Struggling to guide my Star Adventurer GTi with PHD2 Sky-Watcher Star Adventurer GTi · Maxence d. C. · ... · 18 · 554 · 21

Hello,

I first got into astrophotography two years ago and quickly began accumulating gear slowly but surely as the months went by. I recently acquired a guide scope and a guide camera to improve my exposure times, which were previously limited to ~45s to 60s depending on the quality of PA.
Before describing my issue, I'll just list out what my current equipment consists of, in case it matters:

• Main imaging camera : ZWO ASI533MC Pro
• Scope : TS Optics Imaging Star APO65Q (a nice discontinued quadruplet refractor, 420 mm of focal length, 65 mm of aperture)
• Guide scope : SV165 from SVBony
• Guide camera : ZWO ASI120mm mini
• Mount : Sky-Watcher Star Adventurer GTi
• Remote PC to control the rig : OLD Gigabyte Brix S (GB-BXCEH-2955)

Here's a pic of the rig, under Paris' Bortle 9 sky :

and here's a quick picture I managed to get with this mount (unguided, bortle 9, 2h total exposure) just to prove to myself that my mount is at least capable of tracking the sky (the stars here are not perfectly round, but the subs were 60s long):

Now, there's one important thing to know about my mount, it recently had motor issues (basically RA and DEC stopped working) so I sent it back to Optique Unterlinden for repair, and it successfully got taken care of, and it works fine now (well except for the guiding issues which I will get to).
This repair was done before I tried any sort of autoguiding (I did not have guiding equipment at that time) so I don't know if the issue was there before or not.

So, here's my problem:
The guiding errors with PHD2 are simply all over the place. The DEC movement is erratic, RA guiding is just bad… The logs speak for themselves (attached). Here is a sneak peek of the disaster:


(Notice the scale on the left? Yes, it's really that bad…)


But first, here are some important points I've noticed:

• During calibration, the RA steps seem to go great, roughly 11 steps are consistently required to calibrate. Then clearing backlash, not sure how to interpret that, just know that it's always 6-8 steps long. And finally DEC: north steps are fine (around 11 steps), however during the middle of the south steps, the OTA stops moving for like 4-5 steps, then starts moving again, with a sudden drop. I hope you'll be able to notice that with the logs.
• Most of the time, RA and DEC are orthogonal
• I've tried running the guiding assistant, it recommended me to switch the DEC algorithm to low pass, which I did, and it didn't help.
• Measuring DEC backlash yields these unusual results:

you can notice a south plateau, then a sudden drop, almost following the “ideal” line, is this what a typical backlash issue looks like?

… and what I've tried to solve this issue:

• The mount is up-to-date (Synscan says the motor controller board is on version 3.45.0C)
• I'm using GSS to control the mount (I've also tried Synscan, same issue, but Synscan is buggy, so I don't use it)
• No cable are getting caught on the tripod
• The OTA + accessories weighs 4 kg, which is 1 kg under payload limit
• The mount is balanced
• I tried both sources of power to the mount (8xAAA non-rechargeable batteries and 12V3A DC)
• I don't feel any backlash, and I've tried guiding sessions with less tightened and more tightened RA and DEC backlash screws to see if it helped (it did not)
• I've polar aligned the mount with NINA's 3 points PA to within less than 1 minute of total error (though I cannot see Polaris from where I image so I have to trust the software)
• I tried guiding with only the guide scope on the mount, the issue remains (so it's not a payload weight issue nor a cable issue)
• I'm calibrating near where the meridian and the equator lines meet (I also tried calibrating near other areas of the sky)
• I'm using 2s exposures for guiding, to eliminate bad seeing issues
• I've tried tweaking PHD2's settings (mainly DEC aggressiveness and minmo) and starting from scratch with the wizard twice
• PHD2 uses multi-star (I also tried without, same results)
• You can find in the logs sessions for which mount guide output was disabled, to help with the diagnosis

Thank you for taking the time to read this long post. I've tried to be as complete as possible, and thank you for any help and advice you can give me, as I don't know what to try any more.

Logs(I can also provide debug logs if needed):
PHD2_GuideLog_2025-04-17_211516.txt
PHD2_GuideLog_2025-04-21_212326.txt
PHD2_GuideLog_2025-04-23_231622.txt
!!Only guide scope on the mount!! PHD2_GuideLog_2025-04-25_220831.txt

What is the focal length of your guide scope?

120 mm

This is an issue with your mount not something fixable with software tweaks. Best way around is to disassemble, clean, adjust and relube.

Zachary Moore:
I would say check if  dec axis was reassembled too tightly, or a worm gear mesh is too tight, causing inconsistent movement. Does the mount when powerd off feel sticky or inconsistent when you turn it?

I'm not sure if it feels stickier than before the repair, but I do hear slight metal noises when rotating the mount on both RA and DEC, as if it wasn't properly greased. I'll try to record it when I get back home this evening. I'll also take pictures of the gears, as I'm not sure how to properly identify the issues you mentionned.

andrea tasselli:
This is an issue with your mount not something fixable with software tweaks. Best way around is to disassemble, clean, adjust and relube.

Yeah that's what I suspected the most after all those attempts to fix the issue. Honestly not what I wanted to read, but I hope I'll be able to fix it... How would you recommend to go along cleaning the mount?

Maxence d. C.:
Yeah that's what I suspected the most after all those attempts to fix the issue. Honestly not what I wanted to read, but I hope I'll be able to fix it... How would you recommend to go along cleaning the mount?

*Take everything out, clean with solvents, re-grease with the right lube, while putting everything back where it was. But first check that the axes moved smoothly and freely once the motors are out. If they do then probably is best to leave the main assembly alone and concentrate on the gearing and replace the spur gear with belts and pulleys. I'm pretty sure there must be how-tos on how to service this mount as it is quite popular.

Nicolas Deleuze:
Bonjour Maxence, 

Alors je voie que nous avons a peut prêt le même setup, et que tu as les même problèmes que j'avais.

Ce que je peut te dire c'est que 4kilo c'est un peut trop pour cette monture, de mon coté j'ai du réduire le poids 

Comme toi mon setup était : 
Lunette: TS  70 EQ ( 70 /474 )
Caméra : Player-One Poséidon
Lunette guide : ZWO 30F4
Caméra Guidage : Altair 290C

Maintenant : 
Lunette: TS  70 EQ ( 70 /474 )
Caméra : Player-One Poséidon
OAG : Player One
Caméra Guide: Sedna-M



J'ai enlevé la caméra de guidage, la queue d'aronde sur la qu'elle elle était

Même si je pensais que mon équilibrage était bon, en dévissant de beaucoup la vis d en DEC je me suis aperçu que l'équilibrage était pas du tout bon 

En effet je te conseille de dévisser la vis en DEC qui est sous le carter de protection (il me semble, au vu de ce que tu dis que tu sais ou elle se trouve) et de bien faire l'équilibrage, en la dévissant tu vas voir comment c'est bien plus fluide



Pour le resserrage de la vis, je la serre pas trop, je la serre petit a petit jusqu'à ne plus avoir de jeu en DEC et je stop (si trop serrer ton guidage en dec vas faire des pic dans tous les sens)

Je fais comme toi un alignement avec NINA et 3 point alignement, généralement je l'exécute 2 ou 3 fois pour essayer d'avoir 0.20"/arc max d'erreur.

Voila le mieux que j'obtiens (quand le mistral n'est pas présent)


Alors parfois ma courbe en DEC se barre gentiment (et je la laisse faire), je la guide que d'un coté, mais j'arrive à posé largement 300s  avec - de 1 seconde d'arc

Courage a toi, et Bon ciel.

Nicolas.

Bonsoir Nicolas,

Merci pour ces précieux conseils que je vais appliquer pour ce soir, je donnerai des retours demain (si la météo le permet). Effectivement nos setups sont très similaires, donc normalement, je devrais être capable d'aussi beaux résultats que les tiens (modulo le Bortle 9 de Paris quand je ne suis pas à la campagne).
Peut-être qu'effectivement mon DEC était trop serré, là il est au minimum (sans Backlash) et les engrenages de l'axe DEC ont été nettoyés et lubrifiés, je vais voir ce que ça donne.

Bon ciel,

Maxence

I did have a lot of issues as well. What i did like other mentioned, disassembled completely and refreshed RA and DEC gears. The best I got is 1 arc RMS error. Also I was adjusting backlash on DEC and in RA and rerunning guiding assistance to the point the backlash was low, be careful not overnight. Another point is balance, it might look like balanced but cables can affect it, always balance with all cables and with Dec 0. Another point, I usually run polar alignment like 3 times at least. Please share your findings.

One question, have you tried running a guiding session for 1 hour at least, what is your RMS?

Update on the issue :
So yesterday I took apart the DEC axis gears from the mount and cleaned all of them, put some lithium grease on it, and did my best to balance the OTA on the mount. And the issue on DEC is basically solved now! Look at those graphs!


Now, I know it's far from perfect, especially with RA, but the DEC axis is just so much better than before! My RMS over a ~45min session was around 0.9" on the DEC axis and 1.2", for a total RMS of 1.3". Not great, but at least my mount is usable now! I managed to get 3min subs with really close to round stars.

Seeing how effective cleaning the DEC axis was, my next mission will be to do the same to the RA assembly (which is a lot more difficult to take apart, unfortunately). I'll also have to figure some software things out, especially with dithering ; as you can see, sometimes "settling fails". Also, I'm not sure why, but sometimes DEC seems stuck over short periods of time:

Thank you everyone for all the tips, it gave me the confidence I needed to take a hex key and start disassembling the mount. I'm so glad those DEC spikes are not showing up any more. Also, the backlash profile now makes sense:

The mount still has backlash, but I don't think it can be eliminated (maybe I can reduce it a little).
Guiding assistant did report a quite inaccurate polar alignment (7 min), while NINA's three point PA reported <0.5"… Could it be that my guide scope is not parallel enough to the main OTA?


Finally, following Nicolas' advice, I will look into buying a OAG, in order to reduce the payload weight (the above tests were done with the full 4 kg).

Thanks again and clear skies,
Maxence

Update :
I think it's safe to say that this mount is now repaired :

To anyone who encounters this issue : what worked for me is disassembling the mount, cleaning the gears, lubricating the system, and carefully putting it back together. Also, I made sure to have a good balance (by loosening the backlash clutches) and good PA.

Thank you for the help everyone, and Clears Skies!

Maxence