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10542Re: [tmboptical] Re: Final squareness/lens test. Problem found

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  • Richard Chalfan
    Aug 1, 2003
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      --- digidodi <ddierick@...> wrote:

      > Unless as I suspect the error is lateral in the lens
      > or cell. Then
      > you would still have good on axis performance, but
      > as all
      > photographers know, if you move the camera laterally
      > away from the
      > axis for one reason or another, you get uneven
      > aberation because you
      > are putting part of the image zone outside the
      > aberation free image
      > circle. I think this is what's happening.
      >
      Dodi,

      If by lateral error, you mean that the optical axis of
      the lens does not go down the center of the tube (that
      is, the optical axis is offset from the tube's
      mechanical axis, but they are parallel), then yes,
      your CCD images would look like what you posted.

      Your pictures model my earlier suggestion too. The
      aberrations on one side are more pronounced than those
      on the other side if the CCD is perpendicular to, but
      not centered on, the mechanical (and optical) axis of
      the scope.

      But if the lens is perfect and the optical axis of the
      lens is perpendicular to the CCD chip, though
      laterally offset, you should be able to see that in a
      star test.

      The Airy disk rings will only be perfectly concentric
      ON the objective's optical axis. With a laterally
      offset axis, you would see perfectly concentric rings
      off of the center of the field, and non-concentric
      rings when the star was in the middle of the field.

      The reason I earlier suggested an offset chip, is
      because that condition still gives a perfect star test
      in a perfect scope. It is the only condition I can
      think of off hand, where you get a perfect star test,
      but your CCD image is both properly focused and
      abberationally unsymmetrical.

      If the lens is tilted (the objective axis is at an
      angle to the tube's mechanical axis and to the
      eyepiece axis) then you will see an Airy disk with
      off-axis aberrations. And your CCD chip should have a
      band of good focus, going slightly out of focus on
      either side of the band.

      If the lens elements are not mounted properly within
      the cell, then you will not see a "perfect" Airy disk
      even on axis. I think it would look rather
      astigmatic.

      So your symptoms sound slightly conflicting. However,
      if the errors are small the "theoretical" symptoms may
      be hard to detect. I think sending the tube back to
      Marcus is a reasonable start. It is possible though,
      that he will find no problem with the lens or the
      tube.

      Please let us know how this turns out.

      Good luck!!!!

      Richard
      >

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