# Constructing a parabolic dish (for the mathematically inept/lazy) **URL:** \ **Category:** Factual Questions\ **Created:** [April 21, 2008, 3:04pm UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235 "2008-04-21T15:04:43Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 21, 2008, 3:04pm UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/1 "2008-04-21T15:04:43Z")
I want to make a parabolic dish out of plywood or thin MDF - I think it should be easy enough to construct in approximately the right shape by cutting out a big circle, dividing it into segments, then making each of those segment lines as the centre line of a tapered segment such that when gathered together, the edges join to make a big dish. Oh dear, that wasn’t very clear… OK… _I want to cut out a big plywood flower and join the edges of adjacent petals together to make a big plywood dish that is approximately parabolic_ What I think I need to do is to be able to calculate the diameter of the dish at any point in its depth _measured around the profile circumference from the centre_ - (unless there’s a simpler way) - so that at intervals along the length of the petals, I can calculate their proper width at that point ---
**Author:** ![masterofnone](https://avatars.discourse-cdn.com/v4/letter/m/4bbf92/32.png) [@masterofnone](https://boards.straightdope.com/u/masterofnone)\ **Post date:** [April 21, 2008, 3:10pm UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/2 "2008-04-21T15:10:37Z")
I’m not exactly clear what you’re trying to do, but would any of the designs under the parabolic cookers section here help? [http://solarcooking.org/plans/default.htm](http://solarcooking.org/plans/default.htm) ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 21, 2008, 3:24pm UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/3 "2008-04-21T15:24:01Z")
[QUOTE=masterofnone] I’m not exactly clear what you’re trying to do, but would any of the designs under the parabolic cookers section here help? [http://solarcooking.org/plans/default.htm](http://solarcooking.org/plans/default.htm) [/QUOTE] Thanks - yes, well, sort of. this one: [http://solarcooking.org/plans/DATS.htm](http://solarcooking.org/plans/DATS.htm) is sort of similar to the shape I’m imagining, except that it consists of tapered sections gathered at the centre. I want to make a shallower dish from sections joined at the middle. (Shallower means it will focus at a point outside its own volume - like a radio telescope dish) I’m hoping to do a couple of different experiments with it - firstly to try to use it as a sound collector, and after that, to line it with foil or mylar and use it to make things really hot. (Obviously I’m aware that doing these experiments in the other order might result in me cooking my own head). ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [April 21, 2008, 3:25pm UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/4 "2008-04-21T15:25:53Z")
[A Simple Technique of Fabrication of Paraboloidal Concentrators](http://ashokk_3.tripod.com/srinivasan.htm) > [@](#): > > Principle of Fabrication > > Figures 1 and 2 illustrate the principle of construction of a paraboloid starting from a plane sheet of material. Figure 1 is a plot of the parabola Y=X2/4f representing a vertical section through a paraboloid having a focal length of f cm. If the paraboloid is slit symmetrically along eight radial directions and flattened out, then it would appear like an eight petalled flower as in Fig. 2. the non-shaded portion in Fig. 2 represents the reflector part, and the shaded portion that part of the plane sheet which has to be cut out and removed. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 21, 2008, 3:35pm UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/5 "2008-04-21T15:35:33Z")
[QUOTE=Squink] [A Simple Technique of Fabrication of Paraboloidal Concentrators](http://ashokk_3.tripod.com/srinivasan.htm) [/QUOTE] Thanks - that’s exactly the sort of thing I’m trying to do. I was planning to do it with more sections than that example, but the formula given has that as a variable. ---
**Author:** ![The\_Them](https://avatars.discourse-cdn.com/v4/letter/t/898d66/32.png) [@The\_Them](https://boards.straightdope.com/u/The_Them)\ **Post date:** [April 21, 2008, 4:05pm UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/6 "2008-04-21T16:05:18Z")
Took some thought… So, you want this parabola to focus, i.e., have a focal point. Choose one; above the center point (think locating a point 10 cm directly over the center of a dinner plate with a pencil). Doesn’t matter where so much as long as you stick with it. Now, work out some oven-tongs-shaped device with a roller at the apex. Here’s the fun: keeping one end of the tongs pointed at your focus (kitestring should help with that), and the other end straight up (I’m assuming you’re building this on a flat surface), the roller on the tongs will be exactly where your parabolic curve should be. IMPORTANT: you’ll need to keep all your lines straight while doing this, so no slack on that kitestring, or letting your chosen focal point move from however you’ve anchored it. Then, it should be completely no hassle to manipulate your ever-so-easy-to-work-with surface to follow where roller on the tongs is, and parabola away! The reason for the tongs is that it will need to bend a bit to maintain these specified directions for the pointy ends. It will work in 3 dimensions, so once you’ve made it, you can use it for the whole thing. ---
**Author:** ![butler1850](https://avatars.discourse-cdn.com/v4/letter/b/779978/32.png) [@butler1850](https://boards.straightdope.com/u/butler1850)\ **Post date:** [April 21, 2008, 4:53pm UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/7 "2008-04-21T16:53:09Z")
[This guy that runs cockeyed.com](http://cockeyed.com/incredible/solardish/dish01.shtml) may give you some hints. He ended up making a HUGE one out of an old satellite dish, which may or may not be too big for your uses. (He had WAY too much fun… I want one of my own! :D) ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 21, 2008, 6:31pm UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/8 "2008-04-21T18:31:59Z")
[QUOTE=The Them] Took some thought… So, you want this parabola to focus, i.e., have a focal point. Choose one; above the center point (think locating a point 10 cm directly over the center of a dinner plate with a pencil). Doesn’t matter where so much as long as you stick with it. Now, work out some oven-tongs-shaped device with a roller at the apex. Here’s the fun: keeping one end of the tongs pointed at your focus (kitestring should help with that), and the other end straight up (I’m assuming you’re building this on a flat surface), the roller on the tongs will be exactly where your parabolic curve should be. IMPORTANT: you’ll need to keep all your lines straight while doing this, so no slack on that kitestring, or letting your chosen focal point move from however you’ve anchored it. Then, it should be completely no hassle to manipulate your ever-so-easy-to-work-with surface to follow where roller on the tongs is, and parabola away! The reason for the tongs is that it will need to bend a bit to maintain these specified directions for the pointy ends. It will work in 3 dimensions, so once you’ve made it, you can use it for the whole thing. [/QUOTE] That would work well if I was trying to create a form, but what I’m attempting to do is to make a bunch of panels that pull themselves into a parabola shape, when joined along their edges, by virtue of their geometry - sort of like creating a hot air balloon, only not out of fabric. Seems to me that if one of these little 18 inch jobs can cook things, a 4 foot diameter one should be capable of more impressive feats. ---
**Author:** ![Oslo\_Ostragoth](https://avatars.discourse-cdn.com/v4/letter/o/a9a28c/32.png) [@Oslo\_Ostragoth](https://boards.straightdope.com/u/Oslo_Ostragoth)\ **Post date:** [April 21, 2008, 11:41pm UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/9 "2008-04-21T23:41:02Z")
[QUOTE=butler1850] [This guy that runs cockeyed.com](http://cockeyed.com/incredible/solardish/dish01.shtml) may give you some hints. He ended up making a HUGE one out of an old satellite dish, which may or may not be too big for your uses. (He had WAY too much fun… I want one of my own! :D) [/QUOTE] I would **love** to have this guy in my neighborhood. Not so sure about right next door, though. ---
**Author:** ![butler1850](https://avatars.discourse-cdn.com/v4/letter/b/779978/32.png) [@butler1850](https://boards.straightdope.com/u/butler1850)\ **Post date:** [April 22, 2008, 2:03pm UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/10 "2008-04-22T14:03:20Z")
[QUOTE=Oslo Ostragoth] I would **love** to have this guy in my neighborhood. Not so sure about right next door, though. [/QUOTE] He really has a fun site. I love his “how much is in…” series… and I’d like to play with his solar collector. And beat the guy that destroyed an Action Comics #1. ---
**Author:** ![Sage\_Rat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sage_rat/32/399_2.png) [@Sage\_Rat](https://boards.straightdope.com/u/Sage_Rat)\ **Post date:** [April 23, 2008, 5:46am UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/11 "2008-04-23T05:46:45Z")
[QUOTE=Mangetout] That would work well if I was trying to create a form, but what I’m attempting to do is to make a bunch of panels that pull themselves into a parabola shape, when joined along their edges, by virtue of their geometry - sort of like creating a hot air balloon, only not out of fabric. Seems to me that if one of these little 18 inch jobs can cook things, a 4 foot diameter one should be capable of more impressive feats. [/QUOTE] Going through the cockeye stuff, it looks like it might be an idea to build a rotisserie in previous to laying down the reflective surface. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 23, 2008, 7:25am UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/12 "2008-04-23T07:25:53Z")
I definitely need to try cooking something, if I get around to completing this project. I seem to recall seeing big sheets of dense foam with a silvered side at a materials bank I sometimes visit - I’m thinking that might be a better material for my experiment (I had been planning to use MDF or thin ply and attach silvered mylar, but gluing that stuff to _anything_ isn’t all that easy) I love the [cockeyed.com](http://cockeyed.com) site - I’m sure I’ve stumbled across isolated pages from it in the past, but this is the first time I’ve deliberately browsed it - it’s exactly my cup of tea - and I love the style of writing. ---
**Author:** ![Sage\_Rat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sage_rat/32/399_2.png) [@Sage\_Rat](https://boards.straightdope.com/u/Sage_Rat)\ **Post date:** [April 23, 2008, 8:23am UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/13 "2008-04-23T08:23:17Z")
[QUOTE=Mangetout] I seem to recall seeing big sheets of dense foam with a silvered side at a materials bank I sometimes visit - I’m thinking that might be a better material for my experiment (I had been planning to use MDF or thin ply and attach silvered mylar, but gluing that stuff to _anything_ isn’t all that easy) [/quote] Professional photographers often have reflective flexible things (so that they can be compacted down for easy carry.) I’m not personally a photographer, so I don’t know nor recall specifically what all offerings there are, but you might try googling along those lines for a good material. > [@](#): > > I love the [cockeyed.com](http://cockeyed.com) site - I’m sure I’ve stumbled across isolated pages from it in the past, but this is the first time I’ve deliberately browsed it - it’s exactly my cup of tea - and I love the style of writing. If you’re taking any suggestions, I’ll just note that I’d like to see some different marble computers like the [adding](http://www.youtube.com/watch?v=GcDshWmhF4A) [machine](http://www.youtube.com/watch?v=md0TlSjIags). ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 23, 2008, 8:31am UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/14 "2008-04-23T08:31:03Z")
[QUOTE=Sage Rat] If you’re taking any suggestions, I’ll just note that I’d like to see some different marble computers like the [adding](http://www.youtube.com/watch?v=GcDshWmhF4A) [machine](http://www.youtube.com/watch?v=md0TlSjIags). [/QUOTE] Thanks - yeah, I’ve had my eye on that for a while - and on the other stuff that guy does - with wooden gears. ---
**Author:** ![si\_blakely](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@si\_blakely](https://boards.straightdope.com/u/si_blakely)\ **Post date:** [April 23, 2008, 11:41am UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/15 "2008-04-23T11:41:18Z")
Going back a long time ago, I tried to build a parabolic reflector for a school science project (we were building a [photophone](http://en.wikipedia.org/wiki/Photophone)). We started with an accurate parabola in card, and had the woodcraft teacher turn it using the card as a guide. We probably got a good parabola, but could not make it reflective enough for a decent focus. Rather than trying to grind a glass mirror, we shifted to using the fresnel from an overhead projector - worked a treat up to about 100m, but not big enough for what you want to try. Good luck Si ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 23, 2008, 6:10pm UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/16 "2008-04-23T18:10:42Z")
[QUOTE=Squink] [A Simple Technique of Fabrication of Paraboloidal Concentrators](http://ashokk_3.tripod.com/srinivasan.htm) [/QUOTE] Just trying to actually implement my master plan and the old mathematical ineptitude kicked in again. I can’t make sense of the maths in that document and the author skips right through it like a breeze. But I need the ‘for dummies’ version. (It isn’t helped by the diagram being tiny and fuzzy). um… Help! ---
**Author:** ![si\_blakely](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@si\_blakely](https://boards.straightdope.com/u/si_blakely)\ **Post date:** [April 23, 2008, 7:41pm UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/17 "2008-04-23T19:41:19Z")
[QUOTE=Mangetout] Just trying to actually implement my master plan and the old mathematical ineptitude kicked in again. I can’t make sense of the maths in that document and the author skips right through it like a breeze. But I need the ‘for dummies’ version. (It isn’t helped by the diagram being tiny and fuzzy). um… Help! [/QUOTE] OK. Here goes. Not easy without diagrams, but I’ll try… In the equations, f is the focal length of the final parabola, and is a constant. The author works on the principle that a parabola of radius X becomes the petal shaped flattened figure with a radius R (where R is greater than X). You will only need the shallow parabola equation, and he gives the relationship between R and X. To be honest, you don’t really need this. What you do need to know is how much material to remove between the petals. There is a difference between the circumference of circle R and circle X (X is smaller).So you draw your final circle of radius R. Divide it into 16 equal sectors. Then draw concentric circles at regular intervals. Where each circle crosses the sector line, calculate delta W, the difference between circ(R) and circ(X) divided by the number of petals - this formula is given in the text (the last one, and only one you need to know). In practical terms, for each subcircle of radius R, calculate delta W. Set a compass to delta W, put the point on the intersection of the sector line and the circle, and mark the circle on both sides. The area between these two marks can be removed. As R gets smaller, delta W gets smaller quick, until the practical limit is reached about 2/3rds in. I hope this is clear. Set up a spreadsheet, set your focal length and number of petals, then calculate deltaW in increasing steps up to your final size. Si ---
**Author:** ![si\_blakely](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@si\_blakely](https://boards.straightdope.com/u/si_blakely)\ **Post date:** [April 23, 2008, 7:50pm UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/18 "2008-04-23T19:50:56Z")
Bollocks - just noticed that deltaW is calculated in terms of X, the radius of the parabolic object, not R, the plane object. Thats daft - I’ll rework it into something easier. Si ---
**Author:** ![si\_blakely](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@si\_blakely](https://boards.straightdope.com/u/si_blakely)\ **Post date:** [April 23, 2008, 8:38pm UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/19 "2008-04-23T20:38:20Z")
Just an example , for a focal length of 10 units, 8 petals you get, and R is a circle on the plane R deltaW 0\_\_\_\_0 1\_\_\_\_ 0 2\_\_\_\_ 0 3\_\_\_\_ 0 4\_\_\_\_ 0.01 5\_\_\_\_ 0.02 6\_\_\_\_ 0.03 7\_\_\_\_ 0.05 8\_\_\_\_ 0.08 9\_\_\_\_ 0.11 10\_\_\_ 0.15 11\_\_\_ 0.19 12\_\_\_ 0.24 13\_\_\_ 0.3 14\_\_\_ 0.36 15\_\_\_ 0.44 This was just cobbled up in openoffice, and I massaged the X values to give integer R values. My MathsFu is not strong tonight - if R = X + (X[sup]3[/sup]/24f[sup]2[/sup]), what is X in terms of R :- too hard Si ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 23, 2008, 8:38pm UTC](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235/20 "2008-04-23T20:38:38Z")
Thanks - I really came unstuck. I mean, it may be the case that crude methods of construction, disobedient materials etc will make it all moot anyway, but I ought to start from something that should at least _theoretically_ work (if I’ve learned anything at all from the boat project). [Next page](https://boards.straightdope.com/t/constructing-a-parabolic-dish-for-the-mathematically-inept-lazy/446235.md?page=2)