Hi, Anyone know where I can get hold of a mirror with a hole in it ? Its for my lidar, the idea is for the laser to shine through the hole in the center of the mirror, the returned light from the target is then reflected via the mirror onto the PD, thus ensuring ensuring the axis of detection is the same as the beam of the laser.
Diameter needs to be about 30->50mm, hole diameter about 5mm, focul length about 30->50mm and fairly cheep. I could drill a hole in a metal mirror but dont realy want to try it with glass.
Oh and the reflected light from the mirror will be at an angle of about 20' from the incoming light, does this mean the mirror needs to be a different shape for optimum focus or will it make little difference ?
Many people drill holes into a glass mirror from the backside. You won't get a 30-50mm mirror with a focal lentgh of 30-50mm. this is the domain of lenses.
You won't do much better than a handlamp reflector. These days, they are often quite good paraboloids.
Do you mean 20 minutes of arc? If so, no problem. If you mean 20 degrees, yes, you do have a problem. You need an ellipsoidal reflector for best results, and they aren't at all easy or cheap to get.
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2006 is YMMVI- Your mileage may vary immensely.
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
The reflector from a car's headlight or signal light or fog lamp, the kind where the bulb sticks in through the back, might be a cheap way to go, especially from a junkyard. I think they are chrome-plated plastic, which is a front-surface mirror.
In message , dated Mon, 4 Sep 2006, JoeBloe writes
Probably at extremely long prices. Parabolic mirrors are all over the place in handlamps at low prices.
--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
Thanks for the answers so far, the torch type of reflector has too short focul length, and the telescope type looks too long, so I cant see why my focul length/diameter is too dificult for a mirror, the doctors mirror seems quite good if I could find out what the FL would be.
seems idealy I need a Off-axis parabolic mirror wich are "only a section of the parabola form" I saw some from some of the scientific places mentioned but without the hole, expensive though. Front silvered would be preferable, I probably dont need a perfectly focused point as there is a lot of light coming in, but for now I just want to get the prototype working, so could probably get away with a rear coated on-axis mirror for now.
I thought about a bit of shiny alu foil somehow pulled to make it the right shape, I tried metal bashing a bit of steel, got a rough dish shape but havnt got the time or patience to make it very mirror like, didnt seem to be very effective at all.
There are some cheap glass ones but without the hole, such as ebay, this might be ok depending on its FL
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not sure about trying to drill it though, maybe investing in a diamond drill wouldnt be too bad ...
$85.95 is pretty cheap for a telescope if you ask me.
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Though that's a Mak-Cass, so it'll have a spherical rather than parboloid primary mirror. The focal length will be much longer than the desired 30-50mm too.
Drilling holes in glass is very, very easy. Go to any hardware store and ask for a drillbit for drilling glass. You should also use a bit of lubricant, and a slow drill speed.
Other possible approaches (some better than others), & some vague thoughts:
In one of your subsequent posts, you mentioned just using something shiny, stuck onto something with the correct shape. I think you could make a parabolic shape by putting a tray of (something that starts out almost liquid and then hardens) on a turntable and letting it harden while the turntable is running. The rotational speed of the turntable, and the size and maybe initial viscous properties of the material used, would probably determine the focal length. You could try it with water, first, to get an estimate of the necessary speed for a desired focal length. Or, just calculate how far up the side of the container the edge would need to be and crank up the speed until it gets there. Maybe you could use something like Plaster of Paris, or concrete, or even some acrylic or other resin-type stuff.
When done, maybe you'd be able to stick part of a silver mylar "happy birthday"-type balloon on it. Sounds like it would be difficult to get very good quality, though. Maybe there's some other way to achieve a better reflective surface. (Or maybe you could use heat, somehow, to get the mylar to conform to the surface, better.)
A section of a small satellite dish might work, too, if you're able to apply your own reflective surface.
If the focal length isn't too short, you could also investigate the possibility of *deflecting* a cheap, flat, round mirror. You could drill some holes near the center and use some sort of frame assembly so that tightening a nut would pull the center of the mirror back. I'm not sure what exact shape you'd end up with (It "could" be calculated.). But you might also be able to cheaply get some approximation of your off-axis requirements, that way, too. Alternatively, some other material, besides a glass mirror, could be used the same way, if you were also able to find a good-enough way to apply a reflective surface to it. If your shape requirement is difficult to meet, that way, perhaps you could add some mechanical "stops", or "stop rings", to the frame, behind the mirror or material used, so that when it was deflected to the point where it touched the stops it would be approximately the correct shape.
Something else that just came to mind, along those lines: This is a bit more off-the-wall (unless it works well and is cheap): If you used the silver mylar stuff, or anything that's reflective and very flexible, maybe you could make a (circular?) frame to hold it, with an enclosure behind it, such that continuously sucking a little air out of the enclosure would pull the mylar into a suitable shape. But, oops, I guess there would be things in the way of your laser, that way. But maybe some variation would work (e.g. pressure from the front, instead). Again, I don't know, offhand, what shape you might be able to get. Time to dust off the Calculus of Tensors text, I guess (Or was it Calculus of Variations?)
If I were you, I'd also ask in some of the amateur telescope-makers' (ATM) groups, maybe somewhere in sci.astro.
Good luck.
- Tom Gootee
"He who lives in a glass house should not invite he who is without sin."
Along that lines you propose, find something like a bowl about the right diameter and shape then spray it with 3M contact glue. Then using some aluminum powder from a paint store, dust the tack surface with the powder and pat it down. After it has dried well, buff it to a nice smooth shine. Home made front surface parabolic mirror. JTT
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