The automatic guiding is realized with the camera in the spectrograph head. The slit of the spectrograph is not perpendicular on the optical axis but it is a slanted. Moreover it is coated with an aluminium. Thus the camera can scan the picture of the star on the slit.
The picture of the star is cut on two perpendicular axes and the center of intensity of the light is found on these cross sections inependently.
The next step is to de-rotate the picture. As the slit is fixed while the telescope is moving, the visual field is rotating as it is projected on the mentioned camera and this effect must be taken into account for the correct autoguiding. Due to the complicated optical path the visual field executes two rotation per one rotation of the telescope around the polar axis. The de-rotation is computed in the system driving the telescope.
Trick, contraptions For the decreasing the noise we did not
use the full field of the slit camera but we can cut bands with variable
width. Also the slit is eliminated from the calculation of the cross
The visualisation of the autoguiding interface. Green lines depicts bands used to the reckoning the cross section. The cross section and gaussian fits are also depicted (on the right and bottom). The red cross denotes the position on which the star is pointed, the blue cross is the immediate position of the center of the star.