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Focussing with a Bahtinov mask

The Bahtinov mask is a wonderful device invented in about 2004. It is inexpensive, being just an arrangement of slits in a plastic board that hangs over the front of the telescope. You can get one, of the right size for your telescope, from any telescope supplier. Or you can find templates on the Internet for cutting a mask yourself.

A bright star seen through the mask has a diffraction pattern comprising 3 spikes as shown. The magical thing is that as you turn the focussing knob on the scope, the central spike moves through the cross shape formed by the other two spikes. Perfect focus is achieved when the movable spike exactly bisects the other two. With a camera set to magnified live view, this is easy to do.

Smaller Bahtinov masks can be obtained (or made) for camera lenses but I have not tried that.

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