Mount: Losmandy Titan (not the 50:1 gear ratio version that gives more torque), but running 18v 5amp to it, so it has plenty of juice.
After an automated 10 point model run I achieve the following (transcript output from ACP):
13:29:08 Start slew to Map-010...
13:29:10 Az = 358 Alt = 41
13:29:10 (wait for slew to complete)
13:29:36 (slew complete)
13:29:36 Acquiring mapping point...
13:29:48 (taking 10 sec. exposure, Clear filter, binning = 1)
13:30:09 (HFD = 3.09)
13:30:09 (avg HFD = 2.88)
13:30:09 Plate-solve pointing image.
13:30:10 233 image stars found
13:30:10 1742 catalog stars found
13:30:11 Solved! 156 stars matched.
13:30:11 Average residual is 0.87 arcsec.
13:30:11 Pointing error is 1.023 arcmin @ angle 345.82
13:30:11 True focal length is 53.1 cm.
13:30:11 True image center (J2000): 00h 19m 52.8s 10 42' 00.98"
13:30:11 Imager sky position angle is 74.8 deg.
13:30:24 [sync] pointing model updated
13:30:25 Training run completed at 13:30:25 UTC
This is after 10 mapping points. 1' (arcmin) is pretty high (in my opinion). My model now consists of around ~20 points, but I haven't had time to review the logs. I know its considerably less than 1' now, but can't tell you the exact value until I head down to the remote observatory again - sat comms are presently down so can't access the observatory computer.
Tracking - I've been able to do ~10 minute unguided (tracking only) exposures with the FSQ operating at a focal length of 530mm (native). This is quite a forgiving wide field focal length (unlike 2000mm+). The STL11k delivers a 3.5 arcsec/pixel with the FSQ, so the periodic error (PE) ideally should be below this value. In my case, the measured PE (from PEMPro) on the titan is around -/+ 1.5 arcsecs well within the range. If we are talking guided (which isn't what I consider "tracking"), then the longest exposure I've taken is 20min with no signs of field rotation. This was sometime ago and I've since revisited polar alignment. I feel sure I could go for 30min now, but haven't had the opportunity to test this.
Things to note:
- If you're using a SCT or other "sliding" mirror focusing system, there will be some image shift (even if you can't visually detect it). This will produce erroneous pointing results.
- Don't eyeball the pointing model alignment stars, this will result in model inaccuracies. As you can see from the ACP output above, I use a CCD camera to automatically take an image, plate solve it against a known catalogue, then automatically slew to center the star - subsequently the pointing model is updated. In fact, every time a slew to an object and take an image, the pointing model is updated. This ensure accurate pointing across the sky.
- Good polar alignment is key to avoid field rotation at extremely long exposures. Don't think that because your autoguiding, you images will not be subject to field rotation. You'll find with poor polar alignment the image will rotate around the star your guiding on.
- Reduce you mounts PE and backlash as much as possible...and don't forget good balancing of both RA and DEC axis. Software tools are great for PE improvements. Don't forget evil flexure - the hidden enemy.
- You're CCD camera has better vision than you. Use it to your advantage. Use it to polar align as it will detect drift much faster and with greater accuracy. Use it to plate solve and build pointing models (automatically if you choose). Even using software generated cross-hairs on the image output from the camera improves accuracy.
Hope this helps.