There are some tools on the web that lets you figure out the critical focus zone (CFZ) or the "new critical focus zone" (NCFZ) of an OTA.
This is usually stated in mm or µm.
So, what you need to do is to convert that to EAF steps. It is easily done with refractors, by measuring the draw tube, but not as easily done when you don't have a drawtube to measure.
But all is not lost -- just takes longer, and you could even do this using artificial stars indoors. What you do is to move the focus out in one direction so that the FWHM is say about 8 to 12. Then slowly move the EAF in the direction of focus (never reverse direction, since backlash will invalidate your data). Measure the FWHM of stars (or artificial star) at each of this process, recording the EAF step and FWHM at each step. Keep doing this until the FWHM reaches a minimum and then start rising again until it is over 8 again. You now have a V shaped curve when you plot FWHM against EAF step.
The V will not come to a point at the minima, but smoothly flattens out as it nears the focus. The flatten part is the critical focus zone, anything 20% or 30% away from the minimum is probably good enough focus. (i.e., your stars only bloat by 20% to 30% withiin this zone -- small enough for government work).
Now take about a half of this zone and find out how many EAF steps it takes to move that amount.
Use that as your starting point of EAF steps to use. Some auto focus algorithms may prefer a rougher EAF step and some may prefer a finer step, but using half of the critical zone should get you in the ballpark, where you can do further refinements later.
Maybe zwo support could help me out?
You are kidding, right?
Chen
