Re: concave mirror with hole

Note to Colin: scroll down to end of message for an actual mirror
supplier (in the US) and part #'s

colin wrote:
"redbelly" <redbelly98@xxxxxxxxx> wrote in message
news:1157642218.712574.80650@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

JoeBloe wrote:
On 6 Sep 2006 14:35:52 -0700, "redbelly" <redbelly98@xxxxxxxxx> Gave
us:

I doubt it will matter. You're not concerned with forming an image,
you're just collecting as much power as you can on a detector.

Which, to be done properly, REQUIRES a good quality mirror. First
and foremost. It IS "forming an image".

Okay, so it should have said "not forming a **high-resolution** image".

An IR tube "forms an image" of the target onto a 2mm spot inside the
detector transducer.

It IS simple optics, no matter how you want to attempt to simplify
it. No image, no energy is "seen". PERIOD.

It's simple, no matter how I attempt to simply it??? That doesn't even
make sense.

Well from my recent experience it does need to form an image,
not necesarily an undistorted one of course,
but certainly a relativly small but clearly visible and relativly bright
image in the center anyway,
wich means it needs to at least be very shiny, and quite close to the right
shape.

This "image" will be just 1 x 1 pixels, since you have a single
detector element. My whole point was that you don't need a really
good, spatially resolved image. You just need to focus as much of the
power as possible onto a 1 mm^2 area. Aberations become less of a
concern than, say, designing a camera or microscope lens where the goal
is to produce excellant, undistorted images.

My original suggestion was to
1. Find a supplier for the mirror you want, without the hole, and then
2. Find somebody who can cut a hole in the mirror, without ruining the
mirror. You're best bet is to find a local glass shop that can do
this.

the metal mirror I tried,
wich I later found a good way to test by puting a simple led torch some
distance away from it,
forms a clearly visible quite bright image but a rather large one, so most
of the light isnt captured by the 1mm^2 detector.
I just did it with a piece of .15 mm sheet steel and a ball pein hammer and
a flat metal surface,

Even though I said the image needn't be as good as, say, in a camera, I
can't imagine this method being suitable. How do you get a curved
mirror using a flat surface? And even if you had a curved surface, a
hammer will not produce the smoothness you need.

fortunatly I do have a little bit of skill necesary to do it that way.
If I made a wooden dolly of the right shape im sure I could get it close
enough.

However I think any DIY method to make one, although possible, would need
quite a bit of time to make it good enough.
I picked up a CD to play with but actually they dont reflect a great deal of
light, they are partly transparent and also much of the light is difracted.

The lamp mirror im using now forms an image wich is quite bright in the
center 1mm part of it, although its certainly not a just a well defined dot,
I think the much shorter focul length is a big advantage.

It works enough to prove the signal is from the demodulated reflected light
rather than crosstalk,
as moving the target through 75mm (at about 200mm away) clearly cuases 180'
phase change in the signal (at modulation = 1ghz).
now I need to try and get it to work at 2ghz.

In other places I have seen telescope guys make their own mirrors using 2
quite large bits of glass rubbed together with grinding paste,
then silvered in a home made alu sputtering chamber !

Yes, it is possible to grind and polish your own mirror, if you really
want to. If it were me, I would just buy it. For example, from
Thorlabs in the US:
D = f = 25 mm, 42 $US or 44 Euro, part # CM254-025-G01
D = f = 50 mm, 59 $US or 62 Euro, part # CM508-050-G01

Other diameters and focal lengths, plus a reflectance curve, are at
http://www.thorlabs.com/NewGroupPage9.cfm?Guide=10&Category_ID=138&ObjectGroup_ID=1161

Again, finding a local glasscutter to make your hole is up to you.

Regards,

Mark

.

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