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I assume you have done a PEC curve and are using it? Also using direct guide? I am not sure if that reduces guide errors but it may as its more direct and possibly faster.
Modern autoguiders guide at the subpixel level so your resolution of the camera and guide scope are not that relevant. See the writeup at SBIG site about their STi guide kit about this.
The main problem with long focal length is flexure closely followed by seeing.
Poor seeing plays havoc with current autoguiders. Its a flaw in their design really. I have heard of one person who wrote their own software and it was more advanced. I have several concepts for more advanced autoguiders but I am unlikely to take it any further.
Roland Christen posted once that a guide scope should be mounted to the OTA itself not the mounting rings. I guess that reduces differential flexure.
You may also need to play around with your guide exposure length to see what works best. I use 2 seconds with good results on PMX but I used 5 seconds last weekend and got good results where 2 seconds stopped being effective. I use 5 to 6 seconds on my PME.
PMX can be a tad unreliable as well. The cam system to enable the 3 way switch is a source of unreliability and I personally would prefer my PMX did not have this feature and go back to the simple but bullet proof PME system. For example I had my PMX guiding to perfection on 2 nights. I knocked the counterweight shaft - fairly lightly I might add, and the next night I was getting oscillations in guiding which is indicative of the tension screws on the cam not being tight enough. Perhaps they become loose over time - not sure. Anyway, assuming your PMX is behaving itself (which is not certain with PMX) then what you are experiencing may be simply poor seeing. Everything is amplified when you go long focal length but those errors have to be taken in context that you are imaging at a longer focal length - so don't you have to multiply the magnification factor onto the guide errors to match the numbers?
Otherwise you are not comparing apples with apples. Did you get eggy stars or were they still round?
I get round stars at 3 metres and guide errors of .5 pixel or perhaps a tad more occassionally. But thats 3 metres. I get .00 to .3 errors at 1050mm and get round stars too.
STi with the lens guiding kit may be a good accessory and it looks like it would be less susceptible to differential flexure.
Guide scopes are usually cheap scopes and have poor loose focusers and perhaps not the best mounting rings etc so there is plenty of room for flexure.
With autoguiders everything needs to be tight and firm. No slop or looseness.
I often find that selecting the right guide star can make a huge difference. For example I don't always pick the brightest star. If they are too bright I think it makes the software's job of calculating the centre harder. Don't select a double star or a guide star with another similarly bright star in the frame. A nice sharp medium bright star seems to work best. The first thing I do when I start getting worse guide errors is to pick another star. It often works wonders. Errors can vanish and the guiding settles down. Also watch the guide errors and if you are getting - to + errors after every correction then your guider is correcting itself or you have backlash.
On PMX with a good PEC, good polar alignment (this has to be perfect of course and is assumed in the above), good balance, nothing dragging on the camera, you should be able to get round stars at 1800mm without any trouble.
A thread about autoguiding would be good as its critical for imaging.
All imaging roads lead to off axis guiders in the end. Perhaps SBIGs guide sensor in the filter wheel is just as good. Its a bright idea. QSI's combining an OAG in their camera is genius. FLI needs to lift its game with autoguiding. They have no solutions whatsoever. I'd say their marketing is more to the Xray market more than to the astrophotography market.
Greg.
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