Collimating Nexstar refractor?

I've been looking for resources but haven't yet found
what I think I need. I suspect an expect is reading
saa ...

I have a Celestron Nexstar 102 GT refractor. It has begun
showing symptoms of a collimation problem - "comet tails"
on stars at high magnification. There are no adjustments
I can find on the objective cell, and no information in
any of the docs I can find from Celestron.

I've read this symptom on a refractor might also be caused
by a problem with the diagonal or drawtube. I've tried
a friend's diagonal, and also better brands of eyepieces,
with the same symptoms. I don't know how to check the
drawtube for misalignment.

How do I decide which adjustment this Nexstar needs and
apply it correctly? Does somebody have the procedure I
need written up somewhere?

I'll be very grateful for pointers. Thanks.

--
Glenn Holliday