Bahtinov mask
Encyclopedia
The Bahtinov mask is a device used to accurately focus astronomical telescopes.
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. The mask must be removed after accurate focusing is achieved.
It is named after its inventor Pavel Bahtinov. Accurate focusing of telescopes
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
and astrographs
Astrograph
An astrograph is a telescope designed for the sole purpose of astrophotography. Astrographs are usually used in wide field surveys of the night sky as well as detection of objects such as asteroids, meteors, and comets.-Design:...
is particularly of concern to those involved in astrophotography
Astrophotography
Astrophotography is a specialized type of photography that entails recording images of astronomical objects and large areas of the night sky. The first photographs of an astronomical object were taken in the 1840s, but it was not until the late 19th century that advances in technology allowed for...
.
The mask consists of three separate grids, positioned in such a way that the grids produce three angled diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
spikes at the focal plane of the instrument for each bright image element (star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
). 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. The mask must be removed after accurate focusing is achieved.