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SkyWatcher AZ-EQ6 GT mount Periodic Error?

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#1 nikulsuthar

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Posted 19 February 2019 - 10:52 PM

Hello All,

 

I've a problem in getting round star even in 20 seconds of unguided exposure with my SkyWatcher AZ-EQ6 GT mount. What I've observed is that the stars do not form very long trails but they trail short distance on either sides (left side and right side). This happens even if tripod is bubble leveled, polar alignment is done precisely with QHYCCD Polemaster, balancing is done proper, no wind is present and multi-star alignment is done with EQMOD and Cartes du Ciel. That is to say that if the star if found round in first 20 second sub, then in the next 20 second sub it'll trail to left, then in the next 20 second sub it'll trail to right, then in the next 20 second sub it'll go back to almost a round star. After an hour all the stars will remain in the field of view but 2/3rd of the subs will have star trailing captured. This trailing does not happen in up and down directions. The scope I use is not too big and heavy. Its Explore Scientific ED80 Carbon Fiber Apo Triplet refractor and the camera is ZWO ASI 224MC. The field of view is 0.55° x 0.41° and the image scale is 1.51"/px.

 

Could this be a result of periodic error in the mount? If so then is it recommended to record and save the periodic error with EQMOD or use PemPro? I haven't tried PHD2 autoguiding because I'm still studying the native behavior of the mount. Do you think that autoguiding will be a better solution over periodic error correction? 

 

Any input will be highly appreciated. Many thanks for your time in advance.



#2 1DegreeN

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Posted 19 February 2019 - 11:34 PM

Run PHD2 with guide output disabled and that will reveal the actual tracking behavior of the mount which you can then analyze to construct a PEC.


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#3 luxo II

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Posted 21 February 2019 - 12:14 AM

More likely backlash between the worm and wormwheel on the RA axis.

With use the worm slowly wears loose and the result is the scope can slop from side to side. Solution is to adjust the worm to remove the sloppiness.

It is fixable DIY with some simple tools.

Edited by luxo II, 21 February 2019 - 12:15 AM.

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#4 nikulsuthar

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Posted 23 February 2019 - 11:59 PM

 

More likely backlash between the worm and wormwheel on the RA axis.

With use the worm slowly wears loose and the result is the scope can slop from side to side. Solution is to adjust the worm to remove the sloppiness.

It is fixable DIY with some simple tools.

 

Please find my following sample images of star trailing to the left then to the right and back to where the stars were in the beginning. These are only 20 seconds subs back to back. If someone can confirm whether this is the result of periodic error or backlash between the RA axis worm and worm wheel then that would be like a life saver for me. Till then I'm not able to do long exposure imaging. Let me know if any more information is needed...

Capture 00023 20 40 20 result

Perfect round stars...

 

 

Capture 00024 20 40 40 result

Star trailing to the left...

 

 

Capture 00025 20 41 00 result

Star trailing stopped...

 

 

Capture 00026 20 41 20 result

Slight star trailing to the right...

 

 

Capture 00027 20 41 41 result

A little more star trailing to the right...

 

 

Capture 00028 20 42 01 result

Star trailing stopped to round stars...

 

 

Capture 00029 20 42 21 result

Perfect round stars...

 


Edited by nikulsuthar, 24 February 2019 - 12:01 AM.


#5 Stephen Kennedy

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Posted 24 February 2019 - 12:14 AM

To get a good idea of what your PE is you need to take an image of one complete worm wheel cycle which is between 5 and 10 minutes for most mounts so the whole system can be evaluated and deciding how much of the trails are due to PE and which are due to PA errors.
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#6 nikulsuthar

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Posted 24 February 2019 - 12:41 AM

To get a good idea of what your PE is you need to take an image of one complete worm wheel cycle which is between 5 and 10 minutes for most mounts so the whole system can be evaluated and deciding how much of the trails are due to PE and which are due to PA errors.

I can do it but will it isolate any backlash issue just like it can isolate any PA errors? I need to know for sure whether it is backlash issue or PE issue.



#7 Stephen Kennedy

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Posted 24 February 2019 - 12:49 AM

I really do not know anything about backlash since it is not an issue with my Pentax MS-5 GEM.

#8 Auburn80

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Posted 24 February 2019 - 11:24 AM

Back in the day, we used to guide visually with a short fl reticle eyepiece. Backlash was easily seen as a lag in the movement of a star after control input. PE was easily seen as a wobble back and forth of the guide star. PE can be captured with a video of a star at high mag. You might try taking a video of a guide star (like planetary imaging videos) with manual correction and see what's happening.

Clark

#9 1DegreeN

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Posted 24 February 2019 - 11:16 PM

I have the AZ EQ6-GT too. It would be unusual to have backlash in RA, especially if you have the mount tracking while imaging. Backlash in DEC is more common but you would only see that if you were guiding - and I guess you are not, otherwise you could post a guide log. There certainly will be some PE. I don't think you will make any progress until you use a guidescope and run PHD2 either with guide output enabled or disabled. The worm period for the AZ eq6 is around 8 minutes, I would let the mount run for an hour or so. Then check the guide log and it will show you what is happening with your mount. If you're not familiar with PHD2, there is a PHD Log Viewer application for inspecting the guide log. You can also post the log here or over at the PHD forum for some expert analysis. If your aim is to use the mount for unguided imaging you can use the guide log to build a PEC that you can then upload to the mount which should remove at least some of the PE. I image with my AZ EQ6 but use a guidescope with PHD2 and run EQMOD with PEC. I built the PEC curve from a PHD2 guidelog using PECPrep. I'm quite happy with the performance of the mount but I would never attempt unguided imaging with it.


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#10 nikulsuthar

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Posted 15 March 2019 - 01:29 AM

I have the AZ EQ6-GT too. It would be unusual to have backlash in RA, especially if you have the mount tracking while imaging. Backlash in DEC is more common but you would only see that if you were guiding - and I guess you are not, otherwise you could post a guide log. There certainly will be some PE. I don't think you will make any progress until you use a guidescope and run PHD2 either with guide output enabled or disabled. The worm period for the AZ eq6 is around 8 minutes, I would let the mount run for an hour or so. Then check the guide log and it will show you what is happening with your mount. If you're not familiar with PHD2, there is a PHD Log Viewer application for inspecting the guide log. You can also post the log here or over at the PHD forum for some expert analysis. If your aim is to use the mount for unguided imaging you can use the guide log to build a PEC that you can then upload to the mount which should remove at least some of the PE. I image with my AZ EQ6 but use a guidescope with PHD2 and run EQMOD with PEC. I built the PEC curve from a PHD2 guidelog using PECPrep. I'm quite happy with the performance of the mount but I would never attempt unguided imaging with it.

Please find my PHD2 screensshot... Probably the most horrible in the history of Astrophotography. This is for the first time that I set it up.

 

PHD2 guiding graph
 

My mount has some stiction in the RA axis. Could it be because of that? Alternatively could it be because of imperfection in the mount producing this periodic error?



#11 Tapio

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Posted 15 March 2019 - 02:32 AM

All mounts have periodic error - better ones less than cheaper.
That's why most people do autoguiding.


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