@Doug, @Peter, thanks for another guide.

I realised that I should have literally very accurate caliper to be able to measure a difference affected by the movement like i.e. 10 steps. I have two calipers with digital displays and one vernier caliper which I prefer because of its absolute indications. The "digital" ones like to lose their zero position by accidentally pressing the zero or other buttons. The impossibility of making a very accurate measurement prompted me to place my own method based on interpolation here, maybe someone else would like to repeat it. It gave me a very probable result, similar to the results already mentioned in this thread.

Right, so I assumed steps equal to 100 ticks and beginning at 10,000 i Performed 31 steps. Every measure, especially a difference between the actual and the previous ones, even affected by errors related to the inaccurate reading, are summarized and divided by 31 that gave me an average difference between positions equal to 0.37mm for every 100 ticks. I made a chart giving me three positions of the tube where the backlash seems the same: 38mm, 40mm and 42mm. To be honest, I don't know why the backlash looks smaller in the right area of the chart.

Knowing that, I interpolated the values of position of the tube as equal 1/20 their theoretical positions in relation to a 5 ticks step (1/20 of the 100 ticks step) for the three areas. I found values closests of the 38mm, 40mm and 42mm both for the outwards and inwards directions and found respectively values of the backlash: 80, 95 and 95 ticks. Thus the average value is 90 ticks. Doug wrote that it should be between 80 and 100, so it's exactly in the middle between them.



You probably guessed that I like Excel.

Anyway, thanks for encouraging me to perform the experiment and now I'm ready to try the polynomial algorithm as well.

BTW I just checked the version of the Kstars/Ekos which I use, it's 3.5.9. I already performed the "update | upgrade" procedure to be able to test a patch in the INDI driver for OnStep firmware. Unfortunately, after the successful upgrade (observed) I found that both the Canon DSLR driver and the ZWO CCD driver didn't recognise my cameras. I always have at least two microSD cards for my RPi, just to quickly swap them if i.e. file system crashes (often happens in the RPi 3B+, RPi 4 seems OK yet), so I checked whether the cameras work. I described the problem here:

www.indilib.org/forum/ccds-dslrs/12804-i...asi-ccd-crashed.html

As long as the problem is not solved, I don't touch the higher version. It's enough for me to use the linear algorithm interchangeably with the polynomial one.

So, thanks for all now and let's see how it will behave.

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