The Bahtinov mask is a device used to focus small astronomical telescopes accurately. Although masks have long been used as focusing aids, the distinctive pattern was invented by Russian amateur astrophotographer Pavel Bahtinov () in 2005. Precise focusing of

telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

s and

astrograph An astrograph (or astrographic camera) is a telescope designed for the sole purpose of astrophotography. Astrographs are mostly used in wide-field astronomical surveys of the sky and for detection of objects such as asteroids, meteors, an ...

s is critical to performing

astrophotography Astrophotography, also known as astronomical imaging, is the photography or imaging of astronomical objects, celestial events, or areas of the night sky. The first photograph of an astronomical object (the Moon) was taken in 1839, but it was no ...

. The telescope is pointed at a bright

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

, and a mask is placed in front of the telescope's objective (or in front of the aperture). The mask consists of three separate grids, positioned in such a way that the grids produce three angled

diffraction Diffraction is the deviation of waves from straight-line propagation without any change in their energy due to an obstacle or through an aperture. The diffracting object or aperture effectively becomes a secondary source of the Wave propagation ...

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". Optimal 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. The mask works by replacing the

aperture stop In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image of ...

of the optical system (normally the circular shape of the objective itself) with a stop which is asymmetric and periodic. Viewing a point source (such as a star) yields a diffraction pattern at the focal plane representing the Fraunhofer diffraction transform of the aperture shape. This pattern normally would be an

Airy disk In optics, the Airy disk (or Airy disc) and Airy pattern are descriptions of the best-focus (optics), focused Point source#Light, spot of light that a perfect lens (optics), lens with a circular aperture can make, limited by the diffraction of ...

resulting from a circular aperture, but with the mask in place, the pattern exhibits asymmetric spikes representing the transform of the mask pattern's spatial frequency and orientation. A very bright star and very dark sky are required to produce highly contrasted spikes that are clearly visible. The diffraction effect is similar to producing sunstar patterns in landscape photography with ordinary camera lenses, where the mechanical iris of the lens is adjusted to a small polygonal shape with sharp corners. In the example below, the central pattern shows good focus. The central spike is noticeably displaced from the central position in the left and right images. Bahtinov mask example.jpg, Example diffraction patterns produced by a Bahtinov mask
(middle: in focus, left and right: slightly out of focus) Simulation of a bahtinov mask diffraction pattern when focusing.gif, Simulation of the Bahtinov mask diffraction pattern, using "Maskulator". Each frame shows a focus difference of 140.6 μm.

References

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External links

Windows version of the mask generator

Bahtinov mask pattern generator Web Apps

Bahtinov grabber, software tool to detect focus error for any live camera view

Maskulator, free software for diffraction simulation
Telescopes Optical devices

See also

References

External links