Observing system

It was noted that there is an issue with the exposure meter when in use while the command addtime is used to change the overall exposure time. In particular, if the exposure time was reduced, the exposure meter would continue to operate. If not stopped manually this can interfere in subsequent exposures. The exposure command using the exposure meter was modified to allow completely transparent use of the meter. Every time an exposures is started a very long (6 hours) exposure time is given to the exposure meter so that it will count until the desired count level is reach, or 6 hours have passed. If time is added to the science exposure (as long as the total exposure time is no longer than 6 hours) there is no problem as the exposure meter will continue. If time is subtracted, the exposure is aborted, read out or the science exposure reaches its time limit, the exposure meter will be automatically stopped.