Anyone who ever built a dual x printer knows the problem, how to get the difference between the two nozzles down exactly. I tried many things myself, including printed interference patterns, and conduction. All work to some degree, but never reliably.
Then one day i saw one of those cheap USB microscopes and it was pretty clear what will work.
€12 spent on ebay later i was the proud owner of a super cheap USB microscope with the fantastic resolution of 0.3 megapixel.
I printed a small helper, a crosshair for the microscope
With the help of the microscope and the crosshair it was suddenly very easy to calculate the precise offset between nozzle 1 and 2. I just placed nozzle 1 in an easily recognizable place, noted down the coordinates, then switched to nozzle 2 and drove it by hand to the same visual position, et voila, the two coordinates that represent the same exact spot in X/Y. With a bit of work you can do better than 1/10mm, though that should suffice for most uses.
Another advantage of this is that it is now easy to check the repeatability of positioning in X and Y. In the past i had to flexible endstops, the microscope exposes stuff like this easily.
Another advantage is that you can now easily see if a nozzle is worn down.
Thanks to Thomas Kramm, we got this idea while presenting at the Maxon Supermeet 2018