Must say if you have to refocus several times a night - maybe consider an On Axis guider and add their focus lock solution - which changes focus on the guiding chip and hence on the primary acquisition camera in real time as the focus changes with temporature. The physics as I understand it is a single cold mirror will make the guide star a slight oval - and the orientation of the primary axis of the oval will shift as according to the direction you move from focus - this allows the software to re-focus on the fly during every image - nifty!
https://www.innovationsforesight.com...s-guider-onag/