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GEM28 PHD2 Guiding issue, help needed

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#1 snlian

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Posted 12 May 2021 - 12:51 AM

Bought a new GEM28 and it's very light weight and well machined.

 

I'm using a 240mm focal length guide scope + ZWO ASI120MM-S Monochrome CMOS for phd2 guiding. Payload is well below 70%.

 

The iploar tool is perfectly nice, very quick to do polar alignment and super accurate.

 

As I'm using the mount for almost 2 months, I'm getting some weird test results using PHD2 guiding, DEC always jumps suddenly. 

 

Here's a post for my initial testing:

https://www.cloudyni...g-and-thoughts/

 

Capture.JPG

Capture1.JPG

Capture3.JPG

 

I do have several questions:

 

1) is the orthogonality error of 1.8deg normal?

2) When I use guiding assistant to log data for 30mins, the RA axis data looks really weird, may I know why? 

 

I also took a 20mins long exposure pic with my main camera while using guiding assistant. Appreciate any input!

Capture4.JPG

 



#2 michael8554

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Posted 12 May 2021 - 05:21 AM

Hi snlian

 

You have good PA,  1.4arcmins

 

The Calibration looks okay, measured RA and Dec rates are similar, Ortho error is small.

 

Now the Guide Assistant runs the mount with RA and Dec guiding off.

 

So the red Dec line has a little southward drift - no problem, your PA was good enough to guide with.

 

The blue RA line is showing a feature that almost every mount has - Periodic Error.

 

That's caused by the worm and worm wheel not being absolutely round.

 

So the roughly sine-wave appearance of the blue line usually has a period of how long the worm takes to make one revolution, often 4 minutes.

 

And that causes the snaky exposure.

 

It appears to be roughly +/- 15arcsecs in amplitude.

 

iOptron quote +/- 10arcsecs, perhaps your figure is within the range of production tolerances, GEM28 users please comment.

 

If you're going to guide then you're good to go.

 

If not guiding you might want to train the mount's Periodic Error Correction.


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#3 snlian

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Posted 12 May 2021 - 10:36 AM

Hi snlian

 

You have good PA,  1.4arcmins

 

The Calibration looks okay, measured RA and Dec rates are similar, Ortho error is small.

 

Now the Guide Assistant runs the mount with RA and Dec guiding off.

 

So the red Dec line has a little southward drift - no problem, your PA was good enough to guide with.

 

The blue RA line is showing a feature that almost every mount has - Periodic Error.

 

That's caused by the worm and worm wheel not being absolutely round.

 

So the roughly sine-wave appearance of the blue line usually has a period of how long the worm takes to make one revolution, often 4 minutes.

 

And that causes the snaky exposure.

 

It appears to be roughly +/- 15arcsecs in amplitude.

 

iOptron quote +/- 10arcsecs, perhaps your figure is within the range of production tolerances, GEM28 users please comment.

 

If you're going to guide then you're good to go.

 

If not guiding you might want to train the mount's Periodic Error Correction.

Thank you Michael! But do you know what would cause the RA drift up over time?



#4 42itous1

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Posted 12 May 2021 - 10:56 AM

It looks to me like you have alot of backlash for a belt driven dec axis.  I think the belt adjustment is easy--although I never owned an Ioptron mount.  The RA errors will guide out easily. The dec will also guide out easily--but you might want to adjust the bl to get rid of the dec jumps.



#5 snlian

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Posted 12 May 2021 - 06:59 PM

It looks to me like you have alot of backlash for a belt driven dec axis.  I think the belt adjustment is easy--although I never owned an Ioptron mount.  The RA errors will guide out easily. The dec will also guide out easily--but you might want to adjust the bl to get rid of the dec jumps.

I've tightened the belt to the best I could..



#6 OzAndrewJ

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Posted 12 May 2021 - 07:04 PM

Gday snlian

Do you have the PHD guide and debug log???

If so, can you zip it up and post it here.

The calibrate looks quite good for an IOptron :-)

As noted, the RA looks like simple and smooth PE, but it also appears to have a ripple on top of it

that might also be belt related.

 

Andrew Johansen Melbourne Australia



#7 snlian

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Posted 12 May 2021 - 08:57 PM

Gday snlian

Do you have the PHD guide and debug log???

If so, can you zip it up and post it here.

The calibrate looks quite good for an IOptron :-)

As noted, the RA looks like simple and smooth PE, but it also appears to have a ripple on top of it

that might also be belt related.

 

Andrew Johansen Melbourne Australia

Thanks Andrew, log attached. Isn't the RA should be a sine wave? It's keep drifting upwards..



#8 snlian

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Posted 12 May 2021 - 08:57 PM

Gday snlian

Do you have the PHD guide and debug log???

If so, can you zip it up and post it here.

The calibrate looks quite good for an IOptron :-)

As noted, the RA looks like simple and smooth PE, but it also appears to have a ripple on top of it

that might also be belt related.

 

Andrew Johansen Melbourne Australia

Thanks Andrew, log attached. Isn't the RA should be a sine wave? It's keep drifting upwards..

Attached Files



#9 OzAndrewJ

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Posted 12 May 2021 - 09:19 PM

Gday Snlian

The fact the RA is drifting up can be caused by many things

1) Wrong base tracking rate

2) Polar misalignment

3) Flexure

4) Refraction effects

 

As long as it is smooth and steady, guiding will easily fix it.

 

Andrew Johansen Melbourne Australia



#10 StephenW

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Posted 12 May 2021 - 09:30 PM

>As I'm using the mount for almost 2 months, I'm getting some weird test results using PHD2 guiding, DEC always jumps suddenly.

 

Looking at your guide log there are some examples where DEC is jumping e.g.

 

dec_jump.png

 

It looks like DEC backlash may be causing the mount to respond badly - I would try disabling the DEC backlash option and letting it guide again for an extended period of time and reposting your logs.

 

 


Edited by StephenW, 12 May 2021 - 09:31 PM.


#11 OzAndrewJ

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Posted 12 May 2021 - 10:13 PM

Gday snlian

OK, firstly looking at the RA, there is a superimposed ripple of about 1 arcsec pk-pk

that occurs 28x per rev of the worm.

Guided RA.jpg

It is still there and swamped by the main PE when unguided, but clearly shows as a large residual when guiding.

I dont have details for the gears in the 28GEM but the ZEQ25 uses a 28 tooth gear

on the worm. If your mount has a 28 toother on the worm,

then this ripple is more than likely going to be due to belt tension.

Can you see the gears/belt and get the toothcounts/Belt ID when you tension them????

 

As to DEC, you appear to have VERY large backlash.

Have attached a plot of the guided DEC, as well as pulses sent.

DEC guiding.jpg

In the first part of the plot, you were very well balanced in DEC

and thus the mount swung too and fro whilst trying to account for the lash.

On the other side, you had enough DEC drift to ensure that after the first reversal

the mount only ever guided in one direction.

What you can see is the mount drifted for quite a distance before it corrected

and then it slightly over corrected, ( thus giving a spike ),

and then it repeated the process when the drift triggered the limits again.

A smaller min move and tweaks to the guide algorithm should fix that,

but it wont help with the lash.

Andrew Johansen Melbourne Australia


Edited by OzAndrewJ, 12 May 2021 - 10:14 PM.

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#12 snlian

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Posted 12 May 2021 - 11:56 PM

Gday snlian

OK, firstly looking at the RA, there is a superimposed ripple of about 1 arcsec pk-pk

that occurs 28x per rev of the worm.

attachicon.gifGuided RA.jpg

It is still there and swamped by the main PE when unguided, but clearly shows as a large residual when guiding.

I dont have details for the gears in the 28GEM but the ZEQ25 uses a 28 tooth gear

on the worm. If your mount has a 28 toother on the worm,

then this ripple is more than likely going to be due to belt tension.

Can you see the gears/belt and get the toothcounts/Belt ID when you tension them????

 

As to DEC, you appear to have VERY large backlash.

Have attached a plot of the guided DEC, as well as pulses sent.

attachicon.gifDEC guiding.jpg

In the first part of the plot, you were very well balanced in DEC

and thus the mount swung too and fro whilst trying to account for the lash.

On the other side, you had enough DEC drift to ensure that after the first reversal

the mount only ever guided in one direction.

What you can see is the mount drifted for quite a distance before it corrected

and then it slightly over corrected, ( thus giving a spike ),

and then it repeated the process when the drift triggered the limits again.

A smaller min move and tweaks to the guide algorithm should fix that,

but it wont help with the lash.

Andrew Johansen Melbourne Australia

Hi Andrew, really appreciate your detailed explanation! The mount is using B60MXL timing belt with 28 teeth, does this mean the belt is too tight? I also turned on dithering btw.

 

The guiding assistance shows a 20 arc sec DEC backlash which matches your calculation, I will tune the min step and guide on one direction tomorrow to see if it helps.

 

Many many thanks for your time and feedbackbow.gif bow.gif bow.gif


Edited by snlian, 12 May 2021 - 11:56 PM.


#13 OzAndrewJ

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Posted 13 May 2021 - 12:14 AM

Gday snlian
 

The mount is using B60MXL timing belt with 28 teeth, does this mean the belt is too tight?

I had shocking ripple on my AZEQ5 and in my case, i had to tighten the belt.

That said, if you have a 28 tooth pulley on the worm, then the numbers indicate

there needs to be a 14 toother on the motor.

I know that some retrofit kits for synta mounts used small dia belt pulleys

and these caused ripple due to the small dia involved, so not really sure the cause.

Just going to be try and see.

 

I also turned on dithering btw.

Where????? I can see no dither commands in the PHD logs

so is another app sending them ??????

 

Andrew Johansen Melbourne Australia



#14 snlian

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Posted 13 May 2021 - 12:17 AM

Gday snlian
 

I had shocking ripple on my AZEQ5 and in my case, i had to tighten the belt.

That said, if you have a 28 tooth pulley on the worm, then the numbers indicate

there needs to be a 14 toother on the motor.

I know that some retrofit kits for synta mounts used small dia belt pulleys

and these caused ripple due to the small dia involved, so not really sure the cause.

Just going to be try and see.

 

Where????? I can see no dither commands in the PHD logs

so is another app sending them ??????

 

Andrew Johansen Melbourne Australia

Thanks Andrew, although I turned on dithering, I don't really see it from the log, that's another thing confuses me.



#15 42itous1

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Posted 13 May 2021 - 08:30 AM

Thanks Andrew, although I turned on dithering, I don't really see it from the log, that's another thing confuses me.

Dithering is controlled by the acquisition software e.g. SGP-NINA-Voyager, etc.


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#16 larryjh

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Posted 13 May 2021 - 11:26 AM

This is very interesting.  I have a CEM26 which I'm sure has the same motor/belt/worm as the GEM28.  I have also found the 28x (~21.4 sec) harmonic error in my mount.  Here is the topic I posted on it with some comments by a few of our very knowledgeable folks here:

https://www.cloudyni...uency-analysis/

I've been thinking that it might be possible to let the mount PEC correct the 600 sec worm error and get PHD2 to correct the 21.4 sec error by forcing the PHD2 predictive PEC algorithm to the 21.4 sec period.  I have only experimented a little bit with it and so far not had success.  I'm not entirely sure the mount behaves well when PEC is turned on and autoguiding is also on.  I've had one session where it appeared to work with PEC and guiding and one where it did not.  My best sessions so far have been setting PHD2 PPEC to 600 sec and living with the 28x harmonic.



#17 snlian

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Posted 13 May 2021 - 11:35 AM

Dithering is controlled by the acquisition software e.g. SGP-NINA-Voyager, etc.

dumb question: phd2 dithering still needs another sw to do actual dithering?



#18 42itous1

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Posted 13 May 2021 - 11:46 AM

dumb question: phd2 dithering still needs another sw to do actual dithering?

yes



#19 42itous1

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Posted 13 May 2021 - 11:50 AM

dumb question: phd2 dithering still needs another sw to do actual dithering?

Dithers are set for in between image captures.  The software that sets the imaging parameters--such as length of sub, number of subs--also sets when those dithers occur.  For instance--after very capture for 5 min subs.  Or perhaps every 3rd image for 2min subs.


Edited by 42itous1, 13 May 2021 - 11:51 AM.

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#20 OzAndrewJ

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Posted 13 May 2021 - 05:29 PM

Gday Larryjh

I have a CEM26 which I'm sure has the same motor/belt/worm as the GEM28.

I have no first hand reports from users, but the published data would indicate they share the same drive system,

so yes, more than likely the ripple is belt related.

As to PEC built into the IOptron mount itself, no idea but lots of people report it doesnt play well with guiding.

Andrew Johansen Melbourne Australia



#21 OzAndrewJ

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Posted 13 May 2021 - 05:34 PM

Gday 42itous1

 

The software that sets the imaging parameters--such as length of sub, number of subs--also sets when those dithers occur.

Agreed, but in all the logs i have seen to date, the app tells PHD to dither,

and then PHD sets a new lock position, does the dither, and waits for it to settle before starting the next sub.

It logs this process in the logs.

The log currently posted doesnt show any of this happening so maybe the external app

has done the dithers itself by using a manually sent pulse guide command???

ie PHD knows nothing about it

( but if thats the case, then PHD wont relock on a new position so would guide back ??? )

Would need to see the debug logs to see if thats what has happened,

as the guide logs strip all that sort of stuff out.

Andrew Johansen Melbourne Australia



#22 snlian

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Posted 14 May 2021 - 01:41 AM

Gday 42itous1

 

Agreed, but in all the logs i have seen to date, the app tells PHD to dither,

and then PHD sets a new lock position, does the dither, and waits for it to settle before starting the next sub.

It logs this process in the logs.

The log currently posted doesnt show any of this happening so maybe the external app

has done the dithers itself by using a manually sent pulse guide command???

ie PHD knows nothing about it

( but if thats the case, then PHD wont relock on a new position so would guide back ??? )

Would need to see the debug logs to see if thats what has happened,

as the guide logs strip all that sort of stuff out.

Andrew Johansen Melbourne Australia

Hi Andrew, I did more testing today, with smaller DEC steps and disabling the compensation, it does look better. But still jumps here and there.

Attached Files



#23 OzAndrewJ

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Posted 14 May 2021 - 02:25 AM

Gday snlian

The RA still shows good guiding but still has the 28x fundamental, but the DEC was woofy.

The PHD plot shows it was guiding for a while, then DEC deviated from lock but no pulses got sent to correct it.

It looks like it was set to only guide in one direction ( ie set to Guide North only )??

but the polar alignment was good enough that there wasnt enough drift for this to work properly.

As such, when it did drift North by itself, no correcting guides got sent.

 

Andrew Johansen Melbourne Australia



#24 larryjh

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Posted 14 May 2021 - 08:44 AM

Here's a few more thoughts (from a beginner) regarding the 28x harmonic.  I see that you are using the hysteresis algorithm in RA and 4 second frames.  The fact that PHD can't guide out this harmonic says that even though the period is something like 21.4 seconds, it is likely not a smooth curve over that period but rather a repeating step function that has a duration of less than 4 seconds and a period of 21.4 seconds.  That would explain why it doesn't get corrected.  I usually use 2 second frames and generally am not able to correct it either.  I use the predictive PEC algorithm in RA and have tried either letting the period float or setting it fixed to 600 sec.  I have had a few sessions where the 28x component is pretty small (around 0.2" is I think the best I've seen) but it's usually more like 0.4-0.6" in the frequency analysis.  So, I conclude that probably the only way to correct it would be by using the PPEC in PHD or having a very short frame time (probably too short to be useable).

 

So, bottom line is, I would recommend using the PPEC algorithm, the shortest useable frame time, and experimenting with fixed or floating periods for the PPEC and/or trying to get it to work with the mount's PEC (most people say that won't work).  I can't really tell from the PHD documentation whether PPEC can handle multiple periods.  I suspect it can only if one period is truly an exact multiple of the other, which may be the case here.  Anyway, maybe with enough people experimenting we can find a "best practice" for this mount that minimizes the RA errors.


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#25 StephenW

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Posted 14 May 2021 - 09:12 AM

Yeah, DEC was set to guide North only.   Looking back at the previous logs, it has the same problem:  DEC guiding for North only.   "Auto" was enabled later in the first log and improved things a bit - best to just leave DEC guiding as auto for now. 




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