#5000 Jul 17, 2001
There have been several threads concerning what to expect from a
G11/Gemini in terms of pointing accuracy so I thought I would share
my experience. I have a C-11/ST-7E with f/6.3 focal reducer mounted
on the G11. That gives me a field of view of about 9 x 14 arc
minutes. I leave the mount setup with a desert storm wrap over it.
At the start of each observing session I setup the OTA and do a cold
start. I use 4 stars on one side and 2 on the other side of the
meridian. After that I always get the DSO I'm looking for on the
ST-7E chip every time. I've had no failures yet with about a dozen
observing sessions under my belt. Even with long slews which require
a meridian flip, it still gets the object near the center of the
chip. I think this is fantastic, wonderful, and much better than I
Fortunately, I got the above accuracy immediately with very
little tweaking. Just lucky I guess. Now for the interesting part.
A couple of weeks ago, my pointing accuracy went to hell in a
handbasket. Something had changed, but what? I spent quite a bit of
time finding the problem, but I will spare you the gory details.
It seems that the bolts that attach the head to the tripod had
become somewhat loose. I suppose that this was due to repeated
heating and cooling cycles since the mount remains outside
continually. The head could not rotate but it could "wobble" from
side to side. This was a very small amount of motion. I actually
found it by resting my hand on the mount as I did various slews. I
could feel it better than I could see it. I am absolutely amazed at
what a large effect such a small movement could make.
I had looked at all sorts of other explainations before I found
the problem. The head had to be tight. I know, I put those bolts in
myself. Well, just shows you have to check everything - even those
things that couldn't possibly be wrong. After I cranked the bolts
down again, my pointing accuracy problems went away.
For those who are having problems with getting the target on your
CCD chip, check to make sure your head is firmly attached - both to
your shoulders and to your tripod respectively.