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Parabolic mirror to flat mirror to target ??

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  • Parabolic mirror to flat mirror to target ??

    Trying to use light from the sun into a parabolic mirror then to flat mirror and back to target inside the parabolic mirror.



    Parabolic mirror has outside diameter of 14" with 5" hole in the center. Focal length about 9" and distance want to return back to target is about 6-1/4". Target is 1/2" OD copper slug.
    Was using .800" diameter first surface flat mirror with brass plate mounting plate for return mirror. Was unable to focus on target.

    Mirror failed from heat, a test without backing plate showed light was going thru mirror. Not sure if poor quality mirror or focused light burnt mirror material.

    Should there be a concave mirror to refocus on the target? How to figure radius and would polished stainless work for a mirror?



    The purpose of the heat is to heat the hot cap of a Stirling engine to about 500 deg. F and have enough energy to maintain the 500 deg. F. Maybe 400 degrees F could work.


  • #2
    Use polished aluminum, not stainless. SS only reflects about 50%, aluminum 90 to 92% without coatings. To get better than 92% requires high temperature anti-reflective coatings, not cheap and not required for your application. Using ordinary polishing techniques you should be able to approach around 85% reflectivity. A plane mirror will work fine. Your mirror is nowhere near optical quality with the focal length it has so it won't be critical how good the plane mirror is. You aren't trying to make a good image.

    BTW, if the aluminum is getting hot enough to melt then make a piece with heat sink fins on the far side or even easier, use a computer CPU cooler as the mirror.
    Last edited by Evan; 05-15-2015, 01:33 AM.
    Free software for calculating bolt circles and similar: Click Here

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    • #3
      The flat mirror is too small in diameter

      Originally posted by Evan View Post
      Use polished aluminum, not stainless. SS only reflects about 50%, aluminum 90 to 92% without coatings. To get better than 92% requires high temperature anti-reflective coatings, not cheap and not required for your application. Using ordinary polishing techniques you should be able to approach around 85% reflectivity. A plane mirror will work fine. Your mirror is nowhere near optical quality with the focal length it has so it won't be critical how good the plane mirror is. You aren't trying to make a good image.
      To work as you describe, the flat mirror needs to be half the diameter of the main mirror and needs to be 1/2 the focal length from the face of the main mirror. That will put the focal point and the peak heat at a point located about the center of the hole of the main mirror.

      If the second mirror is to be smaller in diameter, either the sterling engine hot cylinder needs to protrude through the main mirror, or the currently flat mirror needs to be convex, similar to the way a Schmidt-Cassegraine telescope is set up.

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      • #4
        There is more than enough area there to not require all the energy to hit the sterling collector. Based on the visible size of the mirror it should be intercepting around a couple of hundred watts on a good day. And yes, the secondary should be a lot larger to focus most of the energy on the sterling from a plane mirror.

        It isn't hard to make all sorts of mirrors from aluminum, both plane and convex/concave.

        Here are some convex mirrors I have made from solid aluminum. These were shaped using CNC but it isn't necessary for the job in question. It shouldn't be difficult to find some online mirror calculators.



        Here is a link to a Masters thesis that includes all the required math to design what you want. It's for radio but the only difference is the size. Just change the scale.

        http://ethesis.nitrkl.ac.in/5438/1/211EE1116.pdf
        Free software for calculating bolt circles and similar: Click Here

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        • #5
          ex amateur astronomer... So much to do, so little time!

          A thin glass (pyrex would be ideal) mirror with a silvered front surface may be a better bet, it'll reflect more of the infra-red light incoming to the target, aluminium's a bit less efficient at the longer wavelengths. Both are way better than stainless! You can silver a glass mirror at home, although it requires some nasty chemicals - look up "Tollens test" for the recipe!

          Evan and JP suggest a convex secondary, I agree: I used to look after some big dishes (30ft and up) and the secondaries were all convex and placed a little closer to the primary than the focal point; if a plane mirror's placed at the focus the incoming energy will be largely reflected back to the primary (raytrace the lines, they'll be reflected back at the same angle, the other side of the normal line (90* to the mirror surface) and you'll have a divergent beam mostly illuminating the primary, so a mildly convex mirror is placed just a little in front of the primary focus and reflects a narrow beam of parallel light into the primary's bore.
          If it were an imaging assembly you'd probably need a correcting plate for phase alignment, for your use no need, luckily!

          Other than that, nice dish, and even nicer Stirliing engine!
          Rules are for the obedience of fools, and the guidance of wise men.

          Holbrook Model C Number 13 lathe, Testa 2U universal mill, bikes and tools

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          • #6
            If you really want to make a great build add a carbide light so it will run with out sun. I have a couple of my dads old cp lights & boy do they work well & the kids get a kick out of watching rocks burn after putting water on them.
            "Let me recommend the best medicine in the
            world: a long journey, at a mild season, through a pleasant
            country, in easy stages."
            ~ James Madison

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            • #7
              Keep in mind that if there are say three mirrors and each has a reflectance of say 90% the first shall be 90%, the second 90 x 90(%) = 81% the third will have a reflectance of 81 x 90% = 72.9" of light intensity of the first mirror.

              There was a loss of 100$ - 72.9% - say 27%.

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              • #8
                Gary where did you get that dish?

                very interesting project - knew Evan was going to chime in from his past projects,

                I have a 1 meter dish I was thinking of using in some way like this, out in the yard far away from my house lol

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                • #9
                  Obviously, your limit on the size of the secondary mirror is the hole in the primary REGARDLESS OF OPTIMUMS. Any bigger, and you begin to lose main collection area. if you HAD to, you could optimize the system to focus on a spot at the mirror surface, but there is no need if the cylinder has some length to it.

                  Once that is set, then you can discover what the best location for the stirling engine is. For that you have at least some options, whereas the optics are unforgiving.

                  Also, you do NOT have to be at the focus, you can be in front of it, or behind it, by an amount up to that which that still lets the entire light "image" hit the engine hot end. You just need the light to not be lost past the engine, there is no need for the system to actually focus to a point on the engine.

                  The entire heat output gets there regardless, if all the light hits the engine. Even if a small area was heated hotter, the mass of the engine will average out the colder and hotter spots.
                  1601

                  Keep eye on ball.
                  Hashim Khan

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                  • #10
                    What will the sun tracking system for the solar power station look like? Any estimates on kW produced as the sun move through it's arc?

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                    • #11
                      You can estimate solar power input as about 1 kw per square metre on a good day. A 14" dish will therefore intercept about 80 or 90 watts, less mirror absorption and efficiency... Say around 20 to 30 watts at the shaft output if all is about average and engine is near max efficiency (and capable of that power).
                      1601

                      Keep eye on ball.
                      Hashim Khan

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                      • #12
                        Thanks for all the replies. Understand now why a flat mirror at the focus didn't work and the reflected light verified that. No sunshine today. Will try flat mirror closer and a small radius convex mirror on next sunny day.
                        Evan, your link wouldn't work for me but that's ok working by trail and error should be good enough.
                        Boomer, no idea of the original use of the mirror was. Got it at a club auction years ago.
                        Thanks for all the input, appreciated.

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                        • #13
                          You could try to mold an hard disk platter into a convex shape. It's already polished.
                          Helder Ferreira
                          SetŮ’bal, Portugal

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                          • #14
                            You either need a convex secondary mirror closer to the primary (google Classical Cassegrain telescope), or a concave secondary mirror further from the primary (google Gregorian telescope).

                            allan

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                            • #15
                              Hi Gary
                              A flat mirror won't work, and your pictures shows why. Follow the leftmost ray as it leaves the parabolic reflector and strikes the flat mirror.
                              Once it strikes the flat mirror, it will not continue downward as shown. It will leave to the right at the same angle it entered at and follow the
                              path of the rightmost ray.

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