Vega in focus with the Bahtinov mask
The Bahtinov mask is a device used to focus small astronomical telescopes accurately. It was invented by Russian amateur astrophotographer Pavel Bahtinov in 2005. Accurate focusing of telescopes and astrographs is particularly of concern to those involved in astrophotography.
The mask consists of three separate grids, positioned in such a way that the grids produce three angled diffraction spikes at the focal plane of the instrument for each bright image element. As the instrument’s focus is changed the central spike appears to move from one side of the star to the other. In reality, all three spikes move but the central spike moves in the opposite direction to the two spikes forming the ‘X’. Optimum focus is achieved when the middle spike is centered between the other two spikes.
I tried it out on Vega last night. Watch the slideshow below (5 images at 5 second intervals) to see how I got on.
way off focus
close but offset left
close but offset right