GBT Open Loop Active Surface

The Green Bank Telescope (GBT) is a 100-meter, fully steerable, unblocked aperture radio telescope presently under construction in Green Bank, WV, USA. Challenging design requirements of large aperture, operating frequencies up to 100 GHz and low cost have led to the implementation of an instrument with an active primary reflector. Following on the heels of adaptive primary optical telescopes as well as radio telescopes which use actuators to make infrequent surface adjustments, the GBT is the first radio telescope to attempt to dynamically control a large primary.

During the design phase of the Green Bank Telescope (GBT), various means of providing an accurate surface (0.220 mm) on a large aperture paraboloid (100 m), were considered. Automated jacks supporting the primary reflector were selected as the appropriate technology since they promised greater performance and potentially lower costs than a homologous or carbon fiber design, and had certain advantages over an active secondary.

The design of the Active Surface has presented many challenges. Since the actuators are mounted on a tipping structure, it was required that they support a significant side-load. Such devices were not readily available commercially so they had to be developed. Additional actuator requirements include low backlash, repeatable positioning, and an operational life of at least 20 years. Similarly, no control system capable of controlling the 2209 actuators was commercially available. Again a prime requirement was reliability. Maintainability was also a very important consideration.

The system architecture is tree-like. An active surface "master-computer" controls interaction with the telescope control system, and controls ancillary equipment such as power supplies and temperature monitors. Two slave computers interface with the master-computer, and each closes approximately 1100 position loops. For simplicity, the servo is an "on/off" type, yet achieves a positioning resolution of 25 microns. Each slave computer interfaces with 4 VME I/O cards, which in turn communicate with 140 control modules. The control modules read out the positions of the actuators every 0.1 seconds and control the actuators' DC motors.

Initially control of the active surface will be based on an elevation dependant structural model. Later, the model will be improved by holographic observations. Surface accuracy will be improved further by using a laser ranging system which will actively measure the surface figure.

Several tests have been conducted to assure that the system will perform as desired when installed on the telescope. These include actuator life tests, motor life tests, position transducer accuracy tests, as well as positioning accuracy tests.

*taken from GBT Memo 184 by Richard J. Lacasse