EPIC

Rotating shutter and exposure times

In front of the filterwheels there is a rotating shutter wheel mechanism (Figure 7) showing 3 slots. The narrow slot is intended for exposures less than 10 milliseconds, the medium slot is used for exposures of 10 to 46 milliseconds, and the wide slot is used for exposures > 46 ms. For 10 to 46 ms exposures, the shutter blade is moved so that the middle sized slot crosses the light path in a single motion. For exposures longer than 46 ms, the wide slot is used. To obtain the various exposures, the blade slows down, then speeds up to complete the exposure. For an exposure longer than about 60 ms, the blade comes to a complete stop in the open position prior to closing. . In the refurbished design, the filter widths were adjusted so that the narrowest slot is not used so as to improve the uniformity of exposure across the CCD.

‏Source epic.gsfc.nasa.gov