Bahtinov mask

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Bahtinov mask example

The Bahtinov mask is a device used to focus astronomical telescopes accurately. It is named after its inventor Pavel Bahtinov. 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 (star). 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 on the star and symmetrically positioned between the other two spikes. Small deviations from optimal focus are easily visible.

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. The direction of this displacement provides information on which way focus needs to be made. The operator will place the mask over the front aperture of the telescope in the same orientation each time. He/she will then become familiar with the direction clue provided by the central spike. Rotating the mask through 180 degrees will reverse the direction of spike movement, hence the need to place the mask on the scope with consistent orientation. Very bright star and very dark sky is required for focusing. Otherwise the spikes are not very well visible or not visible at all. The mask must be removed after accurate focusing is achieved.

Example diffraction patterns produced by a Bahtinov mask
Simulation of the Bahtinov mask diffraction pattern, using "Maskulator".


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