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Drift issue NEQ6

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#1 Astrob0b

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Posted 19 March 2019 - 06:45 PM

Hello,

I am having a problem with the tracking of my NEQ6 Pro and I would truly appreciate if someone with a bit more experience might have a look at the drift in the attached screenshots (I have highlighted the resulting drift of the image with a white frame for better visibility). As you can see in the „Position Drift Scatter“ (left graph) from Sharpcap, there is a more or less constant drift and I am curious how to fix it. I have tried several things already (please see next posts as well due to the file size limitation) but I am still not sure what is causing this drift.

Some basic information:

-I am using a 8“ F/5 Newton + ASI224MC (unguided for EAA) + Baader UV/IR Cut Filter
-The telescope is well balanced and stands (non-stationary) on a field for observing. I have tried different locations so I would exclude that it is caused by the underground.
-I control the mount with a laptop using the SynscanPro Windows app and I am plate solving with SharpCap. However, the drift occurs also when I am using the hand controller. Thus, I would exclude software problems as well.
-The drift occurs with and without cone error correction (see attachments in this post for comparison).
-Interesting observation: The images in post two were taken on the same night. The only difference is that I have turned the camera within the focuser (1:10 crayford focuser from Astro Professional (looks the same as GSO)) and that I have changed the exposure time. This seemed to improve the drift significantly. A close look also reveals that the stars are not 100% round despite focusing with a Bahtinov mask. 

 

So currently I have two theories:

-It is caused by the focuser. If yes, how can I confirm and fix it?
-It is caused by a problem with the gears. So I guess, the drift could be fixed with a belt modification?

What do you think? I am happy about all ideas and answers :-)

Attached Thumbnails

  • Drift_with cone error correction.JPG
  • Drift_without cone error correction.JPG
  • Drift_longterm (NGC 6543).JPG


#2 Astrob0b

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Posted 19 March 2019 - 06:49 PM

Drift pattern before and after turning the camera within the focuser (note: Images haven't been taken on the same night as the images in the opening post).

Attached Thumbnails

  • Drift before.JPG
  • Drift after.JPG


#3 austin.grant

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Posted 19 March 2019 - 08:23 PM

You didn’t mention polar alignment. How accurately are you able to polar align the mount?

#4 Astrob0b

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Posted 20 March 2019 - 04:49 AM

Thanks for your answer. I am not sure about the level of polar alignment that I am achieving. I am polar aligning with the hour angle method (using the „SynscanInit“ app) and I make sure, that the polar star is always in the small bubble in the polar scope. This method is most likely not 100% precise, but the drift occurs on different nights with different levels of polar alignment. E.g. The
The mount drifted in one night around 80 pixels on the Y axis and around 50 pixels on the X axis in 937 seconds. However, on the other night it drifted only 70 pixels on the Y axis and 70 pixels on the X axis in 1241 seconds.
So it seems like there is an influence of the polar alignment but is it really that significant? Also the attached long-term drift (23 min, ca. 3 worm rotation periods) in post 1 (3rd picture) puzzles me since it seems too unstable to be caused by polar alignment. Looking forward to your opinion.



#5 austin.grant

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Posted 20 March 2019 - 12:13 PM

My guess is that it's a combination of polar alignment and periodic error. 



#6 Astrob0b

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Posted 21 March 2019 - 08:02 AM

Thank you so much for your help. I will polar align with Sharpcap and run a PEC training next time.

As you seem much more experienced than me (I am just doing some EAA for around 8 months), I would truly appreciate your opinion on the attached graph in this post (I am very worried about a mechanical problem).
After approx. 1 worm period (point 1), the mount suddenly sacks down, just to recover again. However, at pretty exactly the same spot of the worm wheel (point 2, which is after the 2nd worm period), there is no upward movement.
Could this be caused by periodic error as well or is it a hint that something is wrong with the worm wheel?

Attached Thumbnails

  • Drift_close up.JPG


#7 tomwall

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Posted 21 March 2019 - 08:22 AM

I will be following this topic with great interest as I have exactly the same star shape going on as in your third photo. I'm using an AVX and a new Meade st80 (ASI224 camera, .5 focal reducer, ~1.4* fov). I did the All Star Polar Alignment routine available with Celestron mounts. I had no visually detectable drift over 5 minutes of 30 second stacks (Sharp Cap,Live Stack, using cross hairs for centering). 

 

I'm waiting for my next clear night to check if I have sagging with my maximally extended focus draw tube. I'm hoping that a slight tightening of the focus lock will remove any sagging. Time and another clear night will tell.

 

Good Luck

 

CS

 

Tom,

Tucson


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#8 Phil Sherman

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Posted 21 March 2019 - 08:22 AM

I spent three years trying to fix a similar drift on my Atlas mount and finally traced it to an inadequate polar alignment. Drift from the alignment was less around 1 arcsec per minute which I thought was adequate. I didn't work harder on the alignment because at that level, I was at the limit of the smallest polar adjustments I could easily make. 

 

I finally spent the additional evening adjusting the polar alignment and the 'drift' was eliminated.


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#9 austin.grant

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Posted 21 March 2019 - 08:52 AM

Thank you so much for your help. I will polar align with Sharpcap and run a PEC training next time.

As you seem much more experienced than me (I am just doing some EAA for around 8 months), I would truly appreciate your opinion on the attached graph in this post (I am very worried about a mechanical problem).
After approx. 1 worm period (point 1), the mount suddenly sacks down, just to recover again. However, at pretty exactly the same spot of the worm wheel (point 2, which is after the 2nd worm period), there is no upward movement.
Could this be caused by periodic error as well or is it a hint that something is wrong with the worm wheel?

It could potentially be something mechanical, like a worm issue or some contamination in the mesh. A more likely scenario is 

something like flexure, where some part of your optical train shift ed. Could be at any point: camera in focuser, focuser itself relative to the optical tube, mirror settling, optical tube not mounted adequately.....

 

If it were me, I'd gather several cycles of PE data and analyze that. From there, create a PEC curve and upload it and see if you get improvements. I've used PEMPro several times, as well as PHD/EQMOD to accomplish this. Both do a nice job, one setup is free. 

 

Also, I don't know what type of exposures you are going for, but this mount isn't sufficient for going unguided for anything but the shortest of exposures or the shortest of focal lengths. 


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#10 rmollise

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Posted 21 March 2019 - 09:32 AM

Thanks for your answer. I am not sure about the level of polar alignment that I am achieving. I am polar aligning with the hour angle method (using the „SynscanInit“ app) and I make sure, that the polar star is always in the small bubble in the polar scope. This method is most likely not 100% precise, but the drift occurs on different nights with different levels of polar alignment. E.g. The
The mount drifted in one night around 80 pixels on the Y axis and around 50 pixels on the X axis in 937 seconds. However, on the other night it drifted only 70 pixels on the Y axis and 70 pixels on the X axis in 1241 seconds.
So it seems like there is an influence of the polar alignment but is it really that significant? Also the attached long-term drift (23 min, ca. 3 worm rotation periods) in post 1 (3rd picture) puzzles me since it seems too unstable to be caused by polar alignment. Looking forward to your opinion.

 

This degree of drift is what you can expect with a polar borescope alignment. Want more better gooder?

 

1. Drift alignment (easy to do, but requires patience).

2. Sharpcap (very inexpensive if you have a decent guide camera can be done in just a couple of minutes).

3. Polemaster (as good as Sharpcap if considerably more expensive).


Edited by rmollise, 21 March 2019 - 09:33 AM.

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#11 Astrob0b

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Posted 21 March 2019 - 06:00 PM

Thanks @ all for the input!
@austin.grant: I am going for very short exposures (1 – 4 seconds usually; sometimes 15 seconds) and stack a lot of them. So my objective is just to get some nice „live“ images. However, after 10-15 minutes my pictures have horrible borders from the drift and the stars are slightly egg shaped.

So my next steps will look like this:
1. Improve polar alignment by polar aligning with Sharpcap
2. Gathering PE data
3. Checking for flexure

 

I will keep you all updated about the results :-)


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#12 Astrob0b

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Posted 22 March 2019 - 07:38 PM

Luckily, I had clear skies today and my test setup was running fine:

1. I have achieved excellent polar alignment with Sharpcap (however, I might have rotated the RA axis not 100% accurately about 90°. In the end, I was running of out of battery to check the PA level a second time.)
2. I have collected 1h of PE data with PEMPro (2x 30 min due to trial version restriction)
3. I have checked all screws, the focuser, tripod legs etc. if something is moving. However, everything seemed fine.

Unfortunately, there was again a pretty bad drift which resulted in the much hated black borders (I have highlighted them in red in the attached picture of M42 (( 615 x 1 sec, total exposure: 10m 15s, live stacked in SharpCap) ).

I am not sure about the interpretation of the PEMPro graphs, so I have attached them as well. Two things caught my attention:

1. There was again a “jump” (highlighted with red arrows) which reminded me of the drift pattern from post #6 in this thread.
2. There is a pretty constant Y axis drift (does it mean, I could have messed up polar alignment again?)

What are your thoughts and what might be the next steps?  Any ideas are welcome :-) 

Attached Thumbnails

  • Drift_22.03.2019_highlighted.JPG
  • PEMPro_5 (X)_jump.JPG
  • PEMPro_Y drift 1st cycle.JPG


#13 austin.grant

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Posted 22 March 2019 - 10:17 PM

The PE looks about average for this class of mount. I've seen lower, and I've seen higher. Disregard the variations in your cycle to cycle PE patterns. The drift still looks like polar alignment to me.

 

Your imaging resolution appears to be somewhere around 0.77 arcseconds/pixel. In your last pic, the drift is about 15 pixels over the half-hour run. That's 12 arcseconds, over the half hour. In comparison, your Periodic Error is responsible for around 30 arcseconds of error over an 8 minute period. 

 

I'd say get an autoguider, or get used to these results. It would stop the drift, regardless of the cause. Then you'd only be left with field rotation from polar alignment errors. Only one way to fix that, nail the polar alignment. 


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#14 Astrob0b

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Posted 24 March 2019 - 07:07 AM

Great news: Proper polar alignment with SharpCap solved my problem. Even 16s exposures are no problem! It turned out, that my polar scope is miscalibrated so I could never achieve a decent polar alignment. I am really relived that it was no hardware problem!
So thanks to everybody who helped me.

@tomwall: I would recommend you to check your polar alignment with Sharpcap Pro. Maybe it will solve your problem as well or at least, you can exclude polar alignment from your list of possible causes.

However, my stars are still not round (even in 1 sec exposures, same shape across the complete image -> so I guess it`s not coma). I have attached a screenshot with some details. First thing which is puzzling me are the small spikes in the star pattern using a Bahtinov mask. Can it be caused by reflections or is it a hint for something else? It also seems like there is some dust/dirt on my camera. Can this result in this star shape as well?

Attached Thumbnails

  • Drift_24.03.2019_1.JPG
  • Drift_24.03.2019_2.JPG
  • Star pattern.JPG


#15 tomwall

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Posted 24 March 2019 - 09:58 AM

Thanks Bob. Happy to hear that you solved the issue. I like Sharpcap very much. It's on my shopping list, along with Startools.

 

I was able to get a handle on my elongated star images also. My issue turned out to be a drooping focuser tube. At maximum extension there was a tiny bit of sag that misaligned the light path. Snugging up the focuser lock tube screw lined things up again very nicely.

 

Clear Skies!

 

Tom,

Tucson


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#16 Astrob0b

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Posted 24 March 2019 - 11:34 AM

Great that you could fix your problem as well, Tom!  You have mentioned that you are also using the ASI224MC. While I was cleaning the protect window of mine today, I noticed that my camera is „rattling“ and the black ring around the sensor on the inside is moving around. Is it the same with your camera?

 

CS as well!



#17 tomwall

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Posted 25 March 2019 - 11:49 AM

Yikes!

I haven't had that issue. ASI has a FB group that is very helpful. ZWO/ASI techs checks in there often and helpful. Shake, rattle and roll can't be a good thing. Good Luck!

 

Tom,

Tucson




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