I would assume that if the sample is very flat we have two issues causing a lack of focus. Slide tilt, and the parasitic z-motion from xy-motion.
If we had a reliable way of knowing where we were in the x-y plane then it may be possible to move to the extrema of the scan to calculate slide tilt, and then scan quickly using feed-forward for focus. This is very hypothetical as it would require really good closed loop control. But I think the steps to get there are
- Good ways to “home” xy in the centre. Some great work on this from @siddons
- Closed loop xy motion using the camera as an encoder. I know @r.w.bowman and @JohemianKnapsody are working on this
- Closed loop z. This is harder, and I suppose would be needed as due to imperfect backlash compensation?
I suppose experimenting with missing out the focus step on some scans is simple enough to implement, and may be of use in some applications. We would just need to make it clear that “here be dragons” until it is well tested.