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CGE won't track Luna

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#1 DavidRM


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Posted 07 August 2009 - 04:51 PM

Can anyone tell me if I'm doing something wrong?

My CGE won't track the moon.

I know I've got a good polar alignment - with sidereal tracking, stars stay in the FOV of high-powered eyepieces for 30-45 min. at least. Come to think of it, I don't recall a star ever drifting away, and I've sketched objects for longer than 45 min.

But when I go to the moon, and switch to lunar tracking, I can see the moon creeping out of the FOV. I'm having to manually slew every 1-2 minutes.

Any ideas?


#2 Chris Rowland

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Posted 08 August 2009 - 04:38 AM

How fast is it drifting?
What sort of magnification are you using?
Is Sidereal tracking better?

If you are doing high magnification sketches of a small area then it could just be that the average tracking rate set by the lunar tracking rate isn't the same as the actual rate.
The moon's actual drift rate will vary depending on where it is in it's orbit, the lunar tracking rate is an average. And it also drifts in declination.

Personally I've never tried lunar rate, it's just as easy to leave it a sidereal and press a button occasionally but I don't tend to do long observing sessions on the Moon.

A friend made up a box with a couple of timers and relays to generate autoguide pulses, he uses that to tweak the tracking to keep moving objects centered, it works with comets as well.


#3 JohnH



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Posted 08 August 2009 - 06:18 PM

Of course not. Luna does not quite move at sideral rate. It runs slower by 360 deg X 60 Min/deg X 60 sec/Min / (~28 days X 24 hours/day X 60 Min/hours) which is about 32 sec of arc per hour, or roughly on lunar diameter per hour.

Some mount allowed you to select tracking at this rate, and I sometimes am troubled by it.

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