A bahtinov mask is a device used to focus small astronomical telescopes and lenses very accurately. It was invented by Russian amateur astrophotographer Pavel bahtinov in 2005. Accurate focusing of telescopes camera lenses is particularly of concern to those involved seeking to photograph the night sky (astrophotography.)
The telescope is pointed at a bright star, and a mask is placed in front of the telescope’s objective (e.g. primary mirror).
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 telescope or camera lens 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”. Optimal focus is achieved when the middle spike is centred between the other two spikes.
How to use your Bahtinov mask with your telescope or camera lens?
Set up your telescope and imaging equipment as normal. Then simply locate a bright star (maybe Spica?) with your telescope or Dslr and lens and attach the Bahtinov mask to your telescope as seen in the image on the right and watch the diffraction spike pattern around the star as you adjust the focus. When the middle diffraction spike is centered in the outer spikes, you’re in focus. Very little focusing travel is required to move the inner diffraction spike, which means the focus is extremely accurate.
Where do find I these Bahtinov Masks?
I purchased mine from Amazon for my Dslr i used a 55-90mm (2.2inch – 3.6inch) one. My lenses are 70mm so this covers 55mm to 90mm so is ideal. These are a great bit of kit and provide with you with perfect focus which when using a Dslr and lens combination is quite a tricky thing to do.
If questions please comment below.