowners responsibility once they have installed it. They put it in to get you on board. But if you quit, they won’t take it away. That’s really tough if you’re elderly.

    “In a nutshell, when our satellite dish is installed on a property, it’s considered a permanent fixture of that property,” DirecTV spokesman Robert Mercer said in an email. “Since it’s ‘permanent,’ it becomes the property owner’s responsibility to have the dish removed.”

    http://www.modbee.com/news/local/article3172203.html

    Yeah, these satellite TV folks are as nasty as their cable counterparts. Yet another reason to drop getting TV service from any of them.

  8. Oh btw if you’re looking for a repurposable valve, som vacuum sealed coffee packages have them. Drill lid big enough to epoxy it in the back. Can probably gerry rig adapter to vacuum pump. (old style bicycle pumps you can flip the plunger)

  9. I wonder if you could rig car MAP sensor and aquarium pump plus controller of choice to maintain constant vacuum. Properly referencin the MAP to air should mean barometric changes compensated.

    1. Now I’ve thought of that, I’m trying to think of the biggest circular, more or less rigid thing you could make one out of, garbage can lids weren’t all that interesting, not being orders of magnitude different from plentiful round here sat dishes. Guess it’s mostly dependent on largest piece of mylar you can get, and when that gets enough sag under it’s own weight to mess things up. Could build up circular support out of plywood and seal it really well. Well enough anyway that pump/MAP control could keep it usable.

      Once you’ve done that though you could figure out a range of focus you could pull it through, and let it bleed off through small hole deliberately…. so you you get large duty cycle to hold it’s tightest focus, small duty cycle to maintain longest focus.

  10. This is roughly the classical method of figuring a Schmidt corrector plate for a catadioptric telescope like the popular Celestron. Because a telescope optical system has to be accurate to within a fraction of a wavelength of light, the actual manufacturing process is much more sophisticated. See Wikipedia ¨Schmidt Corrector Plate.”

    If you’re going to play with a Mylar parabolic reflector to concentrate sunlight, I suggest that you leave the Mylar a little wrinkled. If the parabola is really smooth, you can get a very high temperature at the focus. I remember a Texas Boy Scout troop that burned a hole in a garage wall with their very nice parabola. It was a cinder-block garage.

  11. I recall there was a kid… high school kid I believe… that made a literal fortune by coming up with the idea of making telescope mirrors using a turntable to spin a liquid till it set hard.

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