The plots below show the performance of the MDI shutter over the SOHO mission, as devived from the shutter opening and closing times reported in the MDI housekeeping data. The shutter timing information were selected from +/-15 seconds around the TAI minute to insure that exposures were taken during the structure program. This guaranteed that only nominal full disk exposure times were included, and that there were a consistent number of exposures per day (typically about 14,000). Daily mean and standard deviation were determined for the opening, closing and exposure times.
The first plot graphs the daily mean of the shutter opening time, and the second plot shows the difference between the commanded exposure time and the mean of the measured exposure time. The third plot shows the mean shutter exposure time divided by the standard deviation which is a measure of the shutter repeatability or "quality".
There was a significant increase in the opening time beginning on 18 March 2000, probably due to degradation of the shutter bearings. The decrease in shutter opening time in February 2002 was the result of a decrease in Optics Package temperature after turning off one of the fixed heaters. The exposure standard deviation was initially on the order of 10 micoseconds and is currently on the order of 30 microseconds. The shutter repeatability showed a decrease several months before the shutter opening time increased, and a larger drop was seen shortly before the jump in the shutter opening time.
