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I need your help! I am having some weird tracking issues with my tripod (Advanced VX). When I setup and calibrated my scope yesterday I took this picture of NGC6888 (90s exposure): http://imgur.com/3siYrNX As you can see this image looks quite good tracking wise.

However due to unknown reasons it seems that 1 in 3 images is always bad and has tracking issues, like this one: http://imgur.com/NNduJZS This movement always occours in the same direction, it seems that for some reason one axis of the mount is moving faster periodically, causing these tracking issues.

Funny is that is seems to be occouring periodically. I always get 4 images good, 2 images bad, 4 images good, 2 images bad, ...

I tried fiddeling around with the anti-backlash settings (I never touched them before) but that didnt fix it. My current anti-backlash settings are: RA +- 55, Dec +- 60

Does anyone know where those tracking issues could come from and how to fix them? Note I also have a 5s mirror-lock so it should not come from vibrations of the camera.

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  • $\begingroup$ Have you found this forum post? It appears to be about your exact issue. $\endgroup$ – zephyr Jun 23 '16 at 14:27
  • $\begingroup$ @zephyr Thanks for that link, interesting post. But no its not the same, he used a guiding scope / camera and PHD software to track, I dont have any guiding hardware, just the tripod nothing else. I did a polar alignement (2 alignement stars and 4 calibration stars, polar align) $\endgroup$ – RononDex Jun 23 '16 at 14:38
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Looks like you have a periodic error issue. The worm period on the AVX is about 9mins I think. The issue has a period of 90sec * 6 = 540sec (9min).

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  • $\begingroup$ That would make sense, but how can I fix that? is there a way to tackle the issue? $\endgroup$ – RononDex Jun 23 '16 at 15:43
  • $\begingroup$ To solve the problem "properly" would require a the worm to be re-ground (requires specialist equipment) or replaced, even then you are unlikely to get much improvement. To be honest 4 out of 6 good 90sec images without guiding is quite good. You could try PEC (might help a bit), guiding is the only real practical solution. $\endgroup$ – James Screech Jun 24 '16 at 7:24
  • $\begingroup$ Yes it seems PEC can get me up to 90% of good images. I tried to train my mount in PEC yesterday using the PECTool from Celestron. Sadly I could not find any guiding software that was compatible with my Canon 7D Mark II. Do you know any that would be compatible? $\endgroup$ – RononDex Jun 24 '16 at 12:10
  • $\begingroup$ I'm not aware of any guiding software that works with DSLRs, you will need either a webcam type or camera with an ASCOM driver. Could someone from a local astronomy club help you out? $\endgroup$ – James Screech Jun 24 '16 at 12:36
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This is not a solution to the problem, just an idea which may help diagnose it...

Try setting up with the mount deliberately aligned slightly off the pole. Then a time exposure with "perfect" tracking should record stars as a straight line.

Once set up like this take a single long exposure for, say, half an hour and see if the trails on the image are straight with regular bumps (that would probably indicate periodic error as James suggests), or if something more random is going on.

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