9.5 Science Exposure Overheads
Science exposure overheads are dominated by the time required to move OSM1 and OSM2, as well as the time needed to read out the on-board memory buffer at the end of each exposure. Please note that in Phase II the computed overheads may be less than the values presented below as all the interactions inherent in instrument commanding will be accurately accounted for by APT. It is important to plan Phase I with the conservative overheads, especially for detector readout, given below to ensure adequate time for proposal exposures.
The full overhead calculation for science exposures depends upon a number of factors including generic exposure setups (which are detector and observing mode dependent), whether an aperture change is required, whether a grating change is required, whether the grating is not changed but central wavelength setting for the grating is changed, and the directional sense of any required motion to implement an FP-POS change. Table 9.4 lists these additional overheads.
Table 9.4: Science Exposure Overhead Times
Add the values for grating change, wavelength change, aperture change, or segment reconfiguration only if those actions are being undertaken.
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Overhead times (sec)
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FUV
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NUV
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TIME-TAG
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ACCUM
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TIME-TAG
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ACCUM
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Exposure set-up
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Grating change
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Central wavelength change
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FP-POS forward1
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FP-POS backward1
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Aperture change
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SEGMENT reconfiguration
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Memory readout2
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1"Forward" refers to the preferred direction of motion of OSM1 or OSM2 and "backward" to the opposite direction. The preferred direction is toward greater wavelength and toward larger FP-POS value.
2ACCUM mode readout overheads can be hidden within subsequent exposures under certain circumstances, but those are complex to describe. Use these values as safe upper limits for proposing purposes.
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To calculate a complete science (or FLASH=NO wavecal) exposure overhead, start with the desired exposure time rounded up to the next whole second, add the generic exposure setup overhead from Table 9.4; if a grating change has occurred from the previous exposure add the appropriate values from Table 9.2 and/or Table 9.3, if a central wavelength change is made add the appropriate value from Table 9.4, if an FP-POS movement is made add the appropriate value for a preferred direction (toward larger FP-POS) or non-preferred direction move, and for FUV only, if a detector SEGMENT reconfiguration is employed (a change involving any two of BOTH, A, or B in combination) add 330 sec for the associated overhead. Lastly, add the appropriate detector memory readout overhead.
