Building Systems the Old Fashioned Way

Abstract

Most real real-time systems are hybrid systems with certain deadlines which must be met, others which are slippable and other computations which may run as resources permit. In astronomy, computers are used to drive the telescope, rotate the dome, control the instrument and record, display and even reduce (analyse) the data. Some of these tasks have no real deadlines: data reduction can always be done later, off-line, but with a reduction in the ability of the scientist to modify his/her experiment as the evening progresses. Some of these tasks have soft deadlines: rotating the dome may be delayed as long as the slit still clears the telescope field of view; displaying the data need not be 100% up-to-date. Still other tasks must meet hard deadlines: the telescope must be driven consistently across the sky to keep the object centered; the instrument must be controlled as required to record data at precise intervals, etc. But even some of these hard tasks have widely varying time scales. The result is a very complicated system.