concave mirror with hole

(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

The diameter is presumably for capture area. Given your offset and short focal length, the way to do this is probably to use an offset-fed paraboloidal section somewhat like a DirecTV dish.

Possibly suitable aluminum paraboloidal reflectors are available from Carley Lamp.

You can machine away the parts you don't need with the tools of your choice -- laser, waterjet, plasma cutter, milling machine, jeweller's saw, hack saw, whatever turns yer crank.

On Mon, 4 Sep 2006 22:31:41 +0100, John Woodgate Gave us:

Yes, but the focal point is at the wrong location. That is the wrong device as those are ellipsoidal reflectors. The detector would have to be omnidirectional. Sorry.

On Tue, 05 Sep 2006 00:26:56 +0200, Sjouke Burry Gave us:

The back side of most such mirrors are not optical surfaces at all. Most are quite irregular, making the passage of a laser beam quite diffuse to say the least. A hole IS required for his method.

Another method is to use a 45 degree mirror right in front of the focal point/detector where the laser is actually shot out from the second 45 degree mirror.

laser------------------------------\\ |_| \\ | | \\ \\ ____\\__________________________________________

^^^^^tube with convex mirror at the rear, detector at 6 inches.

Target is to the right and cannot be shown in a short usenet text.

6 or 8 inches focal length is good and is what is used in IR thermometers. The tube is approx 3 inches.

Laser aim point is still proper through two 45 degree mirrors. Energy is not blocked by detector array enough to make a difference that cannot be compensated for.

On Tue, 05 Sep 2006 02:45:46 +0100, Dirk Bruere at NeoPax Gave us:

Very lame "stuff". If an energy detection device is to be designed here, spay on mirror stuff ain't gonna cut it.

Sheesh.

On Mon, 4 Sep 2006 23:26:47 -0400, "James Thompson" Gave us:

With so much diffusion characteristics as to not even be useable in this instance.

It needs to be a REAL first surface mirror guys.

On 4 Sep 2006 21:27:04 -0700, snipped-for-privacy@fullnet.com Gave us:

You guys have no clue about surface quality. At least not that which would be required for this device.

On 4 Sep 2006 21:42:05 -0700, snipped-for-privacy@fullnet.com Gave us:

You guys are just plain Nucking Futs!

Yeah, basically just playing around. (You wrongly attributed the "spray-on mirror" idea to me, though, by the way.) BUT:

The OP mentioned ALSO wanting a quick-and-dirty solution, just to see if his prototype might somewhat work. So I was thinking more along the lines of a "junkbox parts" setup that he could try, quickly and cheaply, before sinking real resources into it. And, after all, the OP himself mentioned trying to bang something out of a piece of steel by hand, and trying to shape something out of aluminum foil, stating that it wasn't too critical, as there would be "plenty of light".

So maybe you should lighten up a little.

- Tom

If you use a spherical mirror, then it won't mind being tilted (it'll just behave like a 'section' of a larger spherical mirror). This will though show a lot of spherical aberration. It really does depend on just how good the overall optical accuracy needs to be. Unfortunately, your short focal length will rule out cheap telescope mirrors. Look instead at front surfaced acrylic mirrors. These are sold as part of 'educational' packs by a number of suppliers, for incredibly low prices (unit with about your spec, will typically be about £4 for 10 off...). You need to be fractionally 'off axis' with the laser, or if the reflection is exactly on axis, it'll come back to the hole... You can 'drill' a hole in one of these, with a soldering iron tip. A 'proper' glass mirror with a hole like this, will conversely be expensive. I just spent £300, for a 150mm unit. If you set the mirror at 22.5 degrees, and the sensor at 45 degrees, the educational acrylic units, make great light collectors. I played with some of these when prototyping the larger unit, before spending for the final mirror.

Best Wishes

Ah yes that sounds like a very good idea, plastic front coated and soldering iron for a drill :)) I hadnt realised before that an off axis mirror would just be like a section to the side of a normal. seems obvious now though.

I just want to test the electronics for now, I cant tell if the detector is picking up the reflected laser light or if its just picking up crosstalk, its heterodyne too, and the LO bias for the detector has a lot of feedthrough, need to have a target to move to see the change in propogation.

The target isnt meant to mirror like so light coming back through the hole isnt a problem, infact an advantage as a mirror target would otherwise overload the detector.

Colin =^.^=

I thought some of those ideas were toungue in cheek actually ;)

Yes I was just wanting to test the electronics for now, however after the time spent and wasted on my metal bashing (ok about 1 hour) and the surface doesnt look so bad (so ok i cant see my face in it) however it was very disapointing with virtualy no visible point of focus for reflected light unlike with a lense i was using before.

I did also come accros 'heat sagged aluminium' mirrors on google.

I dont realy have the time (or patience) to waste on methods unless they realy look like they will work easily, I might try the heating the cd to bend it idea though, I bet its not as fun as putting it in the microwave though.

but the plastic mirror idea and the driling hole with soldering idea looks good. Also I might change design slightly so I can use a torch relefctor, obviously the detector needs to be a lot closer due to the short focul length so if I put it at 90' off axis it wil be out ot the way, I can just simply cut the reflector in half and use one side with a new hole of course, or maybe a slot for fine tuning. I believe this will give me the right shape for 90' of axis wont it ?

Il have to make the sheilding real good between laser and detector as its RF modulated.

Colin =^.^=

Thanks, good idea but I was trying to cut down on the number of reflections difractions etc, I could have the laser at 90' and just use one 45' mirror. this is similar to how I was doing it before, I was using a lense for the straight through detector and the laser was positioned at 90' and deflected from a tiny mirror in front of the lense (made out of a polished scalpel blade). this worked ok but i was hoping I could do it with just one mirror, the slightest scattering of light from the laser to the detector severly limited my sensitivty before, although theres a lot more I could of done to reduce this with tubes etc, the simpler the paths the better.

laser light bouncing off a mirror tends to scatter and is relativly high intensity compared to a very weak reflected signal. its surprising how easily it finds its way to the detector.

I might revert back to the lense/45'mirror for now.

Colin =^.^=

the

onto the

the

with

20'

focus or

Interesting choice of weapons, maybe if I leave the laser on for long enough it will cut its own hole.

Gives me an idea tho, to use a plastic torch lense wich I can easily cut a section off to the side and melt a new hole.

thanks Colin =^.^=

the

the

20'

or

I dug up an (very) old bicycle front headlamp, luckily had a front silvered plastic reflector, removed it and cut it in half, driled a 5mm hole for the laser and positioned it so the detector is exactly where the filament would be at about 90' to the beam. looks promising, I can even see reflected laser light on the detector.

putting an led torch where the laser beam is I can see a very bright image at the detector, ideal for focusing, close distance the image is not very focused but this will be compensate by much stronger returned signal.

Colin =^.^=

There ya go! For a look at a similar scheme, see

Ha yes very similar !

I cut the thing directly down the middle, If I was todo it again I would cut it offset so I could glue it onto the pcb without any spacing as i forgot the sensor is proud of the pcb, it is the only thing on the back of the board wich is an otherwise undisturbed gnd plane.

I looked for a cheap torch on ebay but seems theyr almost all LED type nowadays ! then remembered that I had a crappy light that never worked properly as the contacts always seemed to become coroded at the slightest hint of moisture or whatever.

I might take a pic of mine and put it on ABSE, it looks a bit crazy with 4 seperate pcbs all over the place and loads of flying coax leads and smc connectors lol.

Unfortunatly I cant get the electronics to detect anything other than crosstalk atm >.<

Actually if I put the detector pcb in a nice diecast box for screening with the detector just poking through a hole the reflector can sit directly on top of that.

Colin =^.^=

On Tue, 05 Sep 2006 12:43:00 GMT, "colin" Gave us:

A scalpel is NOT a good example of a high quality first surface mirror and will most certainly scatter the laser.

Not at all if it is a first surface mirror. Have you never seen an "optical bench"?

A lense wouldn't be needed. The concave mirror and tube places the laser in front of the entire optical array. The only thing making its way into your "light"/ energy gathering tube should be that which is desired.

Again, proper high quality first surface mirrors are required if you want any success. Find an old IR tube type thermometer for the best example.

We sighted ours with a rifle scope and two 45 degree mirrors just fine. The laser isn't even needed in such a case.

The scalpel blade and the use of a rear coated mirror was the source of your diffuse light castings.

On 5 Sep 2006 00:41:20 -0700, snipped-for-privacy@fullnet.com Gave us:

Except that there would NOT be plenty of light.

Hence the mild dyslexia.