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10726Re: Final squareness/lens test. Problem found

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  • digidodi
    Aug 8, 2003
    • 0 Attachment
      Hi Adrian,

      I don't need any extension with the TV reducer, only one tube, the
      first one to reach focus with my D100 and the TMB.

      But read my other post for interesting info.

      Dodi


      --- In tmboptical@yahoogroups.com, Adrian Catterall <Catterall@o...>
      wrote:
      > 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.
      >
      > Adrian
      >
      >
      > In message <bh0omu+q8jl@e...>, digidodi <ddierick@h...>
      > writes
      > >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!
      > >
      > >Dodi
      > >
      > >
      > >--- 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
      > >found
      > >>
      > >>
      > >>
      > >>   --- 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
      > http://www.observatory.demon.co.uk/
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