Phase 2 Proposal Instructions for Cycle 10

5.15 Time per Exposure [Time_Per_Exposure]

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Enter the exposure time for each separate exposure, using units of either hours (H), minutes (M), or seconds (S). Do not combine H, M, and S on the same line (i.e., use 90M rather than 1H 30M). 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 Number of Times to Iterate the Exposure [Number_of_Iterations]), or with certain Special Requirements. The exposure time entered may be divided among many subexposures, samples, or spectra, 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 (ACCUM spectra with STIS using PATTERN, for example), or to perform a complex, autonomous target acquisition.

If the exposure time is greater than the amount of unocculted time available, the exposure may be broken into a group of subexposures by the planning system; this default splitting may be prevented by using the exposure-level Special Requirement NO SPLIT. The amount of viewing time depends on the targetŐs declination, date of observations, and the HST orbital parameters.

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 B 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; the exposure-level Special Requirements EXPAND, MAX DUR, and MIN DUR (see for details) can be used to control these changes in exposure time. If no preference is specified by the observer, 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.