What is a Bahtinov mask?
Steve says: “A Bahtinov mask is a specially cut visor made from cardboard or plastic that you place on the front of a telescope to help you focus accurately.
The special shape of the cut-outs in the mask produce a set of diffraction spikes in the same manner as the secondary mirror spider vanes on a Newtonian reflector.
However, the Bahtinov mask’s diffraction spikes form a very specific shape comprising a cross with a line passing through it.
Observing a diffracted bright star with this mask in place and adjusting the focus until the line perfectly bisects the cross ensures that accurate focus is achieved.”
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