Does anyone have a technique to make a large spherical mirror? The focal point needs to be in the neighborhood of a foot. If my experiments are successfull, I’ll make it bigger (both longer focal length and larger mirror).
All I really need to figure out is a method of making a section of a sphere. The materials don’t really matter so long as I can coat it with something reflective.
Edmund Scientific used to sell telescope kits where you ground your own mirrors. They and American Scientific usually have surplus stuff that contains mirrors you might use.
If you want the mirror to focus, it needs to be parabolic, not spherical.
A sphere or a section of a sphere can be made with a suitable ball, or balloon, depending on the size you need. Up to 2 or 3 feet, use a ball, for up to about 8 feet (diameter) use a weather balloon. Coat the form with something that will hold its shape for the size - paper mache, polyester or epoxy resin with fiberglass reinforcement, or similar.
For a parabola, a very good estimation can be made by hand. Get a piece of paper large enough to cover the radius of the mirror, and graph a parabola (X = n*y^2, where n is a scaling factor less than 1 that is used to flatten out the parabola enough so it looks like a dish and not a deep bowl.
Once you have the proper or desired curve, cut eight (or more) pieces of stiff cardboard (corrugated, or maybe foam-core) and arrange them around the center, so they form a framework of a parabolic dish. Then fill in the spaces between the ‘ribs’ with thin cardboard. Use glue and/or tape liberally. Then cover the thin cardboard with your reflective material. More ribs makes a better reflector.
If you need an optical quality mirror - almost good enough to be a telescope - then make a form as described above, and then spin it at a very constant speed (There’s a calculation for it, but I don’t have it handy. It was in a NASA Tech Brief from 1983-86 sometime). While it is spinning, pour in epoxy resin. The spinning will cause the resin to level out, but instead of flat, it’ll form a parabola, within a few thousandths of an inch. Keep it spinning until it hardens, then coat it with your material. Or, create a negative form and use that to make as many copies as you like.
Let us know if this isn’t enough. Also tell us how large, strong and accurate it needs to be. What are you using it for?
Don
are you making a fish eye camera???
To make a simple parabolic mirror, use a large pot, reflective mylar, and a vacuum pump.
Stretch the mylar over the open end of the pot, and use the vacuum pump to evacuate the air out of the pot. (You won’t ever cook out of this pot again, as you’ll need to drill a hole in the side/bottom of it and weld a plumbing fitting onto it to pump all the air out.) As you pump out all the air, the mylar will be sucked inward due to the ambient atmospheric pressure and will form a parabolic mirror.
This method will be less “permanent” than pouring epoxy into a spinning mold, but it would be easier to produce a highly reflective surface. With this method, it would also be possible to vary the amount of air you remove from the pot (and hence the focus point of the mirror).
If you guys watch MythBusters, you may remember the solar death ray some time back. Essentially, they were making a mirror that could focus the sun and burn something. The problem that they were all having was that it had a focal point. The target had to sit still while you focus the mirror and it had to stay there.
My solution is to this is to eliminate the focal point. At the focal point of the big mirror, I intend on putting a little mirror that reflects back toward the center of the big one. There’s going to be a hole there that lets the light come out the back. Now I have a death ray with a straight beam. Aiming at the target it is just a matter of using plane mirrors.
So, I don’t need anything fancy. It just has to be easy and cheap. I’ll be happy if it can cook a hot dog.
As for needing to be parabolic, I’m not sure that I understand why. It’s been around 3 years since I’ve taken optics, so maybe I’ve forgotten something.
I don’t think the beam will reflect straight back at the big mirror. I’m not really sure though.
Just because you have a hole in the center of the mirror that is reflecting back from the focal point does not mean you will get a straight beam. The light will simply disperse from that point due to the variety of rays exiting through it from many different points on the mirror. There is a reason why lasers were not invented hundreds of years earlier, without inducing your light through a gas you will never be able to get a beam with an almost infinitely straight path.
In that case my suggestion to you is to review the optics chapter from your high school physics and put a convex lens where you plan on having the hole, that way you can further control where you want your focus to be.
Hope that helps
-Simon Strauss
P.S. how do you plan on getting the necessary amount of light from the sun if you are heating an object that is directly in the path of the mirror’s axis of reflection?
If I remember correctly, having a ray come in parallel reflects it toward the focal point. Having a ray come in through the focal point makes it reflect parallel. The sun is sufficiently far away for us to consider all of the rays to be coming in parallel (or at least enough of it is for the system to do its job).
My bad, I thought you had the hole in the smaller mirror.
In this case good luck because precision is going to be your worst nightmare, you may still want to consider the usage or lenses at the hole so that you can refocus the beam in the same manner as eyeglasses.
-Simon
Phil,
What you are describing is known as a Cassegrain Feed point. It is very common for some of the larger telescopes (Mt. Palomar I think is this type of feed) and for the Celestron line of compact reflector telescopes. The feedpoint (or collector) shape is a little more complex but if you are making a simple planar reflector (one in which the reflector is only a cross section of a full 360 degree parabola you might have an easier time. Many large satellite dishes use this type of feed and one of the design factors is during sun transit. This is when the sun passes behind the geosynchronous device twice each year. We can actually watch the reflection pass across the feedhorn window and it does raise the temperature of the feedhorn during the 15-30 minute transit.
This may be of no use but I recall seeing instructions on the net of how to start a fire with a coke can, chocolate bar, and a stick. You may want to search this but if I recall correctly, you rub the bottom of the can with the chocolate and it slowly pushes into a parabola. Now it supposedly becomes reflective and able to focus sunlight. I just thought this was interesting and thought that though I do not know how to apply it to your current need, perhaps it will give someone ideas.