Slow but Steady Mount Drive Tests
For the first time we got a full evening of drive testing and nothing broke! You’ll recall that mostly we’ve been having a series of problems with the azimuth drive. The last fix that added a shaft dimple for the gear set screw to settle into seems to have resolved the issue with it slowly sliding up and off of the shaft of the servo motor. We had tested that fix under cloudy skies weeks ago.
The other highlight of the night, aside from nothing new breaking, was tracking on Mars for 10 minutes. It wasn’t perfect – the Red Planet moved about a third of the field in the little refractor we have affixed to the back of the mount – but it was encouraging. The tracking success came in the middle of confusing and less promising results for pointing accuracy that will get worked out in future sessions.
For this initial test we simply started up the SiTech software and The Sky 6 and set to work as if we had been doing this for years. We’d done tick determination a couple of times before, so it seemed reasonable that we could skip that step. And we just picked two starting stars, synced Sky on them, and then started trying moves short and long distances.
It was definitely good to see in the first moves after initialization that the scope moved well to the approximate position of the new target with no apparent issues. We had only one move in the night, though, where the target star was in the finder after the move completed, let alone in the refractor field of view. It was in the neighborhood, but not in either of the fields. For the one that worked, the move was a close one. Moving far across the sky seemed to increase the error, so we will go back to tick determination (calibrating the motor encoders against the measured rotation of the telescope) to try and resolve that issue.
Since this was the best result to-date, we weren’t quite prepared to start analyzing the results. From the eyepiece, Mark thought we were seeing more azimuth errors than altitude errors. That could be a problem of slipping in azimuth, errors in the tick determination on azimuth that is greater than in altitude, or simply that we were looking at about the same altitude for all of the objects during the tests.
We also encountered a safety feature of the SiTech system that we haven’t figured out how to escape from yet. From a star in the West, we commanded a move to a star in the East. The scope began to slew toward the South in azimuth and was doing fine until it got to due South and then it stopped. It was protecting us from wrapping the wires up by repeatedly slewing in one direction. I couldn’t intuit what I needed to do to get it to go in the other azimuth rotation direction and ended up slewing to stars more northerly with azimuth in the right direction until it would finally slew correctly to the original target. That won’t do for public observing, to be sure. I know there is a button to press in there somewhere, but I will have to break down and read the instructions.
For the next test session we will organize a process for setting up that will be repeatable, and we’ll start recording the gap between the slew destination and the actual position of the object so we can tackle the mount and software tuning that will get us to acceptable performance. Next session is planned for Wednesday evening, May 16, 2012, at Mark’s shop starting at 8PM.