Re: concave mirror with hole


tomg@xxxxxxxxxxx wrote:
colin wrote:
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 ?

Colin =^.^=

Colin,

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."

(As long as it's "just to get the prototype working":)

Afterthought: For the "deflected mirror" approach, maybe try a CD-ROM
disk. (Maybe, also, see what happens if you heat it, gently, while
deflecting it, or even after it's fully deflected.)

Might appear to have a near-zero chance of working great. But also has
near-zero cost and near-infinite availability, and should be
easy/quick-enough to try.

You could also use heat (and, say, gravity) to "mold" a CD to the
mirror-shape you need, if shape is produced otherwise. And hey! It's
already got a hole! (Albeit a little larger than your spec. But that's
easily rectalfied.)

- Tom

.

Relevant Pages

  • Re: concave mirror with hole
    ... Anyone know where I can get hold of a mirror with a hole in it? ... Drilling holes in glass is very, ... apply your own reflective surface. ...
    (sci.electronics.design)
  • Re: concave mirror with hole
    ... Anyone know where I can get hold of a mirror with a hole in it? ... Drilling holes in glass is very, ... apply your own reflective surface. ...
    (sci.electronics.design)
  • Re: concave mirror with hole
    ... Anyone know where I can get hold of a mirror with a hole in it? ... Drilling holes in glass is very, ... maybe you'd be able to stick part of a silver mylar "happy ...
    (sci.electronics.design)
  • Re: concave mirror with hole
    ... Anyone know where I can get hold of a mirror with a hole in it? ... Drilling holes in glass is very, ... maybe you'd be able to stick part of a silver mylar "happy ...
    (sci.electronics.design)
  • Re: concave mirror with hole
    ... Anyone know where I can get hold of a mirror with a hole in it? ... Drilling holes in glass is very, ... apply your own reflective surface. ...
    (sci.electronics.design)
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