The Acquisition and Guiding Unit (AGU) is mounted on the Cassegrain rotator of the telescope. The AGU provides 5 focal stations for the astronomical instruments, one for the corrected field with 1.05° diameter and four side looking focal stations for the field with 10arcmin diameter. The science fold Mirror directs the light from the telescope to any of the side looking focal stations.
The AGU also provides an offset guiding system (AGS). The AGS patrols a wide field to find bright stars, off axis from the target of interest to the astronomer.
All these functions are to be carried out with reduced vignette of the path of the science beam, or without introducing stray light or extra emission at the science focal plane.