Bahtinov Mask (80mm f/5 refractor)

A Bahtinov focusing mask for a 80mm f/5 refractor (80mm aperture, 400mm focal length). Point the scope at a bright star with the mask on and the three slat grids throw a six-spike diffraction pattern; the central spike slides as focus changes and sits exactly centred at focus — a fraction of a millimetre of focuser travel is obvious in a way no unaided star test matches. The geometry is Pavel Bahtinov's open 2005 design: one half of the aperture carries a grid of parallel slats perpendicular to the dividing diameter, the other half is split into two quadrants whose grids sit at +20° and −20° to it. Slats and slits are equal at 2.67mm, from the usual sizing convention (focal length ÷ 150, the "Bahtinov factor"; anywhere in the 150–200 range works, coarser being brighter but less precise). The skirt is bored 104.6mm to push over a 104mm front end — measure yours, the design is parametric. Prints face-down in one piece, no support. PETG or ASA, in black, and matt if you can: it is used pointing at the sky at night.

from £11.54example, in PLA