The shutter is a two-blade mechanism used to control the duration of the exposure. A listing of the possible exposure times is contained in Table 2.3 . These are the only exposure times which can be commanded.
Current policy is to round down non-valid exposure times to the next valid value.
An exposure time of less than 0.11 seconds will therefore only result in a bias frame being taken. Note that some exposure times in the table do not correspond to commandable values when halved. For the shortest exposure times, it is possible to reconstruct the actual time of flight of the shutter blades. Encoder disks , attached to the shutter blade arms, are timed by means of a photo-transistor. The maximum error is 5 milliseconds.