Experience with a Digitec Laser SCT Collimator

Hi
Here is the magic.
You need a tracking mount or something like Polaris
that moves little over time.
1. Using a high power eyepiece, slightly defocus until
you have a rich amount of diffraction rings. ( not so
far that you see the shadow of the secondary.
2. Move the scope around until you find the location in
the field of view that has the best centering of the
diffraction image.
3. Draw an imaginary line between the center of the field
of view and the location of the star in the field of view.
4. Move the telescope such that the image is now 1/3
of the way out along this imaginary line from the center
to that best location of the star.
( This is because of the magnification of the secondary
is about 3X )
5. Finally adjust the secondary adjustments to center the
image.
6. Repeat if not happy with the first pass or if the star
would have needed to be push out of the field of view
to get the best diffraction centering.

It would seem that steps 3 and 4 are difficult and a little
subjective. This is true but multiple passes will center
in on the an alignment that is as good as you'd be able to
tell the difference.
It'd be good to then check how well your laser correlated
with a good star alignment.
Dwight



Alan Charlesworth wrote in message ...
SCT collimation has always seemed like black magic to me. I only use my
8" LX-90 a couple of times a year at remote dark sites -- like next
weekend's Oregon Star Party. I transport it in a JMI foam case, but the
likelihood of miscollimation seemed high.

I sprung for the $184 Digitec Laser SCT collimator
(http://www.digitecoptical.com/Collimation.htm). It screws onto the rear
SCT threads, and shines a laser beam up through the scope. The idea is
to crank the focus all the way clockwise to the close-focus stop, and
then look to see whether the projected red doughnut hole of the
secondary is centered.

I tried it last night, and the problem was deciding if things were
really centered. I made a target of concentric circles, and put it on a
20' away vertical surface, and leveled the scope to try and get things
perpendicular. Things looked pretty good, and I tweeked a bit with the
Bob's Knobs on the secondary . However, I was disappointed at my
inability to judge perfect centering.

Has any one else tried this gizmo?