Ad Astra Observatory

To do a CCD Polar Alignment, you need to run the mount east at 1x sidereal (the motor runs “backwards” at 1x sidereal), then run the mount west at 1x sidereal (the motor turns off; think about it). Hand controllers call this speed “2x” sidereal, since “1x” sidereal is “tracking” so “2x” must be “moving across the field at 1x”.

With every mount I’ve had before the Gemini, there was a clear setting for this useful rate. For some reason, in the Gemini, it’s tough to find.

The Gemini has 3 rates: Guide speed, Centering speed, and Slew speed. In the hand controller, these can all be set, but are in unitless measurements.

Guide speed seems to be 0.2x to 0.8x sidereal (too slow to do what I’m trying to do; the drive won’t “turn off” in the westward direction).

Centering speeds are multipliers (of what?).

Slew speeds range from 1 to 1600. I’m not certain how these correspond to actual speeds. It’s probably a straight multiplier against sidereal, but it could be scaled in some way. That is, if I’m looking for 2x sidereal, as defined before, would I set it to 1, 2, 100, 200? I have tried a variety of settings and have never heard the actual drive stop. This might be because it’s a servo instead of a stepper?

Anyway, I haven’t found the setting that I’m looking for. So my CCD alignment runs are not quite as accurate as they could be if I could get it to go exactly 2x. But it still works.