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#### Anthony P

##### Well-known member
Does anyone know a dirty trick for determining the radius of curvature for a concave mirror?

#### RedCowboy

##### 0
LPF Site Supporter
Straight edge and a caliper to measure the center depth ?

#### Anthony P

##### Well-known member
If only it were that simple. I am trying to determine the radius of an argon resonator OC mirror. It is about 3/8" diameter and I suspect the radius is around 1.5 meters. I have been trying the thumbtack method and 1/f=1/u+1/v. This is very simple for large mirrors with small radii, but it is proving to be frustrating for such a small mirror at long distance. guess my eyes are getting old!

#### Singlemode Laser

##### 0
Hit the mirror with a well collimated beam at a slight angle so the reflection is separated from the incident beam. Then you can measure the focal lenght pretty well.

Singlemode

#### Anthony P

##### Well-known member
Thanks, that is exactly what I will do. If I want to get fancy maybe I will target some pinholes. I was trying to figure a way to use 2 diiferent color beams and see at what point the image inverts. Just making it more difficult than it has to be. Your technique should be adequate. Thanks again.

#### steve001

##### 0
Hit the mirror with a well collimated beam at a slight angle so the reflection is separated from the incident beam. Then you can measure the focal lenght pretty well.

Singlemode
You sure that's the right answer?

Sometime later.

I think I see what you mean. Radius of curvature formula is R/2. If you know the focal (F) length then multiply that number times 2 will give the radius of curvature (2×F) if I'm not mistaken.

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#### steve001

##### 0
Thanks, that is exactly what I will do. If I want to get fancy maybe I will target some pinholes. I was trying to figure a way to use 2 diiferent color beams and see at what point the image inverts. Just making it more difficult than it has to be. Your technique should be adequate. Thanks again.
See post 6

#### Anthony P

##### Well-known member
The ultimate goal is to order an OC for my homebuilt argon laser. The mirror in question is currently installed in the device. It seems to perfom perfectly as far as radius and alignment. I would like to try an OC with same radius, but a bit higher transmission %. The mirror was surplus from years ago (2 decades) and of unknown specs. I can closely approximate Transmission with diode lasers and power meter. I know a wide range of curvatures will work, but this one works really well. I have a few more experiments with the device before I remove the mirror for testing its specs. I will let you know how it goes. There are photos posted on this website under "gas lasers, sci am argon laser"

#### LSRFAQ

##### 0
All you need is a mercury or low pressure sodium lamp filtered to emit one line and a optical flat.... Helium is also often used but the yellow line is weak.. I'm at work and I don't have the math with me, but it is a simple jump from the interference fringe count to the radius if you know a few simple things about your flat. You can also use an up-collimated HENE, say up-collimated 3 or 5 to 1 situated at least 10 to 20x the HENE cavity length away from the optic under test, and measure the focal point on a business card with the optic slightly tilted. The then find the radius with simple math. A HENE is preferred over a diode laser as there is nearly zero astigmatism in the HENE compared to the massive astigmatism in a diode laser. A green DPSS would be a second choice. For very long radius you can use sunlight passed through a optic sized hole in a piece of cardboard and get "close enough"
Steve

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#### LSRFAQ

##### 0
Also if you can get it to run, you can then pop your radius and distance data for both cavity mirrors, into PSST!'s cavity calculator and see how well you fill the bore, your probable divergence if your not clipping the bore walls, and if your cavity design is possible or impossible, stable or unstable. You'll also get the location of the beam waist, too. Stability as used here has a different meaning then mechanical stability. The math to do that is on-line on various sites, but PSST has a graphical stability cavity stability chart that tells you volumes about how hard it will be to align.

https://www.st-andrews.ac.uk/~psst/

The old way:

https://perg.phys.ksu.edu/vqm/laserweb/Ch-4/C4s4t2p3.htm

Steve

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#### paul1598419

##### 0
Really good information, Vladimir and Steve. I don't need to use it now, but all knowledge is good in my mind.

#### atomd

##### Active member
What I usually do is use autocollimator and try to compensate mirror with known lens and from relative spacings count ROC