I saw an article about replacing the collimation screws, but I could not
find the ones mentioned. Do you have a source?

-----Original Message-----
Doug Walton
Sent: Friday, August 24, 2001 12:12 PM
To: MAPUG; 30cmSCT Group; Sct-User Group
Subject: [M]: 12" LX200 Collimation Questions...


I had my 12" LX200 out last night and while I was at it I replaced the stock
collimation screws with Bob's Nobs. The factory collimation was OK but not
excellent. I had done this once before on a 10" LX200 and had no problems,
and I believe this last installation was fine too. I replaced the screws
one at a time, and made sure to snug down the newly-installed nobs to keep
things in place with the secondary. I then progressively worked my way up
from around 100x all the way up to 500x and a little more, each time
fine-tuning and adjusting. I used Vega for most of the effort.

I noticed two things that I wanted to run by people for opinions or
thoughts.

First, I hadn't realized that collimating while "outside of focus" is
different from collimating while "inside of focus", in terms of which nobs
represent which segment of the secondary, and whether loosening or
tightening accomplishes the desired result. For example, in one case, it
seemed that loosening a particular nob moved the inner donut circle away
from the edge, while in the other case, loosening brought it closer (I
think). The orientation of which nobs affected which part of the secondary
also was different for each of the two focus situation. I was surprised to
see this. Maybe it was getting late and I was hallucinating...

The other thing I noticed was after I got the scope in what I'd call "pretty
darn good" collimation. As I'd move the focuser from a big defocused donut
to a pinpoint, I'd see the central hole stay perfectly centered within the
shrinking donut, almost as though the donut was symetrically collapsing in
on itself. The rings finally just shrink down to a nice focused star, and
you start to get some semblance of an airy disk (depending on conditions).
What I also noticed, though, was that I'd occasionally slew to a different
part of the sky, and then it would appear that the collimation was (just
very) slightly off when I'd defocus. Then, when I'd run the focuser through
the focus point, and then defocus on the other side of focus, collimation
appeared to be perfectly centered again. When I'd then go and move the
focuser back through the focus point to the original defocused side, again,
collimation appeared to be perfect.

Basically I'm wondering if slewing the scope and/or messing with focus can
have a temporary or more permanent effect on collimation settings with this
scope. Also, I noticed that slewing often required that slightly I re-focus
the scope - is this some kind of mirror flop/slop/backlash or whatever? I
don't remember any of these kinds of behaviors showing up on my old 10"
LX200.

Thanks for any thoughts/advice,
DougW

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