Friday, 8 March 2013

Making Spherical Mirrors: Part 2


Making Spherical Mirrors; Part2. Using Modern Materials
This post is about how to construct a 9 piece frame for a segment of a spherical mirror. For the sample frame I used 30cm x 30cm square mirrors from IKEA.
 
Spherical mirrors sets are part of a ball. Imagine you have an orange and you cut a small square out of it's skin: This small 'spherical' square is what we are trying to create. When flat plane mirrors are placed into this arrangement, it will form a segment of a sphere: See part 1 for an explanation of the maths and what's being done.


At first, I tried using shaped plywood and a 16 mirror frame: 


But fairly quickly discovered that in practice it wasn't easy to focus and that a 9 mirror frame would be far cheaper, simpler to construct and easy to focus. To make this, I first constructed frames out of 2 x 1 timber and then fixed pieces of plywood in the centre and either side to make it rigid. At the edges of each part of the central cross, I then placed another piece of 1 timber to raise the edge (where the clamps are in picture below): 


The next step  was to add blocks made of two squares of 1 timber to each of the corners to raise them to the second “level” (see part 1 for explanation). Over the top of this, I put compressible material:


With that task done, the mirrors can be placed and fixed with screws:


When done and the focus checked, the mirror frames can be stacked for use:


This is what the arrangement looks like when complete:


Note that each mirror has a slightly different view of the observer.


The detail & notes
Square mirrors are easy to get. However, other shapes can work as well and can work better than squares. If anyone plans to make this using cast metal, the preferred shape is likely to be an equal sided parallelogram known as a rhombus (and occasionally referred to as a lozenge). I'll describe why later if there's interest. 

End Notes
I have hundreds of photos of the arrangement and how it was built. If there's anything you don't understand, let me know below and I'll add in some detail on how it was done.


Part 3 to follow
Part 1 is here.

6 comments:

  1. .
    Rather clever using the Styrofoam for both shock-absorbers and incremental adjustments. I probably wouldn't have thought of that.
    I would have used a modified sleeve-and-gasket screw fixed to the glass with neoprene gussets.
    Yours is better.

    Neil
    _________

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    1. I would have used a modified sleeve-and-gasket screw fixed to the glass with neoprene gussets.

      So would I if I were doing it on a modern industrial scale to be honest. It's not Styrofoam though! It's a similar material which retains it's elasticity (I found that Styrofoam doesn't work too well because if you mistakenly over-tighten whilst focusing the array, the Styrofoam won't spring back)

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  2. Yes, I am familiar with the product but used 'Styrofoam' cuz I don't remember what the stuff is called!
    Essentially used as a packing material, it sometimes has shrink-wrapped plastic around it. When cut into small squares as you have done makes it the perfect shock absorber.
    Again - nice work Jon.

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  3. Essentially used as a packing material, it sometimes has shrink-wrapped plastic around it. When cut into small squares as you have done makes it the perfect shock absorber.

    How could you tell that from the photos! Bloody well done.. yes, that's the stuff I used.
    Thanks Neil

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  4. [i]How could you tell that from the photos! Bloody well done.. yes, that's the stuff I used.[/i]

    ... because I'm That good!
    lol
    I have been associated with the building trades for almost 40 years.
    For example, I can tell you that in your first attempt, as illustrated on this post, you cut the plywood against the grain, so to speak. Always have your long-axis cuts in-grain with the exterior laminations.

    Plus, being something of a photographer, I have always carefully studied what we call 'Documentation Photographs'. If you present an illustration of what you're attempting to convey, someone will always see a thing that you don't.
    (I have the same chairs on my deck - and what else was in that glass of red liquid?)

    Best wishes,
    Neil

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    1. Unless the pieces you have aren't long enough in the long grain. It was a bit of a bodge the first one. Strange red liquid was Ribena.

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