WFPC2 Instrument Handbook for Cycle 11

Serial Clocks

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The serial transfer registers of the CCDs can be kept running during an exposure (CLOCKS=YES), or run only during the readout (CLOCKS=NO, the default).

The serial clocks are sometimes used on very bright targets where extensive blooming up and down the CCD columns is expected. CLOCKS=YES causes charge which blooms to the ends of the CCD to be read out and disposed of, thus preventing it from flowing back into the image. They will be useful when any single CCD column contains in excess of ~108 electrons. Note that the serial clocks do not actually suppress the blooming process, instead they merely remove any charge that blooms to the top or bottom of the CCD.

For most circumstances, we recommend CLOCKS=NO. The reasons for this recommendation are:

1. CLOCKS=YES is not widely used, so anomalies may exist or develop that we are not aware of. Also, this mode is not as well calibrated as CLOCKS=NO (although we expect the calibration to be independent of the state of the clocks).
2. The shutter open time when CLOCKS=YES can have significant errors. In this mode, there are delays of up to 0.25 seconds in opening the shutter (which are not present when CLOCKS=NO). This means that for exposures of less than ~30 seconds, there may be photometric errors greater that 1%, unless special precautions are taken in the data reduction. Furthermore, if a non-integral exposure time is specified in the proposal, it will be rounded to the nearest second. See below for details.

On the other hand:

1. We do advise CLOCKS=YES if you expect star images to be so saturated that a significant amount of charge will bleed off the chip during the exposure. This would mean that you expect much more than 108 electrons from at least one star in the exposure (more than 1000 pixels would be saturated). Without CLOCKS=YES the bleed-off charge may corrupt other parts of the image.
2. One advantage of CLOCKS=YES is that the overhead time is 1 minute less for exposures longer than 180 seconds. This can be significant if you have a large number of exposure times in the 3 to 10 minute range.
3. Unlike the original WF/PC-1, we do not see a significant variation of WFPC2 dark level with CLOCKS=YES.

In summary:

0-0.8 sec	CLOCKS=NO is required.
1 - 30 sec	Use CLOCKS=NO (or attempt photometric corrections during the analysis of the data).
20 - 180 sec	Use CLOCKS=NO unless more than 108 detected electrons from a single star are expected.
180+ sec	Use CLOCKS=NO unless more than 108 detected electrons are expected, or if you need to save 1 minute of overhead.

 

While exposure times are corrupted for CLOCKS=YES, and are not accurately reported in the image headers, correct values can be computed. Details are as follows:

1. Non-integer exposure times <3 minutes are rounded to the nearest integer (e.g., 1.2 sec and 1.4 sec will actually be 1.0 sec long, 3.5 sec exposures take 4.0 sec). This roundoff is due to the way the spacecraft computer monitors the take-data flag (AP-17 uses its own integer-based timecode). This rounding is reflected properly in the header keywords (keywords UEXPODUR, EXPSTART, EXPEND, EXPTIME, and EXPFLAG in the .c0h file headers, or UEXPODUR and CMD_EXP in the .shh headers).
2. All CLOCKS=YES exposures are also shortened by either 0.125 or 0.250 seconds. This decrease in exposure time is not reflected in the file headers; the amount depends upon which shutter blade was in place at the start of the exposure. The decrease in exposure time is due to the manner in which the application processor (AP-17) in the spacecraft computer operates the shutter blades. When CLOCKS=NO (default), the WFPC2 microprocessor opens the shutter, the AP-17 closes the shutter, and the exposure time is as requested. However, with CLOCKS=YES, the AP-17 opens the shutter, first blade A, then blade B. If blade A is closed at the start of the exposure, the actual exposure begins 0.125 seconds after the AP-17 issues the blade command. If blade B is closed at the exposure start, the exposure starts 0.250 seconds later (after the AP-17 sends the open-A command followed by open-B). The shutter in place at exposure start is given in the SHUTTER keyword in the .c0h file.