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

[Aplanatic@aol.com]

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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