Computers and software in safety-critical applications

The application of computers and software to control continuously or on demand safety critical processes is unavoidable in complex telescopes and associated operation and maintenance support equipment. While the control of the maximum azimuth and elevation movement ranges of a telescope is a rather simple task and hardwired high reliable endstops can be used to practically eliminate any risk associated with the potential exceeding of the range limits, the handling and recoating of a 8 m thin Zerodur mirror is quite a delicate activity. It includes already a number of computer controlled actions which--if no risk reduction measures are established--lead to very high risk. But in complex systems like an 8 m optical telescope, coating facilities or similar the functional control and the control of major safety critical operations have to be performed by computers. Straightforward hardwired interlocks devices are not adequate for the control and monitor of highly complex processes. The still increasing process complexity will also lead to the demand for fast and intelligent risk limitation and reduction devices, and the use of safety related systems based on electrical, electronic and/or programmable electronic technology is unavoidable. But in using computers and software in safety critical applications the applicable standards and norms have to be applied strictly.