A Bathinov mask is a focusing tool for checking precise focus by observing the diffraction pattern formed around stars. Placing the mask in front of a telescope’s objective produces spikes: a perfectly focused star shows symmetrical pattern in contrast to an asymmetrical pattern produced by an unfocused star.
A Bathinov mask for a 4 inch refractor. When not in use, the mask is placed inside the lens cover.
Ideally, the mask should cover the whole aperture of the telescope. Recently, however, I’ve successfully used an undersized mask for a larger telescope. A Bathinov mask allows you to evaluate whether precise focus has been achieved.
Night Sky in Focus © Anthony Urbano | Bacoor, Philippines
Night Sky in Focus 