You wouldn't expect the installation of a finderscope to be much trouble,
but the finderscopes supplied with the NexStar GPS, CPC, CGE and the SCT
AS-GT models can be a little tricky. If assembled incorrectly, the
finderscope will slip around in the bracket with the least touch.
There are two potential brackets you may receive - one has the adjustment
screws in the front ring, the other has the adjustment screws in the back
ring. Brackets on some of the other non-SCT scopes are similar, but
not exactly as I'm describing here.
Adjustment Screws in the Front Ring
The large groove/channel towards the back is to be ignored, don't put the
rubber O-ring in it as intuition would suggest. Rather, push the
O-ring further to the back (eyepiece end) so that it seats firmly in the
back ring of the mounting bracket when the screws are set in the small
groove towards the front of the finderscope.
Adjustment Screws in the Rear Ring
In this case, put the O-ring in the smaller groove towards the front of the
finderscope. Unfortunately there isn't a small groove on the back part
of the finderscope, so things may still slip around a bit. If you have
problems with the finderscope moving around, try putting the O-ring in front
of the smaller groove such that the adjustment screws are up against the
back edge of the large groove/channel.
You may also want to read the article about
Focusing the Finderscope.