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)
Published: May 11, 2020 at 8:08 am
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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."


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


Astronomer Steve Richards
Steve RichardsAstronomer and author

Steve Richards is a DIY astronomy expert and author of Making Every Photon Count: A Beginner’s Guide to Deep Sky Astrophotography.

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