The original motivation for the design used in the monitor and control system for the Green Bank Telescope (GBT) (Lockman, 1998) was to generate a radically modular ``straw man" in order to better understand the limits of modularity1 when implementing a control system for a general purpose telescope such as the GBT. However, to our surprise, repeated scenarios did not break the model, so it was decided to adopt the design for the GBT. There were two other influences affecting the choice of design. We were given the requirement to think in terms of a virtual telescope, i.e., to design a system that was not telescope specific. This was important not so much because there were plans to use the system on additional telescopes, but rather the fact that a telescope does, in fact, become a different telescope over time because of changes in instrumentation. The second influence was that, at the time, I was first becoming impressed with object-oriented methods, and my inclination was to design everything as objects. No regrets or apologies.
The basic design (Clark,1998) is simple. A radio telescope is defined as a laboratory rather than an instrument, i.e., it is a set of devices (of which the antenna is only one) which needs to be configured in novel ways to accomplish observations (or scans). Each device is an autonomous subsystem which can be fully configured prior to a scan through a finite set of control parameters in order to run the scan in coordination with other devices. For purposes of configuration and observing, each device requires no more than four interfaces (control, monitor, message/alarm, and data) which are identical for all devices. The same control interface for user-programs is also used to recursively build a tree of control modules whose root is a generic ``scan coordinator." Any control module of the tree may be used to initiate a scan for the sub-tree under its control because each control module is derived from the same base class (called a Manager) and therefore inherits all of the control protocols required to coordinate and initiate a scan.