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10725Re: [tmboptical] Re: Final squareness/lens test. Problem found

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  • Adrian Catterall
    Aug 8 2:37 PM
      I have forgotten the details of your tests, but one possibility is that
      the focuser is sagging a little. I have this on the TMB 80 and tested
      with an ST10. The focal plane is quite a way back from the focuser.
      Hopefully Markus can make an OTA extension to get round this problem.


      In message <bh0omu+q8jl@...>, digidodi <ddierick@...>
      >Well, it's not the camera either. I made pictures last night with an
      >MK69 Intes, full frame with the D100, and in all directions the image
      >is tacksharp. I used the same camera and adapter. The only thing
      >which is not the same is the Televue reducer. I have now also a Borg
      >flattener to rule out the Televue.
      >It puzzles me as much as it puzzles you. I will do the test with the
      >new tube as soon as possible!
      >--- In tmboptical@yahoogroups.com, "Markus Ludes"
      ><apm_telescopes@w...> wrote:
      >> the tube arrived here. A laser test showed that the mechanical
      >parts have been straight. A coolimation test of the lens showed both
      >reflections to be 100% concentric too, the startest used up to any
      >power was also perfect.
      >> As dominique wished I have send him back his tube with another
      >which passed the test in same way.
      >> To be honest I have no idea whats going on here, but in my opinion
      >it cannot be the tube and it cannot be the lens, otherwise one of
      >both must show a fault.
      >> will be interested to see the new images from Dodi
      >> Markus
      >>   ----- Original Message -----
      >>   From: Richard Chalfan
      >>   To: tmboptical@yahoogroups.com
      >>   Sent: Friday, August 01, 2003 9:30 PM
      >>   Subject: Re: [tmboptical] Re: Final squareness/lens test. Problem
      >>   --- digidodi <ddierick@h...> 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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      Adrian Catterall
      Amateur Astronomer
      Hertfordshire UK
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