Bahtinov mask

Bahtinov mask example
Operation of a Bahtinov mask for refracting optics, showing that rotating it 180° reverses the direction of the pattern

The Bahtinov mask is a device used to focus small astronomical telescopes accurately. It was invented by Russian amateur astrophotographer Pavel Bahtinov in 2005 (http://www.astronomy.ru/forum/index.php/topic,10421.0.html).[1] Accurate focusing of telescopes and astrographs is particularly of concern to those involved in 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 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.

Small deviations from optimal focus are easily visible. For astrophotography, a digital image can be analyzed by software to locate the alignment of the spikes to sub-pixel resolution.

The direction of this displacement indicates the direction of the necessary focus correction. Rotating the mask through 180° will reverse the direction of spike movement, so it is easier to use if placed on the telescope with consistent orientation. The mask must be removed after accurate focus is achieved.

A very bright star and very dark sky is required for to produce clearly visible diffraction spikes.

In the example below, the center pattern shows good focus. The central spike is noticeably displaced from the center position in the left and right images.

See also

References

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