Re: [APML] needed- an excellent sense of direction

[Torsten Clay <tclay@theriver.com>]

Dave- is this correct:

If the camera film plane (oag, T-ring, camera body, etc) is square to the
drawtube, and a star test image in a high-power eyepiece (assuming the
eyepiece isn't skewed either) is perfectly round, then the film plane
should automatically be perpendicular to the optical axis.

Torsten

 On Wed, 9 Jan 2002 Aplanatic@aol.com wrote:
> Hi Bobby:
>
> Precise alignment of the focuser with the tube is not necessary.  It
> only impacts vignetting if the tube is just large enough to accept the
> marginal off-axis rays.  It is still a good idea to get the focuser as
> close to square as possible for this reason, but a non-square focuser
> will not cause the focal plane tilt that you think you are seeing.
>
> An easy way to see this is to image that all you have is a focuser, a
> secondary mirror and a primary mirror in perfect alignment.  Then
> mentally add a tube that is not quite aligned with the optical axis.
> The misaligned tube has no effect on the optical performance
> (collimation), but could cause some vignetting if it starts to cut
> into the marginal off-axis rays on one side or the other.
>
> If you do have a focal plane alignment problem, it's likely caused by
> the laser collimator itself.  I have found that these bloody things
> can change their alignment a lot over time.  The laser diode is
> encased in epoxy and it cold flows over the years causing the beam to
> be considerably off-axis with respect to the barrel.  To check this,
> rotate the collimator in the focuser, locking it down each time, and
> note how large a circle is made by the beam.
>
> In the photos that you presented, I don't really see a strong case for
> a misaligned focuser/focal plane.  If this were the case I'd expect to
> see small donuts in the corners, not elongated stars. (Although with
> coma, I guess one could make some case for a misaligned focal plane.)
> Instead, it looks a bit like you have differential fexture between the
> guider and the film.
>
> Anyway, it's necessary to check the collimator alignment as a first step.
>
> Hope this helps,
> Dave Rowe
>
>
>
>
>
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