In the primary focus of the telescope there are three receivers installed in parallel. When the telescope is prepared for primary focus observations only one such receiver can be mounted in the focus of the telescope and used for observations. A change of an installed receiver normally is possible from day to day.

The first two receivers indicated down as installed (the 21 and the 1.3 cm RX) are permanently installed in the primary focus cabin.

The other prime focus receivers can be used for observations, if  they are exchanged with the one installed in the in the primary focus cabin. Such a receiver exchange is only done some times in the year.