Bahtinov masks.

Bathinov Masks - 2018
Bathinov Masks – 2018

Bahtinov masks are easy to 3dprint, and are a very precise focus tool when preparing an astrophotography session. Grid holes in the mask cause diffraction spikes to appear when looking at the point light from stars. Achieving optimal focus consists in centering the middle spike between the side spikes, so that the figure is symmetric.