AstroDude42 The worst is the SII filter by about 1000 steps off.
I was just thinking about this again, Olli ....
I don't know what your EAF scale is. With my R&P focusers, I see about a 4 µm to 4.2 µm drawtube travel per EAF step. (You can measure it easily by taking 1 thousand or two EAF steps and measure the drawtube travel. It is worth measuring and keeping in your notebook for future references).
If yours is about 4 µm per step (EAF attached to SCTs tend to have many more steps per mm), 1000 EAF steps would represent a whopping 4mm of travel! As they say in German, it is unmöglich!
I assume you are using a Bahtinov mask right? Who knows what the ASIAIR does in autofocus with the halos that are more prevalent with narrow band filters. It could be measuring changes to the halos, instead of the actual Airy point spread function. The Bahtinov works in the diffraction (Fourier) domain, and is always right.
Perhaps you have the typical EAF-to-SCT focuser set up which have EAF scales in the hundreds of EAF step per µm?
Glass thickness is definitely not possible, since it would represent 12mm worth of glass thickness difference for a constant refractive index of 1.5. I also cannot imagine 12mm worth of axial change due to chromatic aberration.
What is your EAF scale?
Perhaps you are measuring with an SCT, or a belt driven helical focuser?
If all the Baader glass have the same thickness and glass refractive index, you should get no focus shift. At the same time, I can't imagine such a large focus shift due to chromatic aberration, either, especially since SII is so close in wavelength to H-alpha. So, it is mysterious why you are seeing a focus shift.
Speaking of the EAF, I have special EAF mounting plates fabricated (Web machine shop in Seattle, Washington that is a subsidiary of a German company; they have a CAD tool for macos - draw it, submit drawing [with money :-)] and back comes a plate after a week).
I couldn't stand the fact that the ZWO mounting has the EAF stick down below the dovetail plate if the R&P focuser is mounted horizontally. This is my plate on the FSQ-85:
Notice the plate with a large hole to accommodate the EAF shaft.
Here is different view of another EAF my FOA-60Q:
Notice that with my Dual Saddle plate, I cannot let the EAF stick below the dovetail.
My EAFs are all "upside down." At this point, between 12V and 5V EAF, I have about 7 of them (perhaps more -- I lost count).