What is Backlash?

[Astro_mark_c]

Last night there was a small break in the cloud so I set up and tried auto guiding for the second ever time using PHD2, as you can see from the image my graph was crazy and it kept warning me about backlash, I have no idea what that is or how to fix it?

I have the Skywatcher EQM 35 pro mount. 

Any help is appreciated,

Thanks

[kens]

Backlash is a common problem in gears that occurs when they change direction. It takes some time for the gear teeth to make contact in the new direction before the driven gear starts to move. On a mount this most often occurs on the declination axis as the RA axis does not change direction while tracking. It can also occur on the RA axis after slewing.

You can adjust your gear mesh to minimise backlash. Then you can try using the PHD2 Guiding Assistant to measure your backlash and then apply the Backlash compensation tool in PHD2. This wil only work if the backlash is not too severe.

You can also force PHD2 to duide in declination in one direction only. For this to work you can deliberately maintain a Polar Alignment error that ensures a steady ddeclination drift in one direction and only guide against that drift.

I suggest you read up on the Guiding Assistant then run it. https://openphdguiding.org/manual/?section=Tools.htm#Guiding_Assistant

You can attach the guide log here or on the PHD2 forum for more advice. https://openphdguiding.org/getting-help/

The guide log location can be found from the Help menu. (Screen shots are generally not useful)

[Seelive]

How free is your declination axis to turn? It should be really free. My first attempt at guiding with my EQM-35 gave a similar poor response in the dec axis which some how also affected the RA axis. It was due to stiction, I loosened the dec axis and all was good. I've not adjusted the gears to reduce the backlash but here's a typical guide graph that I get.

[Astro_mark_c]

5 hours ago, Seelive said:

How free is your declination axis to turn? It should be really free. My first attempt at guiding with my EQM-35 gave a similar poor response in the dec axis which some how also affected the RA axis. It was due to stiction, I loosened the dec axis and all was good. I've not adjusted the gears to reduce the backlash but here's a typical guide graph that I get.

I don't fully tighten the black clutch thing as I read that was bad and could effect your tracking.   I might have tightened it too much, or the dec in and dec out cable might of been restricting it's movement or even the guide cam wire.  I was definitely balanced a long DEC.  I'll make sure to check all this next time!

[Seelive]

24 minutes ago, Sidecontrol said:

I don't fully tighten the black clutch thing as I read that was bad and could effect your tracking.   I might have tightened it too much, or the dec in and dec out cable might of been restricting it's movement or even the guide cam wire.  I was definitely balanced a long DEC.  I'll make sure to check all this next time!

I meant how free does it turn with the clutch fully released (it shouldn't move at all when the clutch is engaged). If you don't fully engage the clutch then the scope could move or not move in response to the motor movement. I think overtightening the clutch is a problem on a different SW mount, not the EQM-35. I lock mine fully.

[Astro_mark_c]

4 minutes ago, Seelive said:

I meant how free does it turn with the clutch fully released (it shouldn't move at all when the clutch is engaged). If you don't fully engage the clutch then the scope could move or not move in response to the motor movement. I think overtightening the clutch is a problem on a different SW mount, not the EQM-35. I lock mine fully.

Ah gotcha, it moves freely when i release the clutch, no stiffness at all. 

[Astro_mark_c]

23 hours ago, kens said:

Backlash is a common problem in gears that occurs when they change direction. It takes some time for the gear teeth to make contact in the new direction before the driven gear starts to move. On a mount this most often occurs on the declination axis as the RA axis does not change direction while tracking. It can also occur on the RA axis after slewing.

You can adjust your gear mesh to minimise backlash. Then you can try using the PHD2 Guiding Assistant to measure your backlash and then apply the Backlash compensation tool in PHD2. This wil only work if the backlash is not too severe.

You can also force PHD2 to duide in declination in one direction only. For this to work you can deliberately maintain a Polar Alignment error that ensures a steady ddeclination drift in one direction and only guide against that drift.

I suggest you read up on the Guiding Assistant then run it. https://openphdguiding.org/manual/?section=Tools.htm#Guiding_Assistant

You can attach the guide log here or on the PHD2 forum for more advice. https://openphdguiding.org/getting-help/

The guide log location can be found from the Help menu. (Screen shots are generally not useful)

Thanks, I'll give those links a look through. 

[newbie alert]

First thing id look at is balance, balance is key in guiding but as yours is osolating both sides of the trend I'd suggest either your settings are way out, so have you run the guiding assistant and apply the settings?  , or if you're pulse guiding you've not calibrated on the right part of the sky? Could be many reasons just got to eliminate them one by one 

[Astro_mark_c]

On 13/01/2021 at 08:12, newbie alert said:

First thing id look at is balance, balance is key in guiding but as yours is osolating both sides of the trend I'd suggest either your settings are way out, so have you run the guiding assistant and apply the settings?  , or if you're pulse guiding you've not calibrated on the right part of the sky? Could be many reasons just got to eliminate them one by one 

I did balance my scope, but i'll do a double check next time (i leave it set up).  For PHD2 I have just been using the default settings.  I guess I'll just have to troubleshoot next time there is a clear night. 

[newbie alert]

3 hours ago, Sidecontrol said:

I did balance my scope, but i'll do a double check next time (i leave it set up).  For PHD2 I have just been using the default settings.  I guess I'll just have to troubleshoot next time there is a clear night. 

Run the guiding assistant and apply the settings.. it be completely different 

[Astro_mark_c]

Swear I’m cursed, any idea what this means? 

[JeremyS]

Backlash? It’s what I do on yet another cloudy night when I can’t get benefit from the £££ spent on my telescopes

🙂

 

[Seelive]

Have a look here https://openphdguiding.org/man-dev/Trouble_shooting.htm and try some of the recomedations for overcoming problems with declination backlash?

[michael8554]

Really need the PHD2 GuideLog to check your setup,  the quality of the Calibration, and your guide settings.

Regarding the Dec Backalsh, if your Dec balance is perfect then the mount is free to wobble from one end of the Backlash to the other.

So balance Dec slightly off so that Dec is always at one end of its Backlash travel.

And a slight PA drift in the same direction will help, PHD2 will cope.

Michael

[Astro_mark_c]

1 hour ago, michael8554 said:

Really need the PHD2 GuideLog to check your setup,  the quality of the Calibration, and your guide settings.

Regarding the Dec Backalsh, if your Dec balance is perfect then the mount is free to wobble from one end of the Backlash to the other.

So balance Dec slightly off so that Dec is always at one end of its Backlash travel.

And a slight PA drift in the same direction will help, PHD2 will cope.

Michael

Where would I find these logs? Are they sorted on my pc or are they lost when I close down PHD2? 

[Seelive]

34 minutes ago, Sidecontrol said:

Where would I find these logs? Are they sorted on my pc or are they lost when I close down PHD2? 

Mine are found in Documents-PHD2.  There are 2 types of file, Debug logs and Guide logs. Finding them is the easy part, understanding them is a different matter! They are just text files so you can open them with any text viewer and you can download a Guide log viewer from

https://adgsoftware.com/phd2utils/

that recreates the guiding graphs and most other information from whichever session you select.

[JOC]

I couldn't resist commenting based on what my first thought was on seeing the thread title: 

Backlash is what you get when the other half finds out you've ordered more astronomy gear!!

[Astro_mark_c]

Just a thought but my ASCOM PulseGuide Settings wouldn’t be affecting my PHD2 would it? When I set it up I kept everything default. 

[Astro_mark_c]

37 minutes ago, Seelive said:

Mine are found in Documents-PHD2.  There are 2 types of file, Debug logs and Guide logs. Finding them is the easy part, understanding them is a different matter! They are just text files so you can open them with any text viewer and you can download a Guide log viewer from

https://adgsoftware.com/phd2utils/

that recreates the guiding graphs and most other information from whichever session you select.

hi, 

found the guide logs and downloaded the guide log viewer, how do you want me to present the info? will i upload the guild logs or can i do something with the guide log viewer?

[Astro_mark_c]

Guide Logs in text files from my last 3 attempts.

 

The attempt with last night, I was having issues with EQMOD stopping my mount from tracking (since sorted this). 

PHD2_GuideLog_2021-01-06_123653.txt PHD2_GuideLog_2021-01-11_221505.txt PHD2_GuideLog_2021-01-16_194038.txt

Edited January 17, 2021 by Sidecontrol

[Seelive]

9 minutes ago, Sidecontrol said:

hi, 

found the guide logs and downloaded the guide log viewer, how do you want me to present the info? will i upload the guild logs or can i do something with the guide log viewer?

I would suggest you upload the logs to a file sharing site and post a link so that people can then download them and do there own analysis.

... but I've just seen you've uploaded the here ...

Edited January 17, 2021 by Seelive
Crossed posts

[michael8554]

I'll have a look at your PHD2 GuideLogs.

Looking at your EQMOD image, you have ASCOM PulseGuide Setting at x0.10.

That's a very low rate which will make Calibration and guiding difficult.

Set it to at least x0.5 (+/- 0.5 of Sidereal Rate)

Michael

[Astro_mark_c]

5 minutes ago, michael8554 said:

I'll have a look at your PHD2 GuideLogs.

Looking at your EQMOD image, you have ASCOM PulseGuide Setting at x0.10.

That's a very low rate which will make Calibration and guiding difficult.

Set it to at least x0.5 (+/- 0.5 of Sidereal Rate)

Michael

Ah great thanks for that. This was never mentioned in the tutorial I was following to set up EQMod... only came across it when researching PHD2 troubleshooting. 

[michael8554]

It all went pear-shaped when you switched to EQMOD with that low Guide Rate setting at 23:31 on the 11th.........

Your guidescope focus could be better, get the HFD figure in the Star Profile to lowest figure, about 3 to 4 pixels 

Look at these from the PHD2 Developers:

https://openphdguiding.org/phd2-best-practices/

https://github.com/OpenPHDGuiding/phd2/wiki/EQASCOM-Settings

Recalibrate at Dec 0 after nudging north as in the best practices, run the PHD2 Guide Assistant and accept the settings, guide for 10 minutes without altering anything, close PHD2, and post that log.

Michael

 

[Astro_mark_c]

13 minutes ago, michael8554 said:

It all went pear-shaped when you switched to EQMOD with that low Guide Rate setting at 23:31 on the 11th.........

Your guidescope focus could be better, get the HFD figure in the Star Profile to lowest figure, about 3 to 4 pixels 

Look at these from the PHD2 Developers:

https://openphdguiding.org/phd2-best-practices/

https://github.com/OpenPHDGuiding/phd2/wiki/EQASCOM-Settings

Recalibrate at Dec 0 after nudging north as in the best practices, run the PHD2 Guide Assistant and accept the settings, guide for 10 minutes without altering anything, close PHD2, and post that log.

Michael

 

Thanks for this, much appreciated!