5.15 Time per Exposure [Time_Per_Exposure]

Enter the exposure time (in seconds) for each separate exposure. It is important that observers consult the Instrument Handbooks, and the tools on the STScI Web pages.

Note that the exposure time is the total time for one execution of a defined exposure. Multiple executions may be specified by entering a value for Number_of_Iterations (see Section 5.14), or with certain Special Requirements. The exposure time entered may be divided among many samples or spectra (subexposures), depending on the Instrument Configuration, Operating Mode, and Optional Parameters. Generally, the exposure time is used only for the collection of photons, but there are exceptions for which instrumental overheads are included in the exposure time. For details, see the specification for each Operating Mode in Part B of these Instructions. In the normal case, the exposure time entered may be used to obtain a single image, to obtain a series of spectra, or to perform a complex, autonomous target acquisition.

Routine calibration exposures will be performed by STScI. If special internal calibrations are required for your program, consult the Internal Calibration Target tables in Part II of these Instructions for information about what exposure times to use.

Use the Target Acquisition sections of the Instrument Handbooks and STScI Web tools to estimate exposure times used for acquisition exposures.

Exposure times may be changed during scheduling by STScI in order to place exposures in observing windows defined by orbital constraints (Earth blockage, passage through the SAA, etc.) and to improve overall efficiency. Changes to exposure times will be such that the exposure time is not changed by more than about 20%; note that increases, as well as decreases, in exposure time are possible.

Target acquisition and peakup exposure times are not altered during scheduling.