MkII Multi-Receiver System for Frequency Flexibility

MkII Multiple Receiver System

The MkII Telescope at Jodrell Bank has been equipped with a multi-receiver system to allow rapid changes of frequency to give MERLIN a frequency flexibility.  This allows, for example, the rapid change of the MERLIN operating frequency from 5 Ghz to 22 Ghz so that the higher frequency observations can be made during the optimum weather conditions which often only persist for a few days.

The MkII is a prime focus instrument and the dimensions of the focus box would not allow a carousel system to be incorporated as for the E-Systems telescopes.   Instead, the receivers are mounted around an off axis spindle which rotates to bring the desired receiver onto the optical axis of the telescope.  The system has to be quite complex as the focal point of the telescope - the point where the phase centre of the feed has to be brought - is outside the focus box. Thus the sequence to the change from one receiver to another requires several steps.

First, the feed of the original receiver has to be retracted into the Focus Box. This is achieved by motors which drive a long screw along which the coupling to the receiver mount can move.  Thus the screw rotation is transformed into linear motion of the receiver mount.   750mm long Linear Displacement Transducers coupled into a Programmable Logic Controller (PLC) ensure that the precise control required to bring the feed to the focal point or stow position is acheived.

Secondly, the spindle is rotated to bring the desired receiver onto the optical axis of the telescope. A 16 bit optical encoder measures the rotation angle to high precision and the rotation is controlled by the same PLC.

Finally, the new receiver is driven along its linear track to bring its feed system out of the focus box to the focal point of the telescope.

For safety purposes, and infra-red "light curtain" protects the complete system so that if personnel in the focus box approach too close, the complete system is shut down.

A second view of the Receiver System

Current Status

At present these movements are controlled manually step by step.   A digital display can be used to show either the linear positions of the receivers or the rotation angle of the spindle as required.   During the 1999 summer engineering work, the PLC will be linked to the MkII Control computer and the receiver change over will be remotely controlled from MERLIN.