New CEM70 mount and poor guiding

[sai...@suddenlink.net]

I have new mount and the best I can obtain for Total RMS is about 0.5.   Here are the logs for my last sessions on M51:

https://openphdguiding.org/logs/dl/PHD2_logs_H9Ws.zip

[Tommy M]

What is your imaging scale?

On Mon, May 18, 2026 at 3:21 PM sai...@suddenlink.net <sai...@suddenlink.net> wrote:

I have new mount and the best I can obtain for Total RMS is about 0.5.   Here are the logs for my last sessions on M51:

https://openphdguiding.org/logs/dl/PHD2_logs_H9Ws.zip

--
You received this message because you are subscribed to the Google Groups "Open PHD Guiding" group.
To unsubscribe from this group and stop receiving emails from it, send an email to open-phd-guidi...@googlegroups.com.
To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/d2b05dd2-eff1-464a-9a74-17671b1f1710n%40googlegroups.com.

[Douglas K Duncan]

Why in the world do you care about better than 0.5”? Your seeing and imaging surely are not that good?

Sent from my iPhone

On May 18, 2026, at 2:41 PM, Tommy M <tomms...@gmail.com> wrote:



[External email - use caution]

To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/CAHZirjeiuO2TPA67k1pVZdMWnc8y33HR%2BgqEG2a-tuC3WRvxiw%40mail.gmail.com.

[Sailmm5]

Stellarvue SV115 (805mm) and ZWO ASI2600MC Pro (3,76 pixel size) = 0.96 imaging scale

Thanks for asking, Tommy.

John

From the iPad

On May 18, 2026, at 19:41, Tommy M <tomms...@gmail.com> wrote:



You received this message because you are subscribed to a topic in the Google Groups "Open PHD Guiding" group.
To unsubscribe from this topic, visit https://groups.google.com/d/topic/open-phd-guiding/e9U1W8c1pf0/unsubscribe.
To unsubscribe from this group and all its topics, send an email to open-phd-guidi...@googlegroups.com.
To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/CAHZirjeiuO2TPA67k1pVZdMWnc8y33HR%2BgqEG2a-tuC3WRvxiw%40mail.gmail.com.

[Sailmm5]

I would like it to be 0.2 to 0.3 which I believe it is capable of doing.

Thanks.

John

From the iPad

On May 18, 2026, at 20:47, 'Douglas K Duncan' via Open PHD Guiding <open-phd...@googlegroups.com> wrote:

 Why in the world do you care about better than 0.5”? Your seeing and imaging surely are not that good?

You received this message because you are subscribed to a topic in the Google Groups "Open PHD Guiding" group.
To unsubscribe from this topic, visit https://groups.google.com/d/topic/open-phd-guiding/e9U1W8c1pf0/unsubscribe.
To unsubscribe from this group and all its topics, send an email to open-phd-guidi...@googlegroups.com.
To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/8DF63BA9-A1AC-47AC-85A1-CC72C0428F2E%40colorado.edu.

[Brian Valente]

Hi John

I think you can improve your mount performance.

Here's what I see, including suggestions if you are really aiming for the best possible:

- the guiding assistant recommended improving yoru focus on the guidestar, it's worth looking into.

- a polar alignment error of 2.4arcmin can be improved. Consider drift alignment

- the GA run (unguided) wasn't long enough to really characterize the raw mount performance. If you're interested more detail here, do successful calibraiton followed by a GA run of 30 min uninterrupted. That will better characterize it

= If you had run the guiding assistant backlash test (looks like you canceled out of it) it would have told you you have enough backlash that auto backlash compensation should be enabled (and it should) consider rerunning including backlash measurement, or if you want to wag it, enable that feature and start at 400ms: it will eventually figure out the correct value. This should be relatively easy to clean up

- on the RA axis, the picture is a bit more complicated. Here is an analysis of segment 13 of PHD2_GuideLog_2026-05-12_130354.txt the using some newer tools:

This shows raw period error in fainter pink, and residual (uncorrected)  period error in green. Note the three numbers correlate to the top peaks listed at the bottom. you can see the effect of how guiding reduces these values to something quite manageable. However, it's about 0.50"

Even with relatively good guiding, your primary PE is still 0.53" on its own. Consider using PPEC at 360 seconds, aggressiveness of 80 on both reactive and predictive, and disable auto adjust period. 

The next two periods are harmonics of your primary: 96 seconds is 0.52" RMS and 172 seconds is 0.33". The 172sec is probably and more acurately a 2x of your primary PE, and likely related to the worm (maybe worm preload, worm bearings, etc)

 the 96 second is also at 0.52" but a bit more challenging because it's further down the drive train, probably in the gearbox/belt. 

I mention these because if your goal is sub-0.5", you are going to at least address the first two, and probably the need to address all three of these. 

Brian

>>>

To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/005B1F3E-9F3D-431E-A75D-863827290B4B%40suddenlink.net.

--

Brian 

Brian Valente

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

[Brian Valente]

PS

Your RMS based on the logs you sent is probably closer to 1-1.5" total RMS on average. 

M51 is a relatively easy target to guide on, being nearer to the pole. The best way to test your mount's performance is the GA run i previously mentioned: where the calibration assistant places your mount: at the intersection of the celestial equator and meridian. You can follow the GA run with 30 minutes guided output as well.

Brian

[John Harris Sailmm5]

Thank you so much Brian for your detailed suggestions. That is very helpful.  Now if you could just do something about this wonky weather we are having here in NW Louisiana.  :)

John

On May 19, 2026, at 12:11 AM, Brian Valente <bval...@gmail.com> wrote:



PS

Your RMS based on the logs you sent is probably closer to 1-1.5" total RMS on average. 

M51 is a relatively easy target to guide on, being nearer to the pole. The best way to test your mount's performance is the GA run i previously mentioned: where the calibration assistant places your mount: at the intersection of the celestial equator and meridian. You can follow the GA run with 30 minutes guided output as well.

Brian

On Mon, May 18, 2026 at 10:06 PM Brian Valente <bval...@gmail.com> wrote:

Hi John

I think you can improve your mount performance.

Here's what I see, including suggestions if you are really aiming for the best possible:

- the guiding assistant recommended improving yoru focus on the guidestar, it's worth looking into.

- a polar alignment error of 2.4arcmin can be improved. Consider drift alignment

- the GA run (unguided) wasn't long enough to really characterize the raw mount performance. If you're interested more detail here, do successful calibraiton followed by a GA run of 30 min uninterrupted. That will better characterize it

= If you had run the guiding assistant backlash test (looks like you canceled out of it) it would have told you you have enough backlash that auto backlash compensation should be enabled (and it should) consider rerunning including backlash measurement, or if you want to wag it, enable that feature and start at 400ms: it will eventually figure out the correct value. This should be relatively easy to clean up

- on the RA axis, the picture is a bit more complicated. Here is an analysis of segment 13 of PHD2_GuideLog_2026-05-12_130354.txt the using some newer tools:

To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/CAJa45i699QQj-yomDB9JZdCFS0PV771A0TaTXBOFDa-S8Ma2ZA%40mail.gmail.com.

[Douglas K Duncan]

Where do you get seeing that good? I’m envious.

Sent from my iPhone

To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/005B1F3E-9F3D-431E-A75D-863827290B4B%40suddenlink.net.