It is clear tonight, after an unusual spell of foul spring weather (rain, freezing rain, snow, sleet). My plan for tonight is to begin to track down the poor tracking and guiding performance I experienced the last couple of sessions.
First, I got the ST-4 guider signals working by checking cables and connectors. I think the connector at the Gemini control box tends to lose connectivity with long sitting – probably through minor corrosion on the contacts. Working the connector in an out several times seems to have cleared it up, and the mount is now responding to guiding signals well.
Then I re-checked the mount’s polar alignment, as that hasn’t been done since last summer. It was quite a bit off. So, using PEMPRO and the QSI camera, I made a couple of passes to improve the alignment.
The azimuth seemed off a bit in a first test, but turned out not to be. The altitude was ‘way off though. Once I adjusted it, another pass of minor tuning to both axes brought them close to pretty good aligment – within 1-2 arc-minutes each. The axes interact, so iterative adjustment is needed, and a bit more is probably worthwhile, but I got cold and stopped for the night. So a little more fine-tuning next clear night and it will be ready for the season.
Oh, a note for myself and others: I had forgotten this, but PEMPRO seems to specify an over-correction on the ALT axis, of approximately double what it should be. So, for example, if I have a “20′ low” error and follow its correction guide, I then have a “20’ high” error. Correcting to half of its recommendation works perfectly. This applies only to the ALT adjustment – the AZ adjustment works fine. Searching the net I’ve seen a couple of other people mention this, but haven’t seen anything about it in the PEMPRO support forums. The factor of 2 error makes me wonder if it has something to do with camera binning, but then one would expect it to apply to both axes. Anyway, just remembering to take the suggested correction halfway works fine.
Next night: one last pass of Polar tuning, then I’ll have to do a Gemini cold-start and new sky model since changing polar alignment destroys the model, and I still want to measure the PE, possibly adjust the worm, and reprogram the PEC.