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Old Aug 27th 2013, 12:53 AM   #1
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problematic spherical mirror focal spot

hello guys,
i monitor with a pixeled camera the focal spot from a spherical mirror but i don't get a nice (even nearly)sperical spot but something weird that shows interference. I woul appreciate a little help. Here are the data (i will make it short, in case anyone can help and need more info i will give it along the way)
1. beam: amplified Ti:sa laser@800nm ~0.7mJ, beam size ~1cm divergent (not too much-on purpose to use for something else)
2. One neutral density filter + one variable attenuation filter,
3. beam splitter@800nm 2% reflection,
4. spherical mirror f/25cm.
5. camera

with this setup i get weird spot images, distorted and showing interference (??). The beamsplitter in theory works cause it is in use in an autocorellation setup, so I wouldn't suspect it. Also the spherical mirror (was just used in an experiment). In the begining I thought it was a tighting optics (beamsplitter) case. But independent of loosening the beamsplitter the spot doesn't improve. Is it possible that the filters cause this interference due to the divergence of the beam? Has any of you dealed with a similar problem in the past? Hope my description is adequate,
thank you in advance
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Old Aug 27th 2013, 09:55 AM   #2
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Distant Lessons Remembered.

If I remember correctly, a spherical mirror does not have a unique clean focal point (assuming a parallel input beam).
A parabolic mirror is required; however the spherical mirror is often used as a reasonable approximation to a parabolic mirror.

I could imagine that the slightly blurred focus of the spherical mirror could lead to interference fringes.
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Old Aug 27th 2013, 10:26 AM   #3
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Thank you very much for your response.
I also thaught this, to switch with a parabolic mirror, and I already had done it. The weird thing is that I still get a bad quality spot! So, it is not due to the focusing optic. Another weird thing, when I moved one of the density filters close to where the laser beam exits from the amplifier (thus, is not too diverged yet), on focus I manage to get a relatively nice and spherical spot. But when I move the camera (while monitoring the spot) out of focus I still see these intereferences and that multiple spots. That is why I start to suspect it is a filter related phenomenon...just can't explain it.
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Old Aug 27th 2013, 05:12 PM   #4
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Without a diagram it's impossible to offer sound advice, but from my experience with telescopes one thing to be aware of is that if there are any objects in the in the light path (such as a camera in the center of the telescope tube, or a secondary mirror) the image of point sources (like stars) will have a corresponding black spot in the middle when out of focus. Hence the best telescope optics try to minimize the secondary "shadow." As for the spherical mirror - these are often used in telescopes but require a correcting lens in the light path to bring the image to proper focus. Otherwise there is no focus point but rather a "focus line."
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