Re: Changing the servo motor encoders from 256 to 512 or 1024. Any reason not to do this?

Peter Boreland

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,


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