Wide Field and Planetary Camera 2 Instrument Handbook for Cycle 14
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. However, an exposure time shorter than the minimum allowed (0.11 seconds) is, instead, rounded up to this minimum value.
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 seconds 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 closing of the shutter 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 Times (Seconds). Exposure times that should not be used for CLOCKS=YES are shaded and flagged with table footnote (a). Exposure times where the PSF is affected by the shutter blade flight time are underlined and flagged with table footnote (b). Exposures normally without loss of lock checking are in italics. Times that are CR-Split by default are in boldface; exposures longer than 5400 seconds must be CR-split. Exposures that take more than one orbit, even when CR-split, are not normally accessible to GOs and are crossed out and flagged with table footnote (c). 0.11 1,2 0.4 1 2.0 10. 40. 200. 900. 1900. 2900. 3900. 4900. 6200. 150003 0.12 1,2 0.5 1 2.3 1 12. 50. 230. 1000. 2000. 3000. 4000. 5000. 6400. 20000.3 0.14 1,2 0.6 1 2.6 1 14. 60. 260. 1100. 2100. 3100. 4100. 5100. 6600. 25000.3 0.16 1,2 0.7 1 3.0 16. 70. 300. 1200. 2200. 3200. 4200. 5200. 6800. 30000.3 0.18 1,2 0.8 1 3.5 1 18. 80. 350. 1300. 2300. 3300. 4300. 5300. 7000. 40000.3 0.20 1,2 1.0 4.0 20. 100. 400. 1400. 2400. 3400. 4400. 5400. 7500. 50000.3 0.23 1 1.2 1 5.0 23. 120. 500. 1500. 2500. 3500. 4500. 5500. 8000. 75000.3 0.26 1 1.4 1 6.0 26. 140. 600. 1600. 2600. 3600. 4600. 5600. 8500. 100000.3 0.30 1 1.6 1 7.0 30. 160. 700. 1700. 2700. 3700. 4700. 5800. 9000. 0.35 1 1.8 1 8.0 35. 180. 800. 1800. 2800. 3800. 4800. 6000. 10000.
1Exposure times that should not be used for CLOCKS=YES
2Exposure times where the PSF is significantly affected by the shutter blade flight time
3Exposure times that take more than one orbit, even when CR-split; these are not normally accessible to GOs
In August 2000, WFPC2 began experiencing occasional anomalies in the operation of the shutter mechanism. The problem was traced to an encoder wheel and photo transistor assembly that serves to sense the position of the "A" shutter blade. This sensor is polled by the WFPC2 computer prior to each exposure. Later, in October 2000, we began seeing a more serious problem where multiple mis-readings would lead to the "A" shutter blade attempting to close even though the "B" blade was already closed, hence causing a collision of the two shutter blades. Since there was some potential for this to damage the mechanism, we ceased WFPC2 observations for several days until corrective action could be taken. On November 8, 2000, we modified the WFPC2 microprocessor software to activate the position sensor 10 milliseconds earlier, thus giving it more time to respond prior to being read by the microprocessor. An extensive series of tests were run on the shutter after the installation of the software patch, and no unexpected side effects or abnormalities in its operation were seen. No further incidences of the anomaly have been seen as of this writing (June 2004).
The anomaly affected only about 0.3% of the images from August to October 2000. In most cases the shutter failed to open, producing a blank image. A few images were also seen with trailed targets, due to the shutter being open prior to the nominal exposure start, or due to the shutter remaining open past the nominal exposure end.
As of this writing the exact cause of the anomaly is still not entirely clear. Much evidence points to radiation damage to the photo transistor, causing its response time to slow, while other evidence points to mechanical wear in the encoder wheel linkage, leading to misalignment of the wheel relative to the photo transistor. In most scenarios the software patch should permanently fix the problem, but there is always some small chance it will reappear.
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