beachbum one guy said 20 did not work for his focuser. He changed it to 40 and it worked. I was going to do that next go to a 40...
Mabuhay Rodney,
On my two Takahashi scopes' rack and pinion focusers, the EAF moved the drawtubes by about 4 µm per EAF step (so, about 250 steps per mm of drawtube movement).
On a belt driven Askar ACL200 lens with the same EAF motor, the lens only moves by 0.44 µm per EAF step. So over 2000 steps just to move the lens just by 1 mm.
If you'd attached the EAF to the slow motion shaft of the focuser, you are likely to see something closer to the Askar lens than to the R&P focuser on the Takahashi.
The number of steps wil also depend on your focal length. In general, the depth of focus is shorter with shorter focal lengths. So there is really no one-size-fits-all solution. You just have to figure it out for your own case.
The HFD I mentioned is the "Half Flux Diameter" of a star; a measurement of how large the star is in pixel units. There should be something that your program should be able to measure for you. It pretty much corresponds to the vertical axis of that hyperbola curve in the screen capture that you posted.
When you focus on a star, geometric optics predicts that the star size will change linearly with focuser travel. So, HFD will change linearly with EAF steps. However, as you approach focus, the star size becomes diffraction limited and focus no longer improves as you get to perfect focus. The diffraction limited region is the Critical Focus Zone of the optics. A good match to the HFD data points is a hyperbola; so that is what is used to estimate the actual focus without having to land on the exact focus while you are building that v-curve.
The one thing that you need to watch out for is the backlash from these stepper motors if you manually focus.
Because of backlash, most of the autofocus programs I know first moves the drawtube in one direction away from the focus point and then step the focuser only in one direction as it measures the star sizes.
Once it is done, and it can now fit a hyperbola curve on the star sizes to determine the EAF position needed for perfect focus, it then again moves the drawtube to that same initial point that it had started before sweeping for the EAF steps. From there, it moves to the final focus setting using the same direction that it makes for the star size measurements. This way, the backlash is cancelled out, and you don't have to worry even about measuring the backlash when using these autofocus routines.
You do need to measure the backlash if you are manually controlling the electronic focuser and rocking it back and forth to achieve focus, otherwise it is an exercise in frustration when looking for focus.
As you said, you will figure it out after you are familiar with how these gizmos work.
Chen