Pete,
I also have G11 and I am using Onstep with the direct drive there too.
Unfortunately, GM8 has only 180 teeth on the ring gear vs 360 for G11, so I believe that doing a direct drive for GM8 would be pushing it.
In any case, I did try direct drive and I still saw bad PE.
Let us know how your mod goes, I was wondering about using the Oldham too. I am currently using the spiral coupler, I believe that most folks do this as well.

Alan,
I will check it, but I think it is optical illusion.

John,
Good point, I will try to loosen up the belt a little.

Michael,
Good point too. Is there an article somewhere, describing how to align worm against the ring?
I am reading this forum on a regular basis and did not see a discussion on this subject.

All,
How consistent is the quality of the brass worms? Does it make sense to get a new one?
I guess I can try to pull one from my G11...

Alex

Alan,? By "main", are you referring to the pulley on the worm?? Given the 480 second PE this seems likely.??

If so, Alexander should check that this pulley is firmly located and tight on the shaft.? There is not much room to make the set screws in the pulley engage with the shaft.

I'm doing an OnStep install on my CG11 and I opted not to do what Alexander has done and use a belt drive.? Instead I am planning to use two new Oldham Couplers to fit the worm shaft in it original position and drill out the 3/16" end of the coupler by 0.235mm to 5mm and then directly mount a 0.9 degree step Nema 17 to that thus avoiding belt drives.? This will be my third OnStep conversion and I hope the best.

Your main pulley is off-center, which gives a 480 s periodic error.

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Alex,

Quite an interesting pic showing your design. If there was ever an arrangement that should remove all gear slop from the stock mounts I would think it would look just like this. Since your deviations are multiples of the worm period, some kind of PEC correction curve should be of benefit. But how to program one into on OnStep I have no idea. You know the brass worms can be bent under the right amount of force, perhaps from your belt tension? If you place a worm in a cordless drill at low speed and stare down the end of the shaft, you should be able to see if the worm end spins in a tight circle, or starts to oscillate out of round. Or some other round piece is doing an ellipse, not a circle.?

You are the trendsetter here, and I certainly hope you can help with a future design model.

Good luck!

John

Hi Michael,

Many thanks for the detailed reply.?
Here is my mechanical setup - I have the reverse worm setup with time gear attached directly to the shaft of the worm. The larger gear is 60 and the smaller is 16.
The aluminum parts form an arrangement, which works as OPW. I drilled and tapped the holes in the bearing blocks in order to make the whole thing as one piece.
So I don't have an Oldham coupler and there is no possibility of misaligning the motor and worm shafts.
I have ABEC 7 bearings and I also tried the Belleville spring, it did not help.
I also tried swapping the worms, looks like it does not help, based on few minutes of testing, before clouds rolled in.?
I have brass worms. I wonder if I should buy new worms.
Right now I am running out of ideas...

Alex

Allan,

The Ra encoder upgrade was a bust I feel. I would not bother. While testing was limited, I did not see a significant improvement. I think it's because you are either slowing down or speeding up what is a continuously moving axis. Better to get? rid of the large period errors natively if possible. Funny thing is my Ra guiding seemed to improve as my Dec guiding improved. I thought that they were independent things?? Now my Dec is so well behaved, my Ra axis, seems to have gotten the hint to get better too?

The reason to upgrade the thrust bearing is not about smoothness parse. If you see a large low frequency spike which for me was around 2600s, I much reduced it with a German made bearing.? If you do not have this error? that's great. The other thing was worm mesh alignment, which proved to be an issue for me, and Michael Herman too.

Peter

Peter

Paul,

I appreciate that, but it's kind of irrelevant, and confuses most people into thinking you are controlling the motor at that resolution. Frankly, I can not recommend enough that Losmandy owners upgrade their Dec encoders and see what results they get. It's such a simple mod. Moreover, people should zero out their backlash. The Allen wrench method is balls on accurate. Zeroing backlash is what 10 micron mount do in their software.? If you change the slew setting to 1200 you can also move just as fast. Sticking with the 25:1 gearboxes make that possible.
?
Upgrading the Ra encoder was a bust. Folks should focus instead on elimination the large period errors. (80s (OPW), 240s and 2600s.? I've shone a light onto those problems and how I dealt with them. If you follow the steps I took you will have no use for PEC. The gearbox upgrade and thrust bearing upgrade are a must if you want a calm Ra guiding curve.? ?

All the best,

Peter

Peter,

Entirely possible and even likely. I'm not arguing with your results. All I was trying to do was to correct some information that was posted earlier on the encoder precision. Gemini PIC controller reads encoders in quadrature, and so the effective resolution is 4x smaller than the (Gemini-addressable) step size.

Regards,

? ? ? -Paul

Paul,

This is where people are getting confused I think.? The positioning step accuracy is 0.5625". Therefore if you what to make a Dec correction it will be that large. You can not move the motor a 1/4 step.? ?If you install a 1024 encoder you can make a 0.1405" step move. The results are night and day as far as Dec guiding performance. In the chart just posted Dec rms was 0.15", about the same size as the step resolution. There is no point fighting the facts, a higher res encoder will greatly improve your Dec guiding.?

Peter

PS. Before this change by Dec guided at about 0.74" rms.

Here is my math:?(360*3600)/(256*25*360) = 0.5625 arc sec per step.This seems rather coarse.?

This is a single step addressable by the Gemini controller. The actual accuracy of the encoder reading is 4x the step or 0.1406" and even smaller for Titan.

Regards,

? ? -Paul

Peter:? since you're doing a deep dive, might be interesting to check the orthogonality of the axis shafts to their top plate. I had an issue in mine leading to some stiff spots when rotating the axis by hand (similar to what one might feel with a wonky needle bearing).? See this thread if interested:??/g/Losmandy_users/message/59944

Keith

Do you have plans to upgrade the optical encoder to 1024. I think you will see a very noticeable benefit. It's a $65 investment and you do not have to remove anything from the drive other than the motor end plate can do everything insitu.? It would be nice to have a second reference point to go by. With a minimum step size of 0.56 arc-sec and I would imagine longer settling times due to the weight, having the ability to make smaller 0.14 arc-sec steps would be beneficial. I've ended up with a min step size of 0.05 in PHD2, compared to 0.4 before I made the alteration. With good polar alignment I would expect on only having to make little tweaks.?

Here is my guide graph from last night showing just how small the corrections are and lack of reactionary overshoots. The scale is +/- 1s if the scale is not readable

I found it useful to remove the motor and use my fingers to judge the worm mesh to get both side touching. It's such a fine adjustment thing. Also, I would highly recommend changing out the thrust bearing. It made a large difference for me.

The encoder upgrade seems like a good thing to do at some point.? Have you tried it on the RA axis too?
I don't plan to do anything about the thrust bearing because it is new and seems to spin smoothly.

I was able to run further tests last night, but smoke moving into our area cut the session short.

I put back the 25:1 McLennan gearbox on the Ra drive and ran a guiding test with the 256 tick optical encoder. Slew rates are now fully restored. I also changed out the Ra thrust bearings to those sourced from McMaster. The net result saw a very significant reduction of 2600s error I previously identified and attributed to the top thrust bearing. As you can see from the attached PHD2 screen shot by guiding rms error was 0.28 arc-sec. Later in the evening I changed out the Ra motor encoder to 1024. I saw no real improved in Ra performance which leaves to with the following overall conclusions:

1. The stock Losmandy thrust bearings are a major source of problems. I attribute stiffness of rotation, worm misalignment, and significantly large 2600s error to them. My suggestion with anyone who buys an older or new mount is to change these to German made bearings sourced from McMaster.

2. The Dec axis greatly benefits from a smaller step size. This can be achieved by switching out the 256 tick encoder to a 1024 tick encoder. Cost is around $65 sourced from US digital. A smaller step size make sense as it leads to a more subtle control, and it most noticed when one's polar alignment is excellent.??

3. Replace the Oldham couplers with rigid couplers. This facilitates alignment of the motor to the worm. This mod it difficult to pull off as the couplers have to be made from two parts, previously listed. Cost is around $20 for both Dec and Ra. Since there are two pivot points that have to be aligned, it requires loosening of the coupler screws and gearbox mount screws and alignment the assembly while rotating the motor. When aligned correctly the motor will not bob up and down, Done right the native 240s error in Ra will be very much reduced and there will be no requirement to used PEC to achieved a 240s guided error of under 0.2 - 0.15 arc-sec.

4. Reduction of the 80s bearing error in the OPW remained stubbornly large. After repeated tests I found the best thing was to used a single Bevell washer arranged to put pressure on the ball race. In my case I reduced the 80s error to under 0.3 arc-sec.

5. Replace the Ra gearbox to the McLennan gearbox (not necessary for Dec) . I saw no real benefit to reducing the step size. This makes sense because the Ra worm is continuously rotating. The reason to replace the gearbox is to eliminate the large 32s error of the Losmandy gearbox.?

6. Spring load the Ra worm ( did not test the locked down cased), and add a 2 lb bias weight to load the worm on the east side. The keeps the worm permanently engaged on the right face of the worm gear both before and after a meridian flip.?

7. Use TVC correctly to eliminate Dec backlash. The ideal arrangement? is to have the worm mash on both sides to the ring gear, but not have the pressure be too great so as not to be able to turn with one's fingers. I found a spring loaded arrange in Dec to be detrimental to achieving a low backlash value. TVC is there to cancel out any backlash. The thrust bearings play a big role here because you want the ring gear to turn easily when the clutch is tightened. If using the 1024 tick encoder then TVC will be 4x larger in value than in the 256 tick encoder case (my setting is 45).?

That it folks! Happy imaging,

Pete

Michael,?

Mark Crossley identifies the issues I"m seeing quite accurately.
I think I'll go back to the thinner thrust bearing to reduce the significant exposure of the needle bearing assemblycaused by the extra thick bearing and?
instead look to putting spacers between the clutch collar and the Losmandy spacer. Moving the wavy spring to be between the clutch collar and the spacer provides more even?
pressure against the thrust bearing which will prevent warping.?

He identifies the Losmandy spacer ring being too narrow as the root cause (narrower than the DSC device it replaces). If that is true then this issue should be common to anyone with this generation G11 that has the polar scope installed. It either manifests as a polar scope that doesn't seat correctly or a clutch collar that feels tight because it's putting pressure on the polar scope not the thrust bearings. That can cause all kinds of issues with the RA axis performance.?

Just thinking out loud here, Michael, but I am wondering if the fresh application of the lubricant grease during a rebuild is not contributing a small amount to the separation of parts. When the lube is new and freshly in place you are creating a hydrodynamic film just from the viscosity of the fluid (grease) over which the parts can ride. Over time the viscosity can break down a bit and and also it gets pushed aside into the surrounding voids as pressure is exerted down on the moving surfaces. Whether the film integrity amounts to the size of the distances you are factoring here I don't know. A measurement would need to be done right after assembly, and repeated after some use period. After displacement of the grease you are left with an ultra thin film and relying more on the chemical nature of the additives over the original lubricant film volume. Or measuring the entire assembly clean and dry versus lube over the the working parts might show this difference. Maybe.

Thanks for the tip Michael.?

I will certainly check out Mark's site.? There is a copper wavy spring under the PS flange. With the thicker thrust bearing there is now enough space for the polar scope to seat correctly.?

I agree that doing the alignment with software tools and an imaging camera makes life easier. The other EQ mounts I have don't have a polar scope at all.? I discovered the problem in the way the clutch was behaving. When the polar scope hit, the clutch collar would feel tight even though it wasn't tight against the bearing. It could have easily damaged the polar scope.?

@Brian V.?

My apology for any inference about Losmandy in this.? As I go through the mount I'm finding more and more oddities that are obviously the previous owner's doing.?

If it was serviced in 2019 it's clear to me now that things were done since then, and none of it for the better.?

I'm was hoping to be able to just do the easy routine of? cleaning clutch plates and partial lubrication. It's better for sure, but I also feel that there's more improvement
to be had.? I'll probably just plan on a full "super tune" type tear down. Mounts like this deserve the effort.?

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?

The vibration settle down from moving the axis may be coming from a different place than when you touch the focus. Perhaps there's some oscillation in the axis that might be reduced with a bit of increased force on the clutch. That clutch sets the load on both thrust bearings.

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