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  • Minimum Measurable astigmatism with Shack-Hartmann WFS

    From David Miller@21:1/5 to All on Thu Dec 7 11:33:49 2017
    Hi,

    I just found this forum, and it seemed like the perfect place to ask about this. I have an older Shack-Harmann WFS from WaveFront Sciences. I was using it to check colimation for a holography set-up, and no matter what, the minimum measured astigmatism
    was ~lambda/10. All other aberations (defocus, coma, spherical) were ~lambda/50. THe RMS wavefront deviation is ~lambda/30, mostly due to astigmatism, it would seem.

    After some digging, I found a Blue Sky Collimeter in the back of a cabinet in the lab, and used this for an independent measure. The beam *blinks* on and off - so it would seem the phase is very flat.

    I tested the calibration of the WFS by placing it several meters from a pinhole, and measuring the radius of curvature. It was within the error of my tape measure.

    Is there a minimum phase error a WFS, or its software, can measure correctly? As in, if the error is too small, numerical noise or something else becomes an issue? Could it be that after all these years, the hardware no longer matches the calibration
    file?

    Thanks for any feedback,
    David

    --
    David Miller
    Graduate Research Assistant
    Department of Electrical Engineering
    University of Colorado

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  • From Phil Hobbs@21:1/5 to David Miller on Fri Dec 8 11:40:37 2017
    On 12/07/2017 02:33 PM, David Miller wrote:
    Hi,

    I just found this forum, and it seemed like the perfect place to ask
    about this. I have an older Shack-Harmann WFS from WaveFront
    Sciences. I was using it to check colimation for a holography
    set-up, and no matter what, the minimum measured astigmatism was
    ~lambda/10. All other aberations (defocus, coma, spherical) were
    ~lambda/50. THe RMS wavefront deviation is ~lambda/30, mostly due to astigmatism, it would seem.

    I don't know that exact unit, but in general Shack-Hartmanns are the
    pits except for adaptive optics, where speed is more important than
    accuracy. Fixed shear plates are also crap, even the shiny models with
    two plates and two cameras. I had one some years ago that showed every wavefront as a potato chip, even when I put a piece of transparent tape
    halfway across the beam. Pure crapola.

    The problem with S-Hs is that the actual data are low-quality
    measurements of local wavefront slope, which have to be integrated
    across the field to get the P-V numbers. It's super easy to get that wrong.

    After some digging, I found a Blue Sky Collimeter in the back of a
    cabinet in the lab, and used this for an independent measure. The
    beam *blinks* on and off - so it would seem the phase is very flat.

    I have two, and have relied on them for 25 years. Highly recommended.

    I tested the calibration of the WFS by placing it several meters from
    a pinhole, and measuring the radius of curvature. It was within the
    error of my tape measure.

    Dunno.


    Is there a minimum phase error a WFS, or its software, can measure
    correctly? As in, if the error is too small, numerical noise or
    something else becomes an issue? Could it be that after all these
    years, the hardware no longer matches the calibration file?

    Thanks for any feedback, David

    Cheers

    Phil Hobbs


    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC / Hobbs ElectroOptics
    Optics, Electro-optics, Photonics, Analog Electronics
    Briarcliff Manor NY 10510

    http://electrooptical.net
    https://hobbs-eo.com

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