Persistent RA Oscillations on HEQ5 Pro (Belt Mod + New Bearings)

[NoahHD]

Dear PHD2 Community,

I am reaching out to get some expert feedback on my guiding performance with an HEQ5 Pro mount. Despite several mechanical upgrades, I am still struggling with a mediocre total RMS and a specific periodic issue in the RA axis.

My Setup:

• Mount: Skywatcher HEQ5 Pro (Rowan Belt Mod installed)

• Telescope: Quattro 150p

• Software: NINA, GSServer, PHD2 (Multi-Star Guiding enabled)

I recently replaced all factory bearings with high-quality SKF and FAG bearings. The Rowan Belt Kit is installed. I noticed a periodic error peak at approximately 122 seconds in my frequency analysis. Suspecting an eccentric motor pulley, I have recently attempted to re-center the motor pinion and ensure the grub screws are not tilting the pulley on the D-shaft. I have adjusted the worm gear to have a tiny amount of felt play to prevent stiction/binding.

My guiding logs show a persistent "sawtooth" pattern in the RA axis. While DEC is relatively quiet, RA seems to struggle with a rhythmic deviation that matches the motor's rotation period (~122s). This leads to a total RMS that is higher than expected for this level of tuning.

Due to bad weather, I haven't had the chance to test the latest pinion re-centering. I am planning to try the Predictive PEC (PPEC) algorithm next.

Looking at my setup, are there any other mechanical "usual suspects" I should check for this 122s oscillation?

Could a slightly over-tightened belt cause this kind of frequency peak, or is it strictly an eccentricity issue?

I have attached my latest Guide Log from a previous session (before the latest pinion adjustment) for reference. Any insights on how to push this mount into the "sub-0.7" RMS range" would be greatly appreciated!

Best regards

Daniel

[Brian Valente]

Hi Daniel

Here's what we can tell from your guidelogs (and this may be recap for you because it sounds like you've done some review of the logs):

you have three high PEs, presumably it's 1x, 2x, and 3x the primary. 1X is highest error and then is reduced with each harmonic, so that's pretty typical:

In the residual (uncorrected) error results, the larger period errors (1x, 2x) are pretty well handled by PHD, and again that's expected since longer periods are easier to guide out

the 122.7 second period is reduced from 1.5" to 0.35", but it is the highest remaining error so it's the constraint to your RA guiding:

As to the nature of this error, that answer is not something that comes from the guidelogs. Skywatcher or other SW users could comment on what is the source of that. You can see some unevenness in the RA calibration (the blue dots are not evenly spaced):

You can certainly use PPEC to at least test/address that error. you would set the period to 122.7 seconds, disable auto adjust, and I would start with predictive and reactive set to 70

You may also want to drop your exposure time to 1.5-2 seconds, since you are aiming for a higher frequency period. 

it takes a few cycles for the predictive part to kick in

Also in the future, please run a guiding assistant for a few worm cycles which produces unguided output and better shows the raw mount performance.

Brian

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Brian 

Brian Valente

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

[NoahHD]

Hi Brian, thank you very much for this detailed analysis! 

I will run the Guiding Assistant for at least 3-4 full worm cycles (approx. 30-45 minutes) without guiding, as you requested. I will test switching to the PPEC algorithm for RA with the settings you suggested and will also reduce my exposure time to 1.5s - 2.0s.

Once the weather allows, i will post the new logs and the Guiding Assistant results. Thanks again for pointing out the unevenness in the calibration dots :)

Best regards,

Daniel