While I was puttering around inside the laser cabinet, I figured it was time to check the mirrors for cleanliness. The first two mirrors looked fine, but Mirror 3 needed help:
A first pass with an optical wipe removed most of the crud:
Gentle touch-up with a little more isopropyl alcohol cleared the rest:
The focus lens required similar attention, but there is no way to get meaningful pictures of a transparent lens.
Realigning the mirrors went well (top before, middle during, lower after):
The diagonal results at Mirror 3 show the XY axes aren’t quite square, but AFAICT it’s close enough. The rightmost tape shows good beam centering in the nozzle and the Focus target shows excellent Z alignment over about 50 mm of travel.