Lunar Tracking Problem

[Jim Smith]

My SkyWatcher AZ-EQ5 GT tracks the stars pretty well but when I come to do lunar imaging I can never seem to get the lunar tracking working. See how far Copernicus drifts in 3.5 minutes...

First frame from video...

Last frame from video...

Here is the set-up...

2,400mm focal length. Altair GPCAM 178C using whole sensor.

I actually get less drift for the moon when using sidereal tracking mode. I balance the scope on the mount quite carefully. This lunar drifting always happens. It's not just a one-off.

Does anyone have any ideas? Thanks in advance, Jim

[malc-c]

No expert, but how good was the polar alignment ?

To be honest drift whilst using a video camera shouldn't be a problem as you typically stack the individual frames, and the registration of each frame is handled by the stacking software.

[Jim Smith]

7 minutes ago, malc-c said:

No expert, but how good was the polar alignment ?

To be honest drift whilst using a video camera shouldn't be a problem as you typically stack the individual frames, and the registration of each frame is handled by the stacking software.

Thanks. Polar alignment is good. The mount lives on a permanent pier. Yes, I do stack the frames but the stackable area gets smaller as time passes. I suppose it isn't a big deal...but it would be nice if it worked.

[malc-c]

If the mount tracks better using sidereal then stick with that.  At the end of the day it's whatever works best to get the results you want.  I don't think I've ever set EQMOD to luna tracking when imaging the moon, 

[Gfamily]

Can you tell whether the incorrect tracking is in RA or Declination?. 

'Normal' Sidereal tracking is in RA only, as the Declination of 'fixed stars' is - well, fixed. However, for proper Lunar tracking, I assume that there needs to be some element of tracking in Declination (in addition to the slightly reduced rate of movement of the RA axis). 

To be honest, it seems unlikely - but it might be worth checking. 

[Jim Smith]

13 minutes ago, Gfamily said:

Can you tell whether the incorrect tracking is in RA or Declination?.

Thanks. Looks like Right Ascension to me.

[jambouk]

Possible causes:

- lunar tracking not selected

- polar alignment not tight enough

- kit not properly balanced

- kit too heavy for mount

I can’t imagine there is a problem with the firmware or the intrinsic lunar tracking rate. 

[Jim Smith]

7 hours ago, jambouk said:

polar alignment not tight enough

Thanks, Of your suggestions, I think the polar alignment might be the most likely. Although I don't notice problems with dso imaging, I do that with only 30s exposures at about 1/4 of the focal length, so it might not be obvious. I will check it again.

[jambouk]

With your DSO images, do you have the first sub and the last sub of a DSO imaging run and flick between those and see if the stars move.

[Jim Smith]

2 hours ago, jambouk said:

With your DSO images, do you have the first sub and the last sub of a DSO imaging run and flick between those and see if the stars move.

You've reminded me of a video I made a few months ago of the periodic error of my mount. https://ishoo.co.uk/video/mounterrorlow.mp4

I think the cycles are about 15 minutes long. Bearing in mind that the above video is of an APS-C sensor rather than the small sensor of my lunar set-up, periodic error might well be the real problem.

Edited January 15, 2022 by Jim Smith
Corrected error

[jambouk]

Possibly. You could be the first lunar imager who has been thwarted by periodic error

 

[Jim Smith]

1 hour ago, jambouk said:

Possibly. You could be the first lunar imager who has been thwarted by periodic error

 

Well, I'm going to train my mount even though that sounds a bit equestrian. Interesting to see if it makes a difference.