Auto Focusing, Step Size



How is the step size used in Auto Focus Options interface calculated? It doesn’t look like it is based on the selected filter (light wavelength), and that is the only way I have seen it calculated. Is this different step size? Thanks.


It is not calculated. You enter it in the auto focus settings:


If you’re wondering how you find it, we have some guidance here:



I actually wasn’t clear with my question. I know how to calculate the value as a function of the wavelength of light and per filter. What I was asking is how do I calculate the number here given that it isn’t associated with any particular wavelength - any particular critical focus zone, or is this step independent from the step size which is a function of CFZ.

Step size = R / CFZ; R = a function of the focuser mechanics, and CFZ a function of Focal Ratio and light wavelength.


I see what you are referring to - and I had read it and forgotten, thanks for the reminder.



Now that I have read the guidance again and think I understand it, I also remember we may have discussed this before.

I know that we calculate a “step size” as a function of the OTA and stepper motor resolution and the wavelength of the light we are capturing. The question I have now is if the step size for which an field is provided in the user interface, and the value that is used to determine how far to rack the tube out first and then back in, is the same step size as I described. The guidance doesn’t say anything about the step size there and how it is calculated.




“Step size” is the number of steps (of the stepper motor) between two successive data points. It is the same parameter that is physically correlated to the stepper motor’s resolution and CFZ. However, there is no need of a high degree of accuracy. If you really want to calculate it - OK, the calculated value will be in the right range. Take it as a good starting point. But then, you have to give it a try and - depending on the result of the auto focus run - you have to tune this value in order that you get a nice V-curve.



Okay, thanks, I will clarify that in my notes.