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AZ-EQ6 ALOT of periodic error


h0lesp

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Hi All!

I have been a member of this forum for some time, but this is my first post!

I bought an AZ EQ6 about 2 years ago, and have been happily imaging with a 1000mm  scope + guiding.

With the short focal length (and good images) I have never bothered with measuring the periodic error.

A few weeks ago i bought an GSO RC8 scope, and i started to see some problems with the tracking.

I Measured the mounts periodic error to over 80 arc-seconds!!! I have yet to find anyone on the internet with this amount of pe on an az-eq6!?

The mount is properly balanced, and there is little backlash.

I am really no expert, but the pe is very periodic, and seems to line up with the worm cycle, so I am hoping that this can be fixed by replacing the worm. Either switch RA with DEC worm, or buy a new one.

What are your thoughts on this?

What would you recommend me to do?

Thanks,

Espen

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80 arcsec is a massive amount of periodic error; worth double checking...having said that..pe peak to peak numbers are not necessarily that important, what is more important is the smoothness of the curve.

80 arcsec over a short worm period the curve would be very steep I would imagine and I doubt its correct.

Hope this helps a little

Ray

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Hi Ray!


Yes, I initially thought I did something wrong when measuring the pe. I have tested with both phd2 and EQMOD (AutoPec and pecprep).

The curve is pretty steep, and phd2 is struggling to keep up.


Here is a image from pecprep: (Captured using phd2 and autopec, EQMOD).


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Hi Espen...I am no expert and maybe someone with more knowledge will assist BT that curve shows little positive pe..its all negative and I see that as odd!!

I would assume that the pe would be broadly equal each side of zero...I accept that I may be wrong here!!!

Apart from that the curve looks quite smooth though at that level of pe its not zoomed, it appears to show a little roughness just at the top of the curve .

As you say the curve is quite steep and if it is indeed correct then guiding at 1x sidereal might be nessasary to keep up with the slope.

I would be inclined to test further rather than go out straight away for another worm, and make sure the set up is free of flexure, balanced with a bias East, and if practical running a long pe session (50min) at Dec zero just East of the meridian.

I can see from the stats that it appears to be at Dec zero and about 50 cycles, it also shows very few harmony's is it belt driven ?

Ray

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You'd really expect a PE of around 30 arcsecs, which is still pretty bad as a stark number but, when it's smooth, it guides out well.

I'd certainly do a careful drift alignment if you can. This explanation is a peach. http://www.andysshotglass.com/DriftAlignment.html If you're doing a software PA process I'd set it aside and do drift because it's a direct measurement and software free. I'd do it using the camera rather than an EP, though, assuming you can have some corsshairs appear on your capture screen.

Then I'd try again.

Olly

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I agree it doesn't look like 'normal' periodic error, perhaps the worm is binding and the motor doesn't have the torque to overcome it. I would be checking the worm to wormwheel clearance (preferably without the motor attached) and just 'feeling' how easily it turns. It may be the worm has an eccentric bearing.

ChrisH

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I'm not sure whether this is related to what you're seeing;

I have the same mount and can't get EQMOD's auto-pec to work and there are other reports of the same problem (the issue also seems to affect the EQ8) in the EQMOD Yahoo group.  I've read that the motor control firmware is reporting an incorrect ratio to EQMOD and auto-pec just doesn't work correctly with the AZ EQ6-GT / EQ8.  I've tried manually setting the mount ratios in EQMOD and it still doesn't work.

I also read that a few people were trying various things to manually create pec curves for their EQ8's to overcome this but there was still no clear route to a successful pec curve creation and results were very variable.

I also doubt that you're seeing a true PE of over 80 arc-seconds, I'd suggest asking on the EQ6 / EQMOD Yahoo groups and see what the guys there have to say.

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Thanks for all replies!
I am no expert and maybe someone with more knowledge will assist BT that curve shows little positive pe..its all negative and I see that as odd!!
I would assume that the pe would be broadly equal each side of zero...I accept that I may be wrong here!!!

I think that is because of the worm position when starting the pec trainging? 

That doesn't look correct at all. Have you selected the correct mount in PecPrep's menu?

Also check your polar alignment- make sure that's correct. Was the guiding corrections turned on or off during measurement?

Can you please tell me what does not look correct?

Yes, I was guiding when doing the AutoPec.

You'd really expect a PE of around 30 arcsecs, which is still pretty bad as a stark number but, when it's smooth, it guides out well. I'd certainly do a careful drift alignment if you can. 

Thanks for the instruction. I have not had time to do a drift aligment yet.

I have the latest motor controller firmware, and Autopec and EQMOD PEC seems to be working. 

I have uploaded 2 more images to the gallery. One with Pectraining over 5 worm periods, and a log from PHD2 showing with and without PEC.

You ca clearly see some drift due to poor Polar aligment I think? But this does not explain my 80 arc second PE. (PE is a lot less when using PEC).

I have testet pec with different focal lengths, and when canging to the correct FL in PHD2, I get the same result every time.. ALOT of PE.

So unless I am doing something very wrong, I think I actually have insane PE.  ??

I agree it doesn't look like 'normal' periodic error, perhaps the worm is binding and the motor doesn't have the torque to overcome it. I would be checking the worm to wormwheel clearance (preferably without the motor attached) and just 'feeling' how easily it turns. It may be the worm has an eccentric bearing.

Thanks. I am thinking about disassembling the mount, and see what i find :)

Espen

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Thanks for all replies!

I think that is because of the worm position when starting the pec trainging? 

Can you please tell me what does not look correct?

Yes, I was guiding when doing the AutoPec.

Thanks for the instruction. I have not had time to do a drift aligment yet.

Its a while since I did a PEC analysis, but from memory you have to have guiding commands disabled when running it. Otherwise you are just measuring your guiding performance.

Is your Polar Alignment good? If not, get that sorted first to prevent field rotation causing an issue.

Thanks. I am thinking about disassembling the mount, and see what i find :)

Why? You don't know if there's a problem yet.

Have you tried imaging with it?

Ripping it apart when you don't know if there even is an issue would seem like a recipe to introduce a host of problems.

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If you had guiding ENABLED whilst carrying out the PecPrep run you will have an incorrect plot. The plot you want is one that shows the actual PE, not one with any corrections.

Thanks for confirming that Steve. I thought that was the case.

EQMOD will allow you to record a PEC graph when guiding is running, but not PecPrep.

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Hi Zakalwe and Steve.

I am not sure if I made this clear, but I recored my PE graph through the feature AutoPec in EQMOD. I only opened the PE file in pecprep to show you the PE graph. (Guding was ON, and has to be ON when doing autopec. http://eq-mod.sourceforge.net/docs/eqmod_vs-pec.pdf.).

I get exactly the same graph if i record thorugh PHD2 (with guiding OFF).. 

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If you had guiding ENABLED whilst carrying out the PecPrep run you will have an incorrect plot. The plot you want is one that shows the actual PE, not one with any corrections.

That advice only applies when using an external guiding application for PE capture. With EQMOD's inbuilt AutoPEC function however  you actively guide on a star and EQMOD will work out your PE from the guide corrections issued to it. This allows you to image and capture PE / apply PEC simultaneously.

Chris.

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Hi Espen,

Have you checked you have entered the correct pixel size and focal length for your camera & scope in to PecPrep.  I have scared my self a couple of times when I punched in the wrong pixel size.

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Hi Espen,

Have you checked you have entered the correct pixel size and focal length for your camera & scope in to PecPrep.  I have scared my self a couple of times when I punched in the wrong pixel size.

I was thinking the exact same thing...

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1. I have the correct pixel size and focal length in PHD2. Have checked this multiple times. + phd is actually detecting the pixel size.

2. I have also measured my PE with EQMOD, AutoPec and PecPrep. You do not need to input Pixelsize or focal length, since EQMOD is using the guideinput to calculate the Error.

So, I am pretty sure my mount actually have a very high periodic error.

From what I can see, this error is very periodic with the worm, so I would like to know if anyone has any idea of how to attack this problem. I don't know much about the Mount mechanics..

Thanks :)

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That advice only applies when using an external guiding application for PE capture. With EQMOD's inbuilt AutoPEC function however  you actively guide on a star and EQMOD will work out your PE from the guide corrections issued to it. This allows you to image and capture PE / apply PEC simultaneously.

Chris.

Thank you for that correction, Chris - if anyone should know the detail, it's you! I have only used it with non-SW mounts in recent times ....

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  • 1 month later...

Hi All! 

Thank you for all your replies!

It has been a while, but I have now successfully dismantled my mount! (Three times!)

I have switched the RA worm with the DEC worm and replaced the worm bearings with SKF, and my periodic error is now about 25 arcseconds, peak to peak.

With Eqmods amazing PEC my "PE" is 8 arcseconds, peak to peak. Very easy to guide :) 

I am very happy with my equipment right now, and hoping to take some good images!

Clear skies!

Espen

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  • 1 year later...
On 25.09.2015 at 22:50, h0lesp said:

I have switched the RA worm with the DEC worm and replaced the worm bearings with SKF, and my periodic error is now about 25 arcseconds, peak to peak.

Espen,

Would you be so kind and share the part number from SKF?

I am also interested in replacing the barings.

Thanks a lot,

Tomasz.

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