10726Re: Final squareness/lens test. Problem found
- Aug 8, 2003Hi 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.
--- In email@example.com, Adrian Catterall <Catterall@o...>
> I have forgotten the details of your tests, but one possibility isthat
> the focuser is sagging a little. I have this on the TMB 80 andtested
> with an ST10. The focal plane is quite a way back from the focuser.problem.
> Hopefully Markus can make an OTA extension to get round this
> In message <bh0omu+q8jl@e...>, digidodi <ddierick@h...>
> >Well, it's not the camera either. I made pictures last night with
> >MK69 Intes, full frame with the D100, and in all directions theimage
> >is tacksharp. I used the same camera and adapter. The only thingBorg
> >which is not the same is the Televue reducer. I have now also a
> >flattener to rule out the Televue.the
> >It puzzles me as much as it puzzles you. I will do the test with
> >new tube as soon as possible!both
> >--- In firstname.lastname@example.org, "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
> >reflections to be 100% concentric too, the startest used up to anyopinion
> >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
> >it cannot be the tube and it cannot be the lens, otherwise one ofProblem
> >both must show a fault.
> >> will be interested to see the new images from Dodi
> >> Markus
> >> ----- Original Message -----
> >> From: Richard Chalfan
> >> To: email@example.com
> >> Sent: Friday, August 01, 2003 9:30 PM
> >> Subject: Re: [tmboptical] Re: Final squareness/lens test.
> >> --- 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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