What is a Bahtinov mask?

BBC Sky at Night Magazine's Scope Doctor Steve Richards answers your astronomy ailments.

Bahtinov mask. Credit: Axleottal (https://en.wikipedia.org/wiki/Bahtinov_mask)

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.

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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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Email Steve your astronomy queries to contactus@skyatnightmagazine.com and they could be answered in a future issue of BBC Sky at Night Magazine.