Small but mighty tuning for the EQ6-R. Change the pulley! Sky-Watcher EQ6-R · Pinguru · ... · 44 · 4025 · 35

Hey everyone,
I recently purchased the Skywatcher EQ6-R. It is the "latest" Version with the USB Interface.

To be clear: My worst guiding sessions with this mount are comparable to my best guiding sessions with my previous mount (Exos-2).

With the help of PHD2 Log Viewer and some curiosity, I have figured out two issues with the belt drive of the two axes.

1. The pulley/belt contact has some amount of inherent backlash (relevant for DEC)
2. The pulley/belt interaction creates a repeated deviation in tracking speed (relevant for RA)

Let me show you what I mean.

Point 1:
The backlash of the pulley can be seen in this little video I have made with my mount:
Video Link to Youtube
You can see that the pulley can move a little bit without the belt moving. The belt tension was already relatively high. This may be amplified by the fact that I usually lube rubber belts with a thin film of silicone oil to keep them smooth and reduce wear.

Point 2:
About every 10 seconds I did see a little spike in the RA guiding. With a longer guiding session this subjective finding became a hard fact. I have a major peak in my RA guiding with a very precise period of 10 seconds. The 10 second period is exactly the freqency of the belt teeth running in/out the pulley. In PHD2 Log Viewer you can analyze the guiding data to get an FFT of the frequency distributon:

The picture above is the data of the RAW RA movement. So no corrections of the guider applied.

The picture above shows the remaining error with the guiding active. The longer period errors can be managed by the guider. But the "fast" 10s period still comes through. The guiding is not fast enough to deal with this.


Analysis of the problem:
The pulley does not seem to fit the belt ideally. This is the main problem and a simple change of the pulley to a new one would most likely solve most of the problems mentioned above. 
Doing my research I found, that the T2.5 belt design used in the EQ6-R is maybe not the best choice. The T2.5 profile can have more backlash than other more modern belt profiles. Apart from this, having just 12 teeth on the pulley means, that just about 5 teeth are working simultaniously. I decided to make a change to the whole belt system and replace it with the very available GT2 belt common in most 3D-printers. This belt profile is optimized for minimal backlash due to its circular teeth shape.

What I did:
I have calculated the drive I could fit into the small installation space of the EQ6-R. Turned out that the smaller pitch of 2 mm of the GT2 belt allows to fit a 15/60 teeth combination instead of the 12/48 of the stock T2.5 belt. This gives me about 6 teeth working simultaneously on the pulley. Not a lot more, but an improvement still.
I have ordered the 15/60 pulleys and the 178 mm (89 teeth) belt from Amazon and Ebay. 
The 60 teeth pulley has two guideplates on the sides. They are too large in diameter to fit in the EQ6-R. They need to be stripped down to have an open pulley, just like the stock 48t pulley. To have everything working smooth and safe, I have chosen the 15t pulley and the belt in 6mm width and the 60t pulley in 10mm width. This ensures that the belt can never fall off the 60t pulley.
https://www.ebay.de/itm/283315656377?var=585025474753
https://www.ebay.de/itm/283302996132?var=585025245636
https://www.amazon.de/dp/B081DBPW21?psc=1&ref=ppx_yo2ov_dt_b_product_details

The gear is relatively cheap so I was not afraid to use some force getting rid of the guide plates:



This is all you need for the refit.

To change the pulleys you will have to open the electronics cover and loosen the motors from the inside to release the belt tension. This is well documented in other sources. I have not taken pictures of this.
Take out the old belt and remove the motor from the mount to take off the pulley. Loosen the grub screws and pull off the pulley. If you are unlucky, the pulley has some glue attached to it and will not easily slip off. I could pull mine off with a little patience, a droplet of oil and gentle force...
For the 48t pulley, you can open the little screw on the top side of the worm carrier. This gives you access to the two grub screw of the 48t pulley. You don't have to remove the whole worm carrier or change your worm play at all. 
The 48t pulley can be glued on as well. Super annoying. On that pulley I have used the two threads in the stock pulley to push it off with two screws. Do this gently. The force is directed through the worm bearings and can easily damage them. I am not conccerned, because I want to replace them in the net weeks. As always, be gentle and patient.
Clean both shafts after you have removed the pulleys.

You can see the marks of the screws around the left shaft from my efford to take the pulley off.

EDIT: having the belt removed is the ideal time to check your fit of the worm gear. Then you can feel the play or friction without the belt influencing your perception!
Spin it all the way round with and without the clutch pressure. You will feel a difference in play between the different positions of the clutch. If the clutch is on the same side as the worm, the play is maximal. If the clutch is opposite from the worm the gears can be mashed together a bit more. Try to find the best of both. This ist the best time to do this :-)
In a future post I may address this topic with a special kind of new bearings (lower radial clearance). But this is a task for future-me.


Replace the pulleys with the new ones and the new belt and apply a good amount of belt tension. You might want to fasten the motor and drive it back and forth without seeing the belt changing tension visually depending on the direction. Close the PCB while doing this to avoid a short circuit.

The big 10mm 60t pulleys fit into the available space, but it is close. You have about 1 mm between the backside and the cover of the housing. In my mount I could make it fit relatively easily without making contact on the front or backside.
Of course you can machine away the front an back edges of the pulley to have a lot of room for aligning. But I don't have access to a lathe at the moment.

Result:

DEC-Axis: The backlash is gone. I don't have proper data to show this, but I can't feel, see or measure it anymore.
RA-Axis: After some hours of guiding I am pretty sure that this improvement was worth the time:


As above: One picture shows you the raw, uncorrected RA and the other gives you the RA with the active guiding. The scale is the same.
The belt period is now at 8 seconds, because of the higher number of teeth.
As you can see, the peak is gone.
I am very pleased with this result. I have expected to see at least some residual peak at 8 seconds.

Conclusion:
A nice little mod for <40€ and <2h of efford. The effect on RA guiding performance is noteable.
You can go all the way like me but I think just replacing the pulley with something like this should be a great first step to try:
https://de.aliexpress.com/item/32794928013.html?spm=a2g0o.productlist.main.5.78433e7a5Pgv3e&algo_pvid=7148bb8b-4230-471a-aacd-d90002aeb72e&algo_exp_id=7148bb8b-4230-471a-aacd-d90002aeb72e-2&pdp_npi=4%40dis%21EUR%2122.93%2119.48%21%21%2124.31%21%21%40211b88f016927950637633993e1dfe%2163980288919%21sea%21DE%214628680248%21&curPageLogUid=jRlaeypeTuqD

Please be so kind and let me know if your EQ6-R shows this FFT peak at 10 seconds as well.
And let me know if just replacing the pulley for a new 12t T2.5 does the same job as my full belt rebuild.

Best regards, Volker


EDIT: all FFT plots above are based on long guiding sessions with about 1.5 hours of continuous guiding + dithering (exclude settling). I am not interested in faking good guiding results with short analysis intervals. The exposure time during those sessions was 300s. The dither command was set to 7.5". The stability criteria was: <1" for 3s with 90s timeout (no timeout in analyzed sessions).

Hi Chris,
I have not focused my evaluation on the guiding RMS (for now). 
At the moment the temperatures, the seeing and my available patch of the sky don't allow an apples to apples comparison.
Right now I just can image near to the equator and the most data prior to the mod was collected at high DEC-values.
The RA-RMS is directly influenced by the DEC coordinates of the target, so I skipped the RMS-part for my post.
I will post them once I have imaged unter comparable circumstances.
Loosening the main preload nuts for both axes a little bit was also something I did during the first days :-)

A regrease of the RA is also on my list. Maybe the final RMS comparison will include more than just the pulley mod.

Best Regards and Clear Skies!
Volker

If you have an idea what could cause this other spike at about 95s period, please let me know:

It clearly happens 5 times during one worm rotation. So it can't be the concentricity of the motor pulley, this would be 4 insted of 5 times.
Having a damaged bearing at the worm shaft (brinelling) would give me an defect order of 4,4 times/rotation.
I am not sure about this one. I am struggeling to find a mechanical correlation to the system.

It doesn't really matter for the guiding. The 95s period is slow enough to be corrected as seen above. Still I am wondering what it could be.

I think have the same on a AZ-EQ 6 pro,
Of course, my guiding and PA wasn't that great that night anyway ! 
Very interesting post, thanks for sharing.

Hi,
one more thing:
If you plan to change the whole belt drive. Consider drilling two holes with threads in the large pulley.
This gives you two advantages:

1. If you are struggeling to get the pulley off again, use the threads to pull the pulley off like mentioned above
2. You can turn the pulley easily with your fingers if you have two screws installed so you can check the worm mashing easily

I have taken my pulleys out again to drill the two holes per pulley


Of course you need to take the two screws out again to reinstall the cover.

@Christian Koll : I think I don't have suitable data to compare the RMS results. I was taking a lot of exposures high in the sky with the EQ6-R prior to changing the belt, but I was taking mosaics. And my strategy is to take just 3 exposures per panel and then move on. This gives my even sky quality for the panels, but just very short guidelogs. I don't like to base a comparison on just 15 minutes of guiding. But a log of a recent testrun gave me the following, very promising results (51 minutes, DEC=0°, just west from meridian)

--> EDIT: I have found a guidelog from me imageing the M16 Eagle Nebula at -14° DEC prior to the mod. Not a perfect comparison, but better than nothing.

picture above: A guide log prior to the mod. Performance is overall good, but you can see the periodic and needle like character of the RA graph.


picture above: A guide log after the mod. Not perfectly compareable, but the super strong periodic error of the 10s (8s) period is down to the overall noise.
PPEC was not active in both of the sessions.

I want to work on the DEC performance, but this is a subject for another topic.

When I got my AZ-EQ6 some 5 years ago, I made the same observation. I increased the belt tension to get rid of that annoying 10 s period (at least, most of it). In normal guiding with >1 seconds guide exposures, that 10 s peak shows up as noise in the guide graph. You can only see it when using short guide exposures. That doesn't mean it's not there with longer exposures. It just means that you can't resolve it in the analysis. The 120 s peak, btw, is one full revolution of the motor pulley. But this can be guided out.

cs,

Wim

Hello,
on my EQ6 R I was able to eliminate the belt play that can be seen in the youtube video by changing the position of the respective motor so that the tension of the belt sections on both sides of the drive sprocket was the same. Then the pull is the same on both sides.
Best regards!

Padraig Farrell:
I think have the same on a AZ-EQ 6 pro,
Of course, my guiding and PA wasn't that great that night anyway ! 
Very interesting post, thanks for sharing.

*** yes, you need to tighten the belt. 10 s periodic error is impossible to guide out, unless maybe you use sub second exposures. Something which I wouldn't recommend, as you'll be chasing both seeing and noise.

cs,

Wim

Wim van Berlo:

Padraig Farrell:
I think have the same on a AZ-EQ 6 pro,
Of course, my guiding and PA wasn't that great that night anyway ! 
Very interesting post, thanks for sharing.

*** yes, you need to tighten the belt. 10 s periodic error is impossible to guide out, unless maybe you use sub second exposures. Something which I wouldn't recommend, as you'll be chasing both seeing and noise.

cs,

Wim

Yes, just tension both belts , it’ll be interesting to see results, 
will post as soon as clear skies 

👍 Wim

Very interesting, indeed! I just ordered the mod. Thanks for sharing this information.  I've never noticed that much play as you showed in your video (or much at all) with my T2.5 system, but I am going to try this and see if I can get my RA to improve.  I am using polyurethane upgraded T2.5 belts already, but could not find any  GT2 belts that are poly ur.  Everything seems to be rubber or fiberglass.

I will report Total RMS updates as soon as I can get this done.  If others who are trying this can report any Total RMS improvement, let us know!

Abundant Clarity-Tom

ooo, loaded question!  If I had the money?  A Paramount or AP or 10-micron.  But on my budget, I would buy the EQ6-R again.  It does well for the price and is easily adjustable, easy to work on and upgradeable.  It handles my heavy rig very well.

The harmonic drives are too new for me to really vote on just yet.  From what I've read, they seem to have comparable guiding figures as the EQ6-R.  I really like the idea of strain-wave drives; they don't seem to be that much of an improvement, however.  They are lighter, for sure.

Can the EQ6-R be better?  Yeah, of course.  But, for what we paid, I think it is a great mount!

Abundant Clarity-Tom

Hey everyone,
I recently purchased the Skywatcher EQ6-R. It is the "latest" Version with the USB Interface.

To be clear: My worst guiding sessions with this mount are comparable to my best guiding sessions with my previous mount (Exos-2).

With the help of PHD2 Log Viewer and some curiosity, I have figured out two issues with the belt drive of the two axes.

1. The pulley/belt contact has some amount of inherent backlash (relevant for DEC)
2. The pulley/belt interaction creates a repeated deviation in tracking speed (relevant for RA)

Let me show you what I mean.

Point 1:
The backlash of the pulley can be seen in this little video I have made with my mount:
Video Link to Youtube
You can see that the pulley can move a little bit without the belt moving. The belt tension was already relatively high. This may be amplified by the fact that I usually lube rubber belts with a thin film of silicone oil to keep them smooth and reduce wear.

Point 2:
About every 10 seconds I did see a little spike in the RA guiding. With a longer guiding session this subjective finding became a hard fact. I have a major peak in my RA guiding with a very precise period of 10 seconds. The 10 second period is exactly the freqency of the belt teeth running in/out the pulley. In PHD2 Log Viewer you can analyze the guiding data to get an FFT of the frequency distributon:

The picture above is the data of the RAW RA movement. So no corrections of the guider applied.

The picture above shows the remaining error with the guiding active. The longer period errors can be managed by the guider. But the "fast" 10s period still comes through. The guiding is not fast enough to deal with this.


Analysis of the problem:
The pulley does not seem to fit the belt ideally. This is the main problem and a simple change of the pulley to a new one would most likely solve most of the problems mentioned above. 
Doing my research I found, that the T2.5 belt design used in the EQ6-R is maybe not the best choice. The T2.5 profile can have more backlash than other more modern belt profiles. Apart from this, having just 12 teeth on the pulley means, that just about 5 teeth are working simultaniously. I decided to make a change to the whole belt system and replace it with the very available GT2 belt common in most 3D-printers. This belt profile is optimized for minimal backlash due to its circular teeth shape.

What I did:
I have calculated the drive I could fit into the small installation space of the EQ6-R. Turned out that the smaller pitch of 2 mm of the GT2 belt allows to fit a 15/60 teeth combination instead of the 12/48 of the stock T2.5 belt. This gives me about 6 teeth working simultaneously on the pulley. Not a lot more, but an improvement still.
I have ordered the 15/60 pulleys and the 178 mm (89 teeth) belt from Amazon and Ebay. 
The 60 teeth pulley has two guideplates on the sides. They are too large in diameter to fit in the EQ6-R. They need to be stripped down to have an open pulley, just like the stock 48t pulley. To have everything working smooth and safe, I have chosen the 15t pulley and the belt in 6mm width and the 60t pulley in 10mm width. This ensures that the belt can never fall off the 60t pulley.
https://www.ebay.de/itm/283315656377?var=585025474753
https://www.ebay.de/itm/283302996132?var=585025245636
https://www.amazon.de/dp/B081DBPW21?psc=1&ref=ppx_yo2ov_dt_b_product_details

The gear is relatively cheap so I was not afraid to use some force getting rid of the guide plates:



This is all you need for the refit.

To change the pulleys you will have to open the electronics cover and loosen the motors from the inside to release the belt tension. This is well documented in other sources. I have not taken pictures of this.
Take out the old belt and remove the motor from the mount to take off the pulley. Loosen the grub screws and pull off the pulley. If you are unlucky, the pulley has some glue attached to it and will not easily slip off. I could pull mine off with a little patience, a droplet of oil and gentle force...
For the 48t pulley, you can open the little screw on the top side of the worm carrier. This gives you access to the two grub screw of the 48t pulley. You don't have to remove the whole worm carrier or change your worm play at all. 
The 48t pulley can be glued on as well. Super annoying. On that pulley I have used the two threads in the stock pulley to push it off with two screws. Do this gently. The force is directed through the worm bearings and can easily damage them. I am not conccerned, because I want to replace them in the net weeks. As always, be gentle and patient.
Clean both shafts after you have removed the pulleys.

You can see the marks of the screws around the left shaft from my efford to take the pulley off.

EDIT: having the belt removed is the ideal time to check your fit of the worm gear. Then you can feel the play or friction without the belt influencing your perception!
Spin it all the way round with and without the clutch pressure. You will feel a difference in play between the different positions of the clutch. If the clutch is on the same side as the worm, the play is maximal. If the clutch is opposite from the worm the gears can be mashed together a bit more. Try to find the best of both. This ist the best time to do this :-)
In a future post I may address this topic with a special kind of new bearings (lower radial clearance). But this is a task for future-me.


Replace the pulleys with the new ones and the new belt and apply a good amount of belt tension. You might want to fasten the motor and drive it back and forth without seeing the belt changing tension visually depending on the direction. Close the PCB while doing this to avoid a short circuit.

The big 10mm 60t pulleys fit into the available space, but it is close. You have about 1 mm between the backside and the cover of the housing. In my mount I could make it fit relatively easily without making contact on the front or backside.
Of course you can machine away the front an back edges of the pulley to have a lot of room for aligning. But I don't have access to a lathe at the moment.

Result:

DEC-Axis: The backlash is gone. I don't have proper data to show this, but I can't feel, see or measure it anymore.
RA-Axis: After some hours of guiding I am pretty sure that this improvement was worth the time:


As above: One picture shows you the raw, uncorrected RA and the other gives you the RA with the active guiding. The scale is the same.
The belt period is now at 8 seconds, because of the higher number of teeth.
As you can see, the peak is gone.
I am very pleased with this result. I have expected to see at least some residual peak at 8 seconds.

Conclusion:
A nice little mod for <40€ and <2h of efford. The effect on RA guiding performance is noteable.
You can go all the way like me but I think just replacing the pulley with something like this should be a great first step to try:
https://de.aliexpress.com/item/32794928013.html?spm=a2g0o.productlist.main.5.78433e7a5Pgv3e&algo_pvid=7148bb8b-4230-471a-aacd-d90002aeb72e&algo_exp_id=7148bb8b-4230-471a-aacd-d90002aeb72e-2&pdp_npi=4%40dis%21EUR%2122.93%2119.48%21%21%2124.31%21%21%40211b88f016927950637633993e1dfe%2163980288919%21sea%21DE%214628680248%21&curPageLogUid=jRlaeypeTuqD

Please be so kind and let me know if your EQ6-R shows this FFT peak at 10 seconds as well.
And let me know if just replacing the pulley for a new 12t T2.5 does the same job as my full belt rebuild.

Best regards, Volker


EDIT: all FFT plots above are based on long guiding sessions with about 1.5 hours of continuous guiding + dithering (exclude settling). I am not interested in faking good guiding results with short analysis intervals. The exposure time during those sessions was 300s. The dither command was set to 7.5". The stability criteria was: <1" for 3s with 90s timeout (no timeout in analyzed sessions).

*** Love the rigorous analysis. I've always had significant declination backlash with my EQ6-R mount and never been been able to fix it. There is no obvious movement in the Dec axis when I tighten the clutches bbut the declination backlash becomes apparent when using the guiding assistant in PhD 2**

A. Chapron:
Hi.

I had the same problem with the Rowan kit for my heq5. A 13,6s period error.
Changing the pulley solved the problem

*** Hi, I think my HEq5 rowan belt mod could do with an upgrade also. Do you mind sharing where you obtained your pulley parts from?  ***