Dust and grease are the biggest enemy of good optics. Of course, this is especially true for microscopes and telescopes.
As a matter of principle, every microscope should be covered with a dust cover after use.
The eyepieces are particularly sensitive, as they are the only part of the optics directly exposed to the user.
I have designed simple protective caps for my ZEISS STEMI SV8 with 42 mm eyepieces to protect them adequately.
I printed the parts with a 20 % infill, a layer height of 0.2 mm and in PETG.
