Achieving focus is arguably the most important aspect of a successful image. This month's project is a focusing aid to help you achieve pin-sharp stars and therefore pin-sharp images. The design, developed by astronomer Paul Bahtinov, is a mask that makes use of diffraction, an optical effect that happens when light passes through small openings. Diffraction effects can be observed with a Newtonian telescope that has a 'spider' support for the secondary mirror, or a camera lens with an iris that has straight edges - in both cases, bright stars appear to have lines (called spikes) radiating from them.
Our mask has three sets of diffracting slits through which the starlight passes. On one side, two sets of angled slits produce a cross pattern, while the straight slits on the other side create a line that passes through the cross. When the image is properly focused, this line passes exactly through the centre of the cross, but when unfocused it is offset to one side.
Cutting it fine
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