Both the Prime and Cassegrain foci are equipped with autoguiders. The two
systems are identical and have FW-130 image dissectors with S-20 photocathodes
as detector.
The autoguiders are controlled by a dedicated Micro Processor, which
continuously analyses the image of a guide star, and provides small
corrections to the telescope tracking (just two numbers) via the interface
polled by the PE 
3220. That is, of course, their main function, but they
also monitor transparency and seeing. The guide-star image and seeing data
can be displayed on a VDU in the Control Desk for inspection.
In order to acquire a guide star, a prism mounted on the end of a
probe is moved to its position, which either can be measured in advance
and entered via the User Interface, or it can be determined
by using a cursor on the finder TV picture (see section 
).