A diffraction pattern that makes focus a symmetry, not a judgement

The best Bahtinov masks for 2026

A Bahtinov mask is a sheet of slots placed over the front of a telescope, and it turns focusing from a judgement into a geometry. The slots throw a bright star into three diffraction spikes, two forming an X and the third crossing them; when the telescope is exactly in focus the third spike passes through the centre of the X, and when it is not, the spike sits visibly to one side. The eye is far better at judging symmetry than at judging when a blur is smallest, which is the whole point, and it is why this small piece of plastic is standard equipment for anyone imaging. Fit is the first question: the mask has to cover the full aperture and sit on the tube without being blown off, and the outside diameter of the tube, not the aperture of the optics, is the measurement that matters. The second question is slot geometry, which is calculated for a focal ratio — a mask designed for a fast telescope produces spikes that are too coarse on a slow one, and the pattern becomes hard to read. Better makers state the range. For a reflector or a folded telescope, the mask has to clear the central obstruction sensibly or the pattern weakens. Read the material: thin plastic warps in cold and takes a set in a bag, while a rigid sheet or a metal mask holds its slots true. We compare aperture and tube fit, slot geometry and focal ratio range, material and rigidity, mounting security and obstruction clearance.