Lunar tracking rate

[Dawson]

I am told that on Skywatcher mounts , lunar tracking mode just tracks in RA, and makes no declination corrections, despite the Moon varying in declination by about 15 arcminutes or so per hour. Even with near perfect polar alignment the Moon will drift out of the FoV of the C14 with a large sensor camera on, in an hour, and for long imaging runs such as the lunar impact flash work, this is quite annoying.

Is there a reason why Skywatcher don’t / can’t add in some declincation correction?

Thanks.

James Dawson

[Dominic Ford]

Random guess: is this due to backlash? RA always moves in the same direction, so you can avoid backlash by always approaching targets from the east. The Moon’s declination moves in both directions, so there would be no way to completely avoid backlash.

[Nick James]

This is a hangover from the old days when mounts didn’t have motors on the dec axis and all you could do to track the sun and the moon was to change the RA axis drive rate to match their average eastward drift rate. Most mounts now have the ability to move in both axes and if you control them via software using, say, ASCOM or INDI, you should find that you can program offset rates based on the true ephemeris or, at least, some calculated offset rate. As Dominic says, there is usually some backlash in the gear system so that reversing direction is problematic but that is only really important for guiding, not in this case where any significant tracking offset is always in one direction.

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