I've been using the CEM120EC for several years, first with a PoleMaster but lately with N.I.N.A.'s plug-in for PA. I use PhD2. Here's my accumulated experience:
1. N.I.N.A. produces a more consistently accurate PA in about the same amount of effort and time. It's also great that visibility of Polaris is not required.
2. Once you think you have proper leveling and PA with the PoleMaster, slew to your target and do the PoleMaster PA again from that orientation, and you may be surprised how much it disagrees with the PA you just did with the scope nearer to its park/zero position, despite being in perfect balance.
3. The bubble level of the CEM120 is too small and poorly calibrated (in manufacturing tolerance of the printed circle to actual level). I found that adding two longer (~2.5"), larger linear bubble levels on E-W and N-S axes to mount base works much better, but check these first and be finicky in selecting known good ones. A trick to testing linear bubble level calibration is to level it on a surface then turn it around end to end, and see if it agrees with itself. If the level is not clearly marked as to which side is supposed to be up, try each of the four sides of the level to find the one that actually works best. Bear in mind that while GPS is accurate to tiny errors in angles and clocktime, the mount software's tracking calibrations assume a truly level mount, and eyeballing the bubble level(s) are by far the major source of error in the overall system design.
4. For whatever reason, my mount tracks much better with just a little imbalance in both RA and DEC axes. The DEC due mainly to wind gusts I believe.
5. In PhD2, experiment with your RA Threshold and Aggressiveness settings. I've had really good luck reducing both, so less error accumulates before a command is sent to the mount, but once sent, it is a small one (and more than one such gentle nudge is required ; 3 or 4 usually). I.e., Don't try to zero the error with one larger push. In general one should not see a corrective pulse in the opposite direction, but a smooth decline and nicely damped error reduction.
6. A perfectly focused guide camera can introduce more guiding jitter than letting it be a bit out of focus, so that PhD has a smoother Gaussian star intensity profile to examine. In a similar vein, for the atmosphere around here, exposure times around 5sec produce better results than 2 or 3 secs, to not excessively chase the seeing. The overall system response does not have the analytical nor mechanical bandwidth to correct for seeing anyway. Enable multi-star guiding.
7. If your target is low in Altitude, set the mount to King vs. Sidereal tracking, to correct for atmospheric refraction as the target rises. This reduces the number and frequency of guider corrections sent to the mount.
8. A too-slow CPU controlling your rig can introduce all manner of oddball effects that are hard to pin down until you realize you've asked too much of it.
9. Once your planning is done and your sequence is imaging, disconnect your rig PC from the internet until morning.
I hope one of those tips helps. All that said I am very very pleased with the overall performance of the CEM120 vs. mounts I replaced with it!
CS, Jim
