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webcam foucault imaging

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Hi all

I am trying to get good clear images of the Foucault test images of my mirrors.

The image of the source at the image plane is about 3 to 4mm across. I purchased a 12mm lens for my pc900 webcam to operate at infinity focus but it can't reach it - the lens unscrews.

So I tried to get an image without a lens and it was really clear - but also really large .


This is taken about 3mm outside of focus using a ronchi grating on my 12" F/2.7 cassegrain paraboloid primary, hence the hole in the middle.

What do others do ?. I don't have enough space for access with a DSLR lens.



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Hi Mike

Not really sure on this one as I tried to get Foucault Images of my mirror whilst testing and found it difficult. I did find it could be done if the camera was set quite far back from the Knife edge (I was using a cheap Samsung L100 camera)

Have you had a look at this website it may provide some answers?




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Thanks for this. I have been trying withe Webcam and 12mm lens I bought for the purpose. The lens wouldn't come to infinity focus though. So I removed the lens and got a very good image without any lens at all.I need to make it smaller though which means getting closer to the focal plane. To achieve this I pulled the casing apart and now have just the boss camera which I'll glue to a tripod mount on a piece of angle.



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When it's complete and working Mike please post pics as it will be very helpful for others.



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I've not tried to do this but have a feeling that the lens needs to be focused onto the mirror not infinity. The reason for changing the lens on a web cam is to get a bigger image as the standard ones are fairly wide angle.

:smiley: The feeling comes from eyeball testing and also messing about with taking photo's from what comes out of an eyepiece where the camera does have to be focused on infinity. I'd be interested to know if I am correct as the idea of a nice big image on a PC screen appeals.

So basically place the camera with the lens a little outside the centre of curvature, focus the camera onto the mirror. Having the knife edge close to the camera is mentioned here for instance. I'm sure I have seen the same thing mentioned elsewhere.


That test might be worth a go but I would cross check with a mask the usual way. A mask on the mirror would help to see if the web cam can be focused.



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Did this ever get done ? The OP was supposed to post a solution, as is very helpful to others with the same problem.

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I'm the op. I eventually used a compact camera on a separate tripod , the zoom on the camera let me get a reasonable image size but still not frame filling. 

I think the  lensless approach worked since you are imaging the ronchi or slit using the primary and that implies you can only image the slit if the slit is inside focus, placing the image of the slit and the camera outside nominal focus. Or I'm making it up since we are talking CoC here rather than focal length. 

I haven't been back to this since making a small bracket with a tripod thread in it and using the web cam glued to this to image the fringes. 

Afterwards I moved to dual pass auto collimation process which is a different kettle of fish. I do know that others have imaged the fringe pattern using the zoom feature of compact cameras. Also  the use of the word infinity above is wrong , I meant imaging at CoC. Which is why the Webcam telephoto lens wouldn't come to focus in the mount I had and needs an extension.



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