Focussing with a Bahtinov Mask

If you have tried astrophotography then you will know how critical and how difficult it can be to find razor-sharp focus – stars appear as point light sources and there is no detailed surfaces to help in achieving focus. A Bahtinov MaskĀ is a simple and widely used tool that can help.

The Bahtinov Mask was invented by Russian amateur astrophotographer Pavel Bahtinov in 2005. The mask is an opaque disk with slots cut out in a specific pattern. The mask is placed in front of the telescope’s objective lens or mirror so that three diffraction spikes appear when the telescope is pointed at a bright star.

The central spike moves in relation to the other two spikes as the focus is changed. Focus is achieved when the central spike is perfectly centered between the other two spikes.

The image above show how the bright star Menkalinan in Auriga appeared when imaged using a Bahtinov mask on a Celestron EdgeHD 8 with a Nikon D5100 DSLR.

A Bahtinov mask should be the proper size to fit your telescope. I have two masks: one that fits over the dew shield of a ED100 refractor and another that fits over the corrector plate of a Celestron EdgeHD 8.

Bahtinov masks can be bought commercially or you can make one yourself – the deep-sky watch website has printable PDF plans for many common telescopes.