While I'm waiting for help on the Marlin Delta driver, I've soldered a 12ga wire brace to the probe arm and shortened it to 85mm. This reduces twang a *lot*. I've mounted a 22ga hypodermic on a croc clip, which clips to the probe arm so I can swap tips. The current tip is bent at 60 degrees so I can sneak it all through the lens turret on the microscope and get closer to vertical contact with the slide.

I've mixed up some machine oil with soot, which approximates the consistency of photosensitive resin. I keep it in a little dish made by wrapping an M8 nut in foil and dishing one side.

A small drop on the microscope slide serves as a reservoir, and contact with the drop gives me good feedback on the probe position (may be useful later, as is seeing the probe shadow). I'm manually tracking the probe until the driver software is sorted. Anyway, this lets me dip the 40μm probe tip then move it elsewhere and touch the slide. Bear in mind the tip is elongated on a hypodermic. This dipped tip gets me several consistent dots, about 30μm in size. Here's one viewed through the microscope: