10700Re: [tmboptical] Re: Final squareness/lens test. Problem found
- Aug 7, 2003the 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
----- Original Message -----
From: Richard Chalfan
Sent: Friday, August 01, 2003 9:30 PM
Subject: Re: [tmboptical] Re: Final squareness/lens test. Problem found
--- 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.
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
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
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
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
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
Please let us know how this turns out.
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