Instrument Description

2.5 Shutter

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.

Some exposures should be split into two (CR-SPLIT) in order to allow cosmic ray events to be removed in post-processing. By default, exposures of more than 10 minutes are CR-SPLIT. If an exposure is CR-SPLIT, the exposure time is divided into two fractions and then rounded down. Normally the fractional split is 50%/50% but, unless constrained by the user with CR-TOLERANCE, the ratio may be up to 70%/30%, as allowed by the default CR-TOLERANCE=0.2. Note that some exposure times in the table do not correspond to commandable values when halved. In preparing a proposal containing an exposure that is to be CR-SPLIT, the simplest procedure to use in order to be sure of a given total exposure time, is to enter double a legal value, and impose CR-TOLERANCE =0.

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. The necessary information is contained in the WFPC2 engineering data stream, however, this information is not in the processed science header.

Diffraction effects from the edges of the shutter blades affect the point spread function for very short exposures. It is advisable to use exposure times greater than 0.2 second when obtaining point spread functions in support of long exposure observations (see the WF/PC-1 IDT OV/SV Report, Chapter 9, for further discussion in the spherically aberrated case).

The control of the initial opening of the WFPC2 shutter during an observation is held by the internal WFPC2 microprocessor in all cases. However, control over when the shutter is closed is held by the microprocessor only for exposures less than 180 seconds in duration. For longer exposures, control passes to the Application Processor (AP-17) in the NSSC-1 spacecraft computer. The consequence of this arrangement is that loss of guide star lock will result in the WFPC2 shutter being closed only for those observations with planned durations of 180 seconds or longer. The AP-17 always controls the shutter closing if the serial clocks are enabled during the exposure (CLOCKS=YES), which then has a minimum planned duration of 1 second, and exposures are rounded to the nearest second. If guide star lock is reacquired prior to the end of the planned observation time, the shutter will reopen to obtain a portion of the planned integration. As discussed in the next section, CLOCKS=YES should generally not be used with exposures shorter than 30 sec., if 1% or better photometric accuracy is needed.

Table 2.3: Quantized Exposure Time (seconds). Exposure times where the PSF is affected by shutter flight time are underlined. Exposures normally without loss of lock checking are in italics. Times that are CR-SPLIT by default are in bold. Exposures that take more than one orbit even when CR-SPLIT are not normally accessible to GOs and are crossed (and exposures longer than 5400 seconds must be CR-SPLIT). Exposure times that should not be used when CLOCKS=YES are shaded.