OK so the bottom line: reducing the air assist does reduce engraving residue.
All of these are engraves in solid maple, 750/full/vary power/270 lpi
No reduction. This is a deep engrave, so you expect a lot of residue.
Level 4 reduction, still a bit of airflow but severely reduced. Noticeably less residue.
Level 5 reduction, completely blocked. Even less than level 4.
Side by side comparisons of the full power/no airflow engraves. Notice the reflection on the tarry residue.
Full air assist power:
No air flow:
The residue is still present but it’s far less thick. My optics seem fine, not excessively dirty at all. There was no flareup, the smoke was quickly evacuated just via normal exhaust airflow. Of course flareup and other properties may be different depending on the material, but hardwood maple presented no problem. A cleaner-engraving wood like walnut might have even better performance.