Help with guiding on a iOptron CEM120

[Lance Smith]

Hi all,

I am asking from some help with my guiding.
My logs

https://openphdguiding.org/logs/dl/PHD2_logs_nz3x.zip

I am using an iOptron CEM120 with an OAG and an ASI174mm guide camera.
The telescope is a GSO RC 14, so I am running a long focal length. 2278mm

When my telescope is East pointing I get this strange DEC spikes at what appears to be random intervals. The next spike will always be the opposite direction of the previous.  I cant workout if this is backlash or stiction or something else.

When telescope is West facing, everything appears much more stable which makes me think it is a balance issue, but this is my first time with a telescope this size.

I have tried a few things myself.
- Rebalanced everything in both RA and DEC.
- Redone my polar alignment many times with tools like Nina 3 point alignment and also drift alignment.
- Many tweaks to algorithm in PHD2 with lower/ higher MinMo, aggressiveness etc.
- Cable snags. I have strapped down all the cables so that they shouldn't shift.

I really hope I am missing something obvious.

I appreciate help.
Regards Lance Smith

Quick image of issue and logs linked above.

[Brian Valente]

Lance

Well nothing is ever easy or simple with a setup like a 14" RC ;)

What are the specifics of your CEM120, do you have encoders on the Dec axis?

At first blush it looks to me like some stiction (possibly exacerbated by imbalance), but i'm not sure why you are using lowpass2 for the Dec algorithm and not resist switch

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Brian Valente

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[Bruce Waddington]

Hi Lance.  I think you have a mistaken impression of the problem, I don't think it's particularly tied to the side-of-pier position.  Here's a look at the Dec behavior when you were pointing to the west (side-of-pier east):

Unless you have a high-res encoder on the Dec axis - which I doubt - I think these big excursions are going to be originating in the payload that's riding on the mount rather than the mount drive system.  In any case, they aren't caused by guiding.  It strikes me that a 14-inch RC system is probably very heavy so is it supportable with the mount you have?  You may need to send a picture showing the scope and imaging payload, the cables, and how everything is connected to the mount.

Regards,

Bruce

[Lance Smith]

Thanks for the replies so far, it has me thinking for sure.

In terms of weight, the iOptron CEM 120 is supposed to handle 52kg excluding counterweights. My scope and equipment is around the 35kg mark. Heavy for sure but I "hope" workable.

After Bruce's comment about West not being great either I went back and checked older PHD2 logs as well as rechecking my cables. 
- West was definitely inconsistent, sometimes really good, sometimes less so.
- Opps I noticed the guide camera itself was slightly loose in the OAG. I had missed this when tightening everything else up. Sure it only moves fraction of mm but no doubt that adds up at 2200mm focal length.
- Therefore I am now hoping all my issues are just the loose guide camera and maybe why I saw different results West vs East were because of the angle the OAG is positioned on the imaging train and may tug slightly different.

On next clear night I am going to run it again and fingers crossed.

I'll keep you all posted.

Regards Lance

[Lance Smith]

Update:

I have been making some adjustments - 

- I re-balanced the scope to make sure it was slightly tail heavy.
- Made some small adjustment to the DEC worm gear tension.
- As per Brian's comment I have set algorithm to Resist Switch, I had gone with lowpass2 just because GA had recommended it.
  - Aggressiveness 65; MinMov 0.90

  - I do have backlash compensation on after a recommendation from Guiding Assistant 1500ms

and Things are looking better!

I do think I need to temper my expectation somewhat with the guiding accuracy. This is NOT an encoder mount, so I think RMS below 1" is good at 2200mm+ ?

Thank you for the help.

Regards Lance Smith.

[Brian Valente]

Hi Lance

Can you please upload your guidelogs again using the log uploader, we can offer you additional feedback

Regarding total RMS, there are three things : The accuracy you want based on the image scale of your imaging scope, and the accuracy you are able to achieve with your equipment and settings, and what is the limits of your skies/seeing conditions

Very generally speaking, you want your guiding to be sub-pixel accurate. For example, if your imaging scope has an image scale of 0.5"/pixel, you want your guiding to be less than or equal to 0.5". For your RC 14, I estimated the native resolution at 0.27"/pixel. It's unlikely your skies support anything close to that, so but keep this in mind

Next, your guiding total RMS is a rough indication of your tracking accuracy. If you are at 1", the error is about 4 pixels (1"/0.27'). Guiding is a combination of your seeing conditions and your settings. You can only do so much tweaking on your guiding before you run into the sky limitations. From your screenshot i think there's still more you can do to improve things. 

Finally, your skies and seeing conditions for this kind of setup are likely to be the limiting factor. Typical sky conditions for backyard imaging are maybe around 1.5"/2". You didn't say where you are imaging from.  

So basically you have a setup that theoretically has a very high resolution, but you are currently limited by your tracking performance (including guiding) and likely your seeing conditions.

I would probably look at binning your imaging camera 2 or even 3, that would put your system closer to what's possible

Hope that helps

Brian 

To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/ca717f0d-da66-4b1c-afaa-ed84ab8207b3n%40googlegroups.com.

[George Shoup]

Excellent note Brian.

George

Sent from my iPhone

On Jan 3, 2026, at 8:34 AM, Brian Valente <bval...@gmail.com> wrote:



Hi Lance

Can you please upload your guidelogs again using the log uploader, we can offer you additional feedback

Regarding total RMS, there are three things : The accuracy you want based on the image scale of your imaging scope, and the accuracy you are able to achieve with your equipment and settings, and what is the limits of your skies/seeing conditions

Very generally speaking, you want your guiding to be sub-pixel accurate. For example, if your imaging scope has an image scale of 0.5"/pixel, you want your guiding to be less than or equal to 0.5". For your RC 14, I estimated the native resolution at 0.27"/pixel. It's unlikely your skies support anything close to that, so but keep this in mind

Next, your guiding total RMS is a rough indication of your tracking accuracy. If you are at 1", the error is about 4 pixels (1"/0.27'). Guiding is a combination of your seeing conditions and your settings. You can only do so much tweaking on your guiding before you run into the sky limitations. From your screenshot i think there's still more you can do to improve things. 

Finally, your skies and seeing conditions for this kind of setup are likely to be the limiting factor. Typical sky conditions for backyard imaging are maybe around 1.5"/2". You didn't say where you are imaging from.  

So basically you have a setup that theoretically has a very high resolution, but you are currently limited by your tracking performance (including guiding) and likely your seeing conditions.

I would probably look at binning your imaging camera 2 or even 3, that would put your system closer to what's possible

Hope that helps

Brian 

On Fri, Jan 2, 2026 at 11:14 PM Lance Smith <la...@maxmiko.com> wrote:

Update:

I have been making some adjustments - 

- I re-balanced the scope to make sure it was slightly tail heavy.
- Made some small adjustment to the DEC worm gear tension.
- As per Brian's comment I have set algorithm to Resist Switch, I had gone with lowpass2 just because GA had recommended it.
  - Aggressiveness 65; MinMov 0.90

  - I do have backlash compensation on after a recommendation from Guiding Assistant 1500ms

and Things are looking better!

I do think I need to temper my expectation somewhat with the guiding accuracy. This is NOT an encoder mount, so I think RMS below 1" is good at 2200mm+ ?

Thank you for the help.

Regards Lance Smith.

To view this discussion visit https://groups.google.com/d/msgid/open-phd-guiding/CAJa45i6%2BydHgeKMAhaLqbz2ayjONo%2B_DHECYmNMtox4EL1Yf-w%40mail.gmail.com.

[Lance Smith]

Here are my latest logs https://openphdguiding.org/logs/dl/PHD2_logs_EMah.zip
Log section 1-6 I was still making physical tweaks to the setup before the main imaging session that went both West and East side of pier.

In terms of seeing, I am in a bortle 2-3 area in regional Australia. Summer here and the imaging I started last night was with atmospheric conditions in the mid 20-c temperature and most likely some dust haze in the air.

Regards Lance.