Sorry, I see that I missed some words in my text. I never use the Canon's built in autofocus. I have either a PC with APT to control the focus inside the lens or a Raspberry with Indi drivers. None of them can control the lens focus motor as I wish. It's because of Canon's limitations.
If I want control the Canon lens motor I can use an Canon Remote app in my smartphone if I enable the WIFI in the Canon 6D. But then the camera can not communicate over the USB. And after it's focused I must switch the lens' button to manual focus again. To have perfect focus during a night I need to refocus about every 45 minutes.
But with the Astromechanics adapter (the link on my page) I can separate the camera (other cameras with shorter backfocus) and lens and control the focus independent. There is a way to do it in a similar way with the DSLR also, I have talked with a friend about this.
I have updated my page:
A question: When you used the 300mm f/2.8, did you use it at full aperture or stopped it down ?
When I compare auto focus lens with manuell lenses I have noticed that auto focus lenses can focus more beyond the infinity which is good for astro use.