Observation Strategies

7.5 Choosing Exposure Times

The choice of
exposure time generally depends on the signal-to-noise ratio required to meet the science goals. This can be assessed using information in Chapter 6 or Appendix B herein, or by using the on-line WWW Exposure Time Calculator tool.

However, when packing orbits, one must often compromise somewhat and decide which exposures to lengthen or shorten. Table 7.4 may be helpful in this regard. It shows the total time required to execute a single CR-SPLIT=NO exposure, excluding any time needed to change filters.

Note that the most efficient exposure times are those whose length approaches or equals, but does not exceed, an integral number of minutes plus 40s. Figure 7.6 illustrates event timings during a typical 60s WFPC2 exposure, similarly, Figure 7.7 illustrates events during a (more efficient) 100s exposure. (See "Overhead Times" on page 29 for more information about exposure timings.)

Figure 7.6: Event Timings During a 60s WFPC2 Exposure.

Figure 7.7: Event Timings During a 100s WFPC2 Exposure.

Due to the various overheads, shortening or lengthening an exposure can have unexpected effects on the orbit packing. For example, shortening an exposure from 400s to 350s has no effect on orbit packing; they both require 9 minutes to execute (CLOCKS=NO, the default setting). On the other hand, shortening an exposure from 180s to 160s trims the execution time by 2 minutes (again CLOCKS=NO, the default setting).

CLOCKS=YES may have some advantage in a long series of exposures whose lengths are 180s or somewhat greater. Each savings of 1 minute can add up to a few more exposures per orbit. The down side is that most calibrations are derived for exposures with CLOCKS=NO, so the calibration maybe slightly compromised. The largest calibration error is expected to occur on the dark current, where there may be a slight increase near the top and bottom of each CCD. In many situations this error may be acceptable, such as a small target near a CCD center, or broad band filter images where the sky completely dominates the dark current. CLOCKS=YES will have more impact on calibration of narrow filters, or situations requiring an extremely flat background. (Also, see "Serial Clocks" on page 27 for discussion of exposure time anomalies associated with CLOCKS=YES, though these are most important for exposures <30s.)

An exposure with CR-SPLIT=YES would simply require the total time for each sub-exposure as given by Table 7.4, again, plus any time needed to change filter before the first exposure. However, the default CR-SPLITting allows schedulers some latitude in dividing the exposures (CR-TOLERANCE=0.2 is the default) so the exact overheads are unpredictable. For example, a 700s exposure with CR-SPLIT=0.5 (the default) could be split into a pair of 350s exposures totaling 18 minutes, or a 300s and 400s exposure totaling 17 minutes.

Table 7.4: Basic Time to Execute Single Non-CR-SPLIT Exposure.