What is the axis of a telescope

The azimuthal mounts

But now we know from daily experience that a star - when it rises in the east - appears above the horizon and then slowly rises in an arc over the course of the night until it reaches its highest point above the horizon in the south and then slowly towards the west to sink back to the horizon. The star does not move, but the movement of the star is simulated by the rotation of the earth around its axis of rotation.

If you enter the coordinates of a celestial object in the azimuthal coordinate system, time information is also included - e.g. at 9:30 p.m. the object is at the azimuth angle of 172 degrees and the elevation angle of 32 degrees. Both angles change continuously with the observation time due to the rotation of the earth.

Azimuthal mounts are available in different designs, depending on how the telescope is attached to the mount. In the vast majority of cases, these are so-called fork mounts, where the telescope is attached between two fork arms. With smaller telescopes, you can save the second fork arm and attach it to one fork arm on one side. In the simplest case, both axes have to be set manually and can then be fixed using clamping devices - when the telescope is aligned with the object to be observed. If you now observe your set observation object, you will notice quite quickly how the observation object disappears from the field of view as a result of the earth's rotation. So you have to readjust both axes permanently for a longer observation time in order to follow the observation object.

Technically more sophisticated azimuthal mounts then have so-called worm gear drives - flanged to both axes - which allow the telescope to be adjusted slowly and very sensitively over both axes and thus to compensate for the rotation of the earth. Since the star describes an arc in the sky, you have to readjust both axes at the same time, which is not easy and quite annoying because you concentrate more on the steady movement than on the actual observation.


Keyword worm gear:
A worm gear practically consists of a worm wheel (similar to a gear with very wide teeth) and a worm shaft. The worm shaft engages in the gear. If you turn the worm shaft one revolution, the gear wheel rotates one tooth further. In principle, a worm gear drive is a reduction gear. This achieves a large reduction in order to achieve the necessary slow compensation movement for the rotation of the earth.

Guide value:
a star moves roughly 15 arc seconds per time second as a result of the earth's rotation.

Keyword arcsecond:
A degree is divided into 60 arc minutes and each arc minute in turn into 60 arc seconds. One degree corresponds to 3,600 arc seconds.

Reference value:
The sun and moon appear from the earth at an angle of roughly 0.5 degrees, corresponding to 1,800 arc seconds.

If azimuthal mounts become even more convenient, both axes then have drive motors that are moved via a controller in such a way that azimuth and height are permanently readjusted and the object under observation always remains in the center of the field of view - with occasional manual corrections.