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,appear to have lines (called spikes) radiating from them.
