Guide scope
In astronomy, a guide telescope or guide scope is a telescope with a long focal length for checking long-exposure images of the sky .
The guide scope is mounted parallel to a second telescope or an astrograph and monitors the manual or mechanical tracking of the daily rotation of the starry sky. If small deviations occur, either the tracking speed is adjusted (this can also be done automatically with CCD sensors) or a small change in the angle of the transverse axis ( declination ) is made. The aim is to achieve a precisely punctiform image of the stars. For this, the focal length of the guide telescope should be at least twice as long as that of the camera (with a double telescope , the situation can be different). Modern electronic tracking systems that work with "subpixel accuracy" can also use optics with a relatively short focal length as a guide telescope. There one speaks of so-called searcher guiding.
The above method requires equatorial mounting , where the "hour axis" points to the celestial pole . Newer, often GPS-controlled instruments are mostly azimuthally mounted (vertical axis of rotation similar to a theodolite ). Here a rotation of the image field occurs (see parallactic angle ), which has to be corrected additionally or (after a few minutes) becomes noticeable as short traces of the stars.
The direction control is carried out with a guide star of suitable brightness, which is held by the observer precisely on the crosshairs of the guide telescope. In the sky, the guiding star should be approximately in the direction of the sky image (i.e. the center of the image). If its angular distance is too great, the astronomical refraction , which changes with height, causes the guide star and the photographic axis to drift slightly apart.
Modern large telescopes can also monitor each other's direction if they have a coupled control - for example with the Large Binocular Telescope (LBT).