Reflectors in flashlights are getting hit with light from behind. Or, you could say, from the back edge of the parabola. The light is travelling in a specific way and the parabola forms it a specific way in order to focus a beam outward. Here, we’re talking about a light source pointing toward the front of the curve, not from the back. That makes things a bit different. If you think about a perfect sphere, and a point source at the center, all the light is focused back to center, as you said before. If you move the point source, the light will still be focused but not to the same place or at the same angle. That is because the angle that the light hits the curve is no longer perpendicular or 90 degrees to the surface of the curve.
I just did a quick online search and found this webpage. The “Lesson Three” talks about concave mirrors and how light reflection works with them. I was unable to find specific mention of a distinction between parabolic vs spherical surfaces. But, the way the text was worded suggests that they are assuming a spherical mirror shape.
I wonder if you could shape a parabolic mold negative(think beehive shape) then use it to cast a Fiberglas and epoxy reflector. Do your best Richard Dreyfus imitation. Whom knows? You might even be able to pick it up. Kind of like the way integrating spheres are done but in a parabola.
Here’s a parabolic mirror on ebay that is in bidding. As of 1:00 PM EST, it has one bid and is at $9USD. Listing says it will ship to Australia. Has free shipping.
Believe me, I spent a lot of time searching on-line for something suitable, with a reasonable price.
I'm choking on forking out for four XHP's, never mind ~$150 for a reflector...
I have some "97%" reflective flexible adhesive mirror squares in the mail, so I'm really just looking for a suitable shape to coat with them, although a 'ready to go' reflector would be nice.
Explain to me why I can't use the satellite dish if it's tilted on an angle, or if the emitters are off-center?
This seems to be the best option I can find in a pre-made solution. Almost perfect size for the body I have, good focal length, only costs half the earth... I know I can get it in a couple of days, being local to me. Not sure what impact the small hole in the center would have, but I could patch that with some reflective material.
Of course, the other solution is to make something.... after a bit more searching, I came across this site, which made understanding the parabola easier for me.
I'm also planning to make a few cardboard parabola cut-outs that I can take to the dollar-store, in the hope that they may have a dish/bowl that just happens to have a parabolic contour.
That Edmund Optics mirror looks so perfect! Have fun trying to find/make something. Hope it goes well for you. I’m looking forward to seeing your finished product!
That 40.5cm mirror I linked earlier is still at one bid $9USD and has 14 hours left to go. It will probably end up well below the $62 price of that Edmund Optics mirror. If nobody else bids on it, somebody’s gonna get it for $9USD. It can ship to Australia, and shipping is free.
Maybe for a long focal length like that, you could use a microwave dish. It seems the main problem would be getting it smooth and shiny. A simple way might be to cover it with aluminum foil.