Field rotation

As a field rotation in which it is astrophotography the disturbing effect called that during long exposures, the image field around its center rotates and the stars will no longer be accurately imaged point-shaped.

With the altazimuthal mount, the rotation of the field of view is caused by the fact that the earth's rotation is compensated around a vertical axis instead of the celestial axis (parallel to the earth's axis). The rotation is noticeable after a few seconds with long focal lengths.

An equatorial mounting of the telescope or astrograph avoids this effect if it is aligned exactly to the celestial pole (see also Scheiner method ). In the event of a small deviation from this, the tracking can be corrected using the guide telescope , but outside the center of the image the constellations can be slightly distorted tangentially.

The rotation of the image field can be calculated mathematically by means of the parallactic angle , which depends on the star location, the time and the latitude . More precisely, the field rotation is the time derivative of the parallactic angle η. For a given azimuth A and a given altitude h on the latitude it is φ ${\displaystyle {\dot {\eta }}}$

${\displaystyle {\dot {\eta }}=\Omega {\frac {\cos(\varphi )\cdot \cos(A)}{\cos(h)}},}$

where Ω is the sidereal rotational frequency of the earth, i.e. the speed with which the earth rotates beneath the starry sky. This amounts to

${\displaystyle \Omega ={\frac {1{\text{ Umdrehung}}}{\text{Sterntag}}}={\frac {360^{\circ }}{86164{,}091\,{\text{s}}}}=4{,}178\cdot 10^{-3}\,{\frac {\text{deg}}{\text{s}}}=15{,}04\,{\frac {\text{Bogensekunden}}{\text{Zeitsekunde}}}.}$