Pointing accuracy w/Gemini - Fantastic

Jul 17, 2001



#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.


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