ATM Laser Collimation/Focuser Question -- Update: JMI Mini 3 Focuser Problem Solved!

["Al Klayton" <aklayton@pcisys.net>]

In response to the NGF Mini 3 focuser image shift problem described below I
called JMI. They knew just what I was talking about. It turns out the
composite bearings used on the NGF Mini 3 (and Mini 4) focuser are the
problem, and in fact the JMI Web site clearly recommends NOT using laser
collimators with these focusers because of the small amount of image shift
 which they say is not a problem visually). JMI sells an upgrade kit for $29
which just consists of 4 true roller  bearings which are used to replace the
composite bearings ( unlike the true roller bearings, the composite bearings
don't contain any ball bearings as far as I can tell). This kit in effect
upgrades the NGF Mini 3 to a NGF Mini 2. I found the bearings locally for
about $19 and put them in -- no more image shift, no more wandering laser
collimator dot!

For those who might want to do the same, the replacement bearings are
designated SSR-2-ZZ in the "bearing world". The SS stands for Stainless
Steel. I don't know what the rest of the code means but the guy in the
bearing shop recognized what I wanted. BTW, the fellow at JMI was very
helpful and volunteered the bearing info in case I wanted to try and find
them locally -- which saved a few bucks and some time.

Al Klayton

Original post follows:

When using a laser collimator in a 1.25" JMI Crayford focuser [ NGF Mini 3]
on my 8" Dob I observe the following: If I start with the focuser racked all
the way out, and then slowly rack it all the way in while observing the
laser beam on the
primary mirror, the beam moves first in one direction .... and then wanders
BACK even though the focuser tube is still moving in. Although the degree of
beam movement is fairly small, about + - 3/8 inch or so (the beam stays
within the donut ring used to mark the center of the mirror), I wonder if
this is normal? Seems to me that even if the laser collimator is slightly
out of collimation itself, or if the JMI focuser is not perfectly square to
the OTA, or the secondary mirror is not adjusted perfectly, I would still
not expect the wandering back and forth type movement of the beam -- I'd
expect beam movement in one direction only since the focuser tube is only
moving in one direction. I conclude the wandering motion reflects
imperfection in the movement of the JMI focuser tube ( I've tried adjusting
the focuser).  If this type of situation was normal, then the accuracy of
the laser collimation method would depend on the position of the focuser
tube, which can't be right.
Am I being to critical -- do others see this type of beam movement?
Or...what?
Thanks,
Al