Trouble getting stable guiding graphs in PHD2

[Luca Bartolucci]

Hello everyone,
My name is Luca and I am new to the forum.
I’m writing because I need some help—I can’t get a proper guiding graph in PHD2 to take decent photos.
I started using PHD2 with the Star Adventurer 2i mount and a 120 mm focal length guide scope. With that setup, I had no problems and never needed to change the program’s default settings.
Since June, I’ve bought a used Bresser Messier EXOS-2 GOTO mount and an RC 6” telescope. For guiding, I have a 70/400 mm guide scope and an ASI 120 MM camera. I know that for long focal lengths like the RC 6” it’s better to use an off-axis guider (which I already have), but I’m still struggling to get the guide star properly focused. Meanwhile, I’m testing the 70/400 mm guide scope.
Unfortunately, I’m not getting usable guiding graphs to properly run my imaging sessions.
Below is a link to my PHD2 log files. I’ve shared multiple guiding sessions with the respective Guide Assistant reports. In the latest log (06/10/2025), I tried adjusting several parameters like hysteresis, aggressiveness, minimum move, and calibration step size.
I would also mention that the mount recently had professional maintenance, including gear lubrication, and I was told all the ball bearings are intact. I verified the ST4 cable is working properly, and I cleaned the contacts on both the mount and the guide camera.
If there are existing forum threads about similar issues, I’d be happy to follow them and ask for clarifications if needed.
Thanks in advance for your help!
Best regards.

https://openphdguiding.org/logs/dl/PHD2_logs_wtAn.zip

[Bruce Waddington]

It's hard to give you a very complete analysis here.  You're using ST-4 guiding so we don't have any visibility into where the mount is pointing.  This is a substantial operational handicap.  The fact that there is evidently no supported ASCOM driver for the mount is probably a strong hint that the mount wasn't intended for deep-sky photography.  If you do a Google search on ' Bresser Messier EXOS-2 mount', you will see a lot of online discussions that point to the difficulties of trying to use the mount for imaging.  Someone saying that "all the ball bearings are intact" sets a pretty low bar - it's kind like buying a used car and being assured it still has all four wheels.  If we look at your results for one of the guiding sessions, we can see a number of problems (RA in red, Dec in green):

You can see the sidereal tracking in the mount is very rough, with lots of abrupt errors of 5-10 arc-sec.  I would say PHD2 actually did a reasonable job trying to control this, achieving a guiding RMS of 1.8 arc-sec.  But this is still very poor and probably won't meet your needs unless the imaging scope is lightweight and has a short focal length.  You can also see that the Dec axis is very unresponsive to guide commands, probably because of some combination of backlash and stiction.  

If we look at the native sidereal tracking without guiding corrections, it looks like this:

You can see there is at least a 60 arc-sec peak-peak periodic error, which is very large.  Even worse, you can see the numerous parts of the curve where the tracking errors become very steep.  It is very difficult for guiding software to cope with these kinds of things.  These results are pretty consistent with the online comments I found and mentioned earlier.

In its current mechanical condition, I don't think the mount is going to do any better than this and fiddling around with the guiding parameters is not likely to accomplish much.  I strongly recommend that you take a close look at the online discussions I mentioned earlier and decide if you have both the time and the mechanical skills to make the needed corrections to the mount.  Most mount "tune-ups" have no way to quantitatively measure the relatively tiny errors that can still cause substantial problems for auto-guiding and imaging.  Errors at these measurement scales require specialized equipment or, more likely, testing under the sky with actual guiding on a star.  A 10 arc-sec tracking error on your setup can be caused by a 20 micron displacement of the guide camera sensor - that's less than 1/2 the thickness of a human hair.

Hope you can figure out a workable path forward,

Bruce

[steve]

Hi and welcome.

I'd recommend replacing the bearings, cleaning and re-greasing.
Use either the ASCOM driver:
 https://github.com/Spitzbube/EXOS-2_GoTo_HandController/releases/download/v2.3/BRESSER_GOTO_SYSTEM_2018-06-05.zip

Or the INDI driver which is now integrated into the mainstream distribution:
https://docs.indilib.org/getting-started/
There's also a flatpak if you just want to get started quickly:
https://flathub.org/en/apps/org.kde.kstars

HTH and CS

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[Luca Bartolucci]

Dear Bruce and Steve,

First of all, I apologize for my late reply—I have been extremely busy with work lately.

Bruce, thank you very much for your thorough explanation and for all the advice you have provided. I truly appreciate the time you took to analyze my situation in such detail. I will read through the online discussions you mentioned and carefully consider what steps I should take next.

Steve, thank you as well for your suggestions. May I ask you: do you think that using the ASCOM drivers or switching to INDI could significantly improve the guiding performance with this mount, or would these changes make little difference given the mechanical limitations?

Thanks again to both of you for your help and support!

Best regards,
Luca

[Bruce Waddington]

ASCOM pulse-guiding doesn't usually produce more accurate guiding results but it greatly simplifies imaging operations.  To understand the benefits of using it, please read the User Guide section.:

https://openphdguiding.org/man-dev/Basic_use.htm#ASCOM_Benefits

In your case, it won't help because your problems are coming from mechanical issues with your setup.  That's what I was trying to explain earlier, you can't pave over these problems with software options.

Good luck,

Bruce

[Brian Valente]

Ascom connection also provides more diagnostic data to help with troubleshooting. 

Brian 

Brian Valente

astronomy portfolio https://www.astrobin.com/users/bvalente/ 

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