Hi Gene,

The problem is as Ihoujin mentioned if you are approaching the final focus point from one direction as in linear it won't matter, but the problem is that (1) IN and OUT backlashes could not be equal, and (2) This confuses the algorithm as at the final position, the algorithm is not equally going IN and OUT, and that is the cause of the shift of backlash that causes the problem.

Hi Mohamed,

True backlash in opposite direction can vary but more than likely comes from drawtube tilting than gear train backlash, which is more repeatable -if- the gears are fixed to the shaft and do not ride on the shaft they are mounted to. Coming from drawtube tilt also implies amount may change depending on how far the drawtube is extended.

Here is a video I did of a stepper motor head used, gears only in the head. Repeatability is good considering any slip that may have occurred of the dial indicator on the moment arm and the 5 pound weight hanging from the 2 inch moment arm

Stepper motor head video from 2012

One thing you can try:
Most (all?) focusers have an option to reverse the direction.  So do an auto-focus run with the linear algorithm (that works around a possible backlash) in the normal direction, and write down the value (also look at the curve to verify it is a reliable one).  Then activate the 'reverse' option in the focuser driver, and repeat the AF run.  This time, because of the reversal, it will always end moving outward, and you get the focus 'coming from the other side'.  The difference between the two values you get that way would be your backlash.
 

Hi Peter,

Thank you for your suggestion, but when I tried it, I got variable readings. Please find the below table...

 

Hi Gene,

Thank you for the video, but I couldn't grasp how it was related to the focuser backlash. From what I can tell your stepper motor worked well

 

If you look at the values, both show a clear trend for decreasing values.
My suspicion would be that there is slip somewhere in your focuser train:  Either it is a Crayford-type that works just by friction between the axis and the tube, or you have connected the focus motor to the fine drive knob that usually also has some intrinsic slip.

But if your focuser has slip, backlash correction is quite futile anyhow.
 

Hi Peter,

These readings were from the WO Z61 with a 2" Rack/Pinion focuser, and I'm connected directly to the course knob (not the fine). Any ideas what else can be wrong?

 

In that case it could be temperature variation.  If you put out the telescope from a warm room into the cold, it will change focus until it's temperature stabilized.  For a test like the above, I'd wait at least an hour after having brought the telescope out (My 140mm needs almost 2 hours...)
Or the coupler between motor and focus axis isn't solidly fixed.  Apart from that, I'm out of ideas....
 

Hi Mohamed,

This is on your WO Z61 with focuser motor under comtrol of the MyFocuserPro2?

How fast are you driving (steps/microsteps per second)? What type of stepper motor.

Drive too fast and miss steps, not enough motor torque and miss steps.
Not all clamped down on the shafts and slippage as Peter mentions.

What was the orientation of the scope, high alt, low alt ?

Gene
PS: For my video, sorry, it was meant to show a low repeatable backlash stepper motor gearhead.

Hi Gene,

Please find my answers below...

>>This is on your WO Z61 with focuser motor under comtrol of the MyFocuserPro2?
Correct. It is MyFocuserPro2

>>How fast are you driving (steps/microsteps per second)? What type of stepper motor.I use STEPPERONLINE 27:1 Planetary Gearbox Nema 17 , and I configured it to 1/4 step


>>Drive too fast and miss steps, not enough motor torque and miss steps. Not all clamped down on the shafts and slippage as Peter mentions.>>What was the orientation of the scope, high alt, low alt ?I did this focusing test close to the meridian & equator (South, ALT about 40-50degrees)


 

Hi Mohamed,

Thanks!

For the 'steps/second' part of the question, when you did the testing, on the Main tab of the driver, what value did you have for 'Speed' Focus Speed?

Source code allows values of 0, 1 or 2

Which looks to directly correspond to Slow or Medium or Fast
as defined in the documents on sourceforge myFP2-307-3.pdf

Had you done any tuning of the motor speeds as given in the document section
FINE-TUNING THE STEPPER MOTOR SPEED
?

Did you happen to try the backlash experiment at different speeds or different stepping, which you mentioned was set to 1/4 step?

This is one area that can cause strange results, how fast driving based on chosen step mode, just looking to rule it out.

Another dealing with stepping is power supply to the phase coils of the motor drooping.

ALT: With your angle of 40-50 degrees, this would seem to present leverage on the drawtube when changing directions, giving it a chance to tilt.

Gene

Hi Gene

Thank you for your response. The speed was always set to "2". I shall perform the test again today and share the results