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Re: Collimation diffraction ring question

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  • dmwmpd
    Hi Mike, Your photos show a classic Out-Of-Colimation condition. The overall shape of the diffraction rings should be round, unless there is astigmatism
    Message 1 of 16 , Mar 4, 2009
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      Hi Mike,

      Your photos show a classic Out-Of-Colimation condition. The overall shape of the diffraction rings should be round, unless there is astigmatism (which you probably do not have). The off center star image is the key that it is not colimated. Look at Covington's book "Astrophotography for the Amateur" page 172 for diagrams. Only perfect colimation will show perfect concentricity of the star image and difraction pattern.

      Hope this helps,
      Don

      --- In ccd-newastro@yahoogroups.com, "Mike Dodd" <mike@...> wrote:
      >
      > Would someone please take a look at this photo
      > <http://astronomy.mdodd.com/files/offset_diffraction_rings.jpg> and
      > offer an opinion about it?
      >
      > I'm trying to check the collimation of my TMB-130SS APO 910mm FL f/7).
      > Seeing tonight was terrible, so I didn't attempt any adjustments --
      > just took some diffraction-ring photos.
      >
      > The photo illustrates what I saw visually in a 3.5mm eyepiece.
      > Starting with an in-focus star (Elnath, nearly straight up in the sky)
      > I slowly moved the Feather Touch electric focuser outward to defocus
      > the star. As the star went out of focus, the diffraction rings began
      > to form, but they were not concentric with the star -- they formed off
      > to one side, as seen in the photo.
      >
      > The bright spot at the 4:00 position is the star, and the bright halo
      > surrounds the outermost diffraction rings.
      >
      > I'm very puzzled by this behavior, and wonder if it represents a
      > defective telescope or if something in my technique caused the offset
      > pattern. The star was very well centered in the eyepiece (there was
      > enough moon glow to easily see the eyepiece FOV). The camera was
      > securely mounted on a bracket attached to the eyepiece, so it didn't move.
      >
      > I always thought diffraction rings form concentrically around the
      > star. Seeing was too poor to even see the inner diffraction rings, so
      > I don't know what they look like. What do the experts say about the
      > offset rings? Thanks in advance.
      >
      > Mike
      > -----
      >
      > Mike Dodd
      > Montpelier, VA USA
      > http://astronomy.mdodd.com
      >
    • Mike Dodd
      ... I hope so; I just ordered a Glatter laser collimator, since the prospects for good seeing any seem poor any time soon. I need to resolve this issue. --
      Message 2 of 16 , Mar 4, 2009
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        dmwmpd wrote:
        > Hi Mike,
        >
        > Your photos show a classic Out-Of-Colimation condition. The overall
        > shape of the diffraction rings should be round, unless there is
        > astigmatism (which you probably do not have). The off center star
        > image is the key that it is not colimated.
        I hope so; I just ordered a Glatter laser collimator, since the
        prospects for good seeing any seem poor any time soon. I need to resolve
        this issue.

        --
        Mike

        Mike Dodd
        Montpelier, VA USA
        http://astronomy.mdodd.com
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