Pressure-formed Parabolic Mirror from a Mylar Blanket

Parabolic reflectors are pretty handy devices. Whether you’re building a microwave antenna or a long-distance directional microphone, suitable commercial dishes aren’t that hard to come by. But a big, shiny mirror for your solar death-ray needs is another matter, which is where this pressure-formed space blanket mirror might come in handy.

Pressure-forming was a great choice for [NighthawkInLight]’s mirror. We’ve covered pressure-formed plastic domes before, and this process is similar. A sheet of PVC with a recessed air fitting forms the platen. The metallized Mylar space blanket, stretched across a wooden frame to pull out the wrinkles and folds, is applied to a circle of epoxy on the platen. After curing, a few puffs with a bicycle tire pump forms the curve and stretches the film even smoother. [NighthawkInLight]’s first attempt at supporting the film with spray foam insulation was a bust, but the later attempt with fiberglass mesh worked great. A little edge support for the resulting shiny taco shell and the mirror was capable of the required degree of destructive potential.

We doubt this process can be optimized enough to produce astronomy-grade mirrors for visible light, but it still has a lot of potential applications. Maybe a fiberglass radio astronomy dish could be pressure-formed directly with a rig like this?

71 thoughts on “Pressure-formed Parabolic Mirror from a Mylar Blanket

  1. Interesting methods for assembly. All nice hacks – our motto, and a great result, carefully thought out and not to difficult for anyone to accomplish. I’m sure you could find a more reflective surface material than the space blanket, but inexpensive works, although not to the level of a dielectric mirror, and certainly not wavelength specific – this is quite useful I would guess.

      1. ‘When I use a word,’ Humpty Dumpty said, in rather a scornful tone, ‘it means just what I choose it to mean — neither more nor less.’

        ‘The question is,’ said Alice, ‘whether you can make words mean so many different things.’

        ‘The question is,’ said Humpty Dumpty, ‘which is to be master — that’s all.’

      1. I don’t think so.

        Catenarys form under the influence of gravity acting along a evenly weighted chain which is usually a parallel downwards effect (other than for very long chains).

        This is divergent through out the length and not even locally parallel.

    1. (1-r**2)**2 is just the deflection perpendicular to the plane. You also have to use the second formula. r was the distance of a point on the membrane from the center before the membrane was stretched.

      Using the variable names from the NASA formulas, the points on half a slice of the membrane have the coordinates (r+u, w) for 0 <= r <= a.

  2. Very cool hack. I wonder if would have worked better to cut a ring from PVC (or MDF?) and glue that down on top of the blanket and clamping the ring to the base (with a dose of caulk to make the clamp joint airtight). Then it could possibly have been a one-step process. The ring becomes the support.

    1. I thought so too…. As a solar collector mirror, it would be plenty good. Lossy and with poor focus, but you would still get most of the energy focused where you’d want it.

      Many astronomy mirrors are spherical, but they have very long focal lengths. The focus generally tends to be several mirror diameters in front of the mirror. In this configuration, they approximate parabolic mirrors.

      1. The long focal length of an astronomy mirror might actually be easier to achieve than the short one he went for. If the backing could be made to not distort as it cured it might even be good enough to do schliren photograhy (a hoolahoop sized mirror would be awesome for this)!

  3. I imagine you would get much better results by mounting it to an airtight frame and using a vacuum pump. If you look at the surface when it is under pressure, you can see that it’s much smoother than after he glasses it. Sure it would be harder and the shape would change with the temperature, but I’m sure you could come up with a way to compensate.

    1. I’m with you on this, no need to fix anything just use a vac pump to alter focus while using it. Using a bicycle wheel rim would also give a good, straight, flat outer frame and innertube and tyre would hold it in place really well with no overtensioned spots.

    2. Dan Rohas on youtube did the negative pressure thing. IIRC it worked well enough and you could go 2 by 2 meters. Also i believe with pressure variations it should be possible to do a variable vocal length.

      Instead of glueing the the Mylar to the frame he used Tape which allowed to carefully stretch the mylar into a very flat surface which would be better for Astronomy stuff. You would need a ring that blocks the light comming from the edge though since the image there would be distortet.

      I believe it was this video: https://www.youtube.com/watch?v=FyCLOXF1188

    1. Not saying the resulting dust is good for you, nor a warning not made, but people have been using abrasive power tools on fiberglass for decades. Even with a cigarette hanging out of their mouth.

  4. I would think holding down the mylar with the ring a little smaller than the glue circle and glassing to the ring itself would yield a cleaner edge. Thus a better parabola.

    Glue is the air seal, but the ring sets your parabola quality.

  5. Hmm I’m wondering if bedliner spray would work too instead of the messy/heavy fiberglass. Or were the imperfections a result of the leaking air/heating with hairdryer?

  6. I’m reminded that a decent parabola can be made using a vertical axis turntable with a sufficiently turbulence-free fluid. Start with a rough parabola and a thin, slow-cure resin and the right RPM and it will lay out smooth and parabolic. Then the mylar can be vacuum pulled to match the resin for a reflective surface. The cool thing is the focal length is dependent only on RPM, not diameter, for a given local acceleration.

    1. How do you minimise the amount of resin used to avoid all the nasty heat problems with it when it cures? Could you have a thin plastic membrane over water and then just cast a resin shell over that, or can you cast directly onto the water because the resin floats?

      1. Rough form for the tray works. sometime in the next month I need to do another, larger than my previous, and am working on a ribbed wood form. It will be either plaster (what I used last time) or epoxy. Plaster is less $$$, but the support must be rigid and can’t be removed.

          1. That I don’t know. I can think of a few ways (the most DIY would be Tollens reagent to silver plate) but few are doable without some nasty chemistry or a vacuum chamber sized for the paraboloid.

          2. Spaz Stix Ultimate Mirror Chrome paint. Comes in tiny spray cans or they have bottles for airbrush application. If applied to a mirror smooth surface it will produce a reflective mirror finish – on both sides of the coating. Thus it works for making back or front surface mirrors.

            Rustoleum has a Mirror Effect paint but it’s mainly for use on glass. It does not make a reflective coating on the paint side so can’t be used for front surface mirrors. It also attacks styrene so unlike the Spaz Stix paint you can’t make a mirror out of a CD case lid.

            If you have a lot of surface area to spray, there’s ALSA Killer Chrome. It comes in spray cans with a knob on the bottom. Twisting the knob releases a catalyst into the paint. Shake it up and spray, and spray until you’ve used it all before it hardens in the can. Unusable for small jobs, unless you have a crapload of little things to chrome paint. People have sprayed whole cars with this, dunno how many cans that would take.

      2. Slow curing resins in thin layers don’t build much heat. The closer the original form the less resin required.

        There was a full size mirror in glass that was spun while cooling to get near finished shape to cut down on the amount of grinding and the amount of glass poured and the time required to strain relieve it.

        https://en.wikipedia.org/wiki/Spin_casting_(mirrors)

        There is also this: https://en.wikipedia.org/wiki/Liquid_mirror_telescope

      3. You use a slower curing resin which heats up a lot less. I’d like to know why that’s called exotherm when the heat comes from *inside* the hunk of curing resin.

        Mammals have endoskeletons *inside* them. Insects and arachnids have exoskeletons on their *outside*. Why does stuff that generates heat *inside* it like resins and concrete get to use the incorrect Latin prefix?

        1. Because when reacting the chemicals release energy to the outside world.
          The opposite thing is an endothermic reaction, which only happens when you add energy from the outside to the reaction, e.g. by having a heat source to keep the reaction going.

    2. I remember that, too, but for the life of me I can’t figure out how to take advantage of it without freezing my ass off.

      Another way to do it might be in sand or cement. Cut a profile out and revolve it around the center.

  7. Vacuum port in center is cut off by sheet with a little rubber seal in center.
    Self regulation of shape. An old idea.
    Think large and shallow, sphere and parabola are close enough, with long focal length.

  8. Instead of using a fixative to convert this air pressure induced parabolic curve into a permanent solid mirror why not just take this same idea but simply maintain air pressure and use the mirror as is?

    Instead of using a flat surface use a ring and stretch transparent Mylar across one side and reflective Mylar across the other side, then pump in air between them from a great many tiny multiple ports in the ring. Why multiple ports? To keep the ring equally stable around its circumference. Now the mirror can be used right through the transparent Mylar because the transparent Mylar is not in the reflective Mylars focal plane

    The pressure that the transparent and reflective Mylar disks exert as they stretch due to the inflation of air between them imposes an equal and opposite force on the ring preventing it from warping.

    the degree of air pressure determines the focal point of the mirror which can therefor be adjusted at will by adjusting air pressure. In space such an apparatus could be huge with a ring of fishing rod like materiel and very little air pressure

    A drum head like circular stretching apparatus can be used to evenly stretch the Mylar before gluing it to the ring to possibly get a telescope grade mirror if the Mylar is consistent enough and the stretching even enough.

    has this been thought of? who has attempted this?

  9. Great video.
    I was actually going to try aluminum foil and spray foam, but the proof that it won’t work is very clearly shown. I want to try out the ALSA Killer Chrome that [Galane] suggested.

    There are quite a few choices of mylar blanket at the fishing and camping store…

  10. Schmidt corrector plate are made by applying low vaccum to a flat optical plate then polish it.
    I remember years ago that someone tried parabolic astronomic grade mirror by using back pulling with a ring. Don’t know the outcome of this.

      1. Can’t remember the episode(s), but I recall the Mylar mirror has appeared twice, once when an university guest team visited the show and Grant tried to achieve something using the same method.

  11. Very nice. But: “not too bad for being made from a 1-dollar space blanket”?
    Coming up next: A nice car, being made from a 1-dollar battery (that is in the remote keys)

    1. Is that the same as the stuff they call mirror window tint film? You can get it in rolls 1 meter across but it must be made in wider rolls for these guys to have made this,

  12. Hah, I’ve seen videos of people manufacturing home satellite from cut discs of reclaimed aluminum or galvanized steel on a simple rotating spindle against a pre-cut form using simple hand-held tools. The result is quite good supposedly. The resulting “dishes” are mainly used in Islamic regimes that (try to) outlaw satellite TV. These people churn these things out in stacks.

  13. Theres is a neat little book called Unusual Telescopes by Peter L. Manly which describes, among other weird things, practical astronomical telescopes made by pressure-stretching mylar and thin plate glass (!). While the mylar one was surprisingly good it required a very elaborate and expensive to fabricate mount to keep the mylar evenly supported at the edges. Manly also covers a practical rotating mercury-bath telescope which is used for precise zenith observations.

  14. How about taking a Dish Network or DirecTV dish and using it for the form? They are better designs anyway with the off-axis focal point so whether you’re concentrating solar or cooking or whatever, your focal point isn’t obstructing the beam. Note also, a perfect smooth surface is not exactly what you want – a bead-blasted finish with slight dimples can give you a nice Gaussian beam – SureFire I believe invented this – only to be rapidly copied by the Chinese then obsoleted by LED lamps. For probably over a century, flashlights used perfect reflectors that cast nicely focused shadows of their filament holders. SureFire roughened the surface and created a perfect beam with no shadows.

    I’m just peeved I had to dispose of my 73″ and 62″ Fresnel lenses from my junk DLP TV’s when I moved…

    1. The ‘satellite dish’ method is a pretty common hack actually. And it looks like it works really well. :)
      http://hackaday.com/tag/solar-cooker/

      “I’m just peeved I had to dispose of my 73″ and 62″ Fresnel lenses from my junk DLP TVs”
      Evey time I go to the dump I look for one, but haven’t had any luck.

      I eventually plan on making a large solar cooker/steam generation unit, hopefully with working sun tracking.

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