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 (russian: Павел Бахтинов) in 2005. Precise focusing of

telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observ ...

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, ...

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 1840, but it was no ...

. The telescope is pointed at a bright

star A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth ma ...

, 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 defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...

spikes at the

focal plane In Gaussian optics, the cardinal points consist of three pairs of points located on the optical axis of a rotationally symmetric, focal, optical system. These are the '' focal points'', the principal points, and the nodal points. For ''ideal'' ...

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, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An opti ...

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 In optics, the Fraunhofer diffraction equation is used to model the diffraction of waves when plane waves are incident on a diffracting object, and the diffraction pattern is viewed at a sufficiently long distance (a distance satisfying Fraunhof ...

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- focused spot of light that a perfect lens with a circular aperture can make, limited by the diffraction of light. The Airy disk is of importance in physics ...

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