[seszett]

> Normally you’d focus an enlarger visually, perhaps with a grain focuser—maybe at this point you’d be focusing manually with the grain focuser in the visible region and then making some calculated adjustment for UV

Yes, I have some experience with this (see my other reply in this thread) and it's a real problem but there is an easy fix.

Instead of using a grain-focuser (which would also be dangerous for the eyes anyway, and good quality UV lights don't leak much into the visible) you use plain white paper, cheap paper that is treated with optical brighteners.

These brighteners glow in the UV to make the paper look whiter, which means that under your UV enlarger, they turn the projected UV into a visible image. You can then focus using a regular looking glass to look at the paper. And UV-blocking sunglasses (you can easily test if they actually block UV: the paper will not glow at all if UV-blocking glasses are in the way).

Some lenses are corrected for UV-Vis focus shift, but they are generally extremely expensive.