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NICMOS Camera 2 ACQ Exposure Times

NICMOS Camera 2 ACQ exposure times

The ACQ exposure times (keyword exptime) in the science headers are incorrect. This is due to a reformatting/software problem in OPUS and not a commanding problem. The ACQ exposure times as specified by the PI in the Phase II are being executed. An Operations Problem Report (OPR) has been filed and the problem is being worked.

This problem only affects the on-target ACCUM exposures used by the flight software to determine the location of the target. The background and flat field observations obtained in support of the target ACQ have the correct header exposure times.

NICMOS coronagraphic observers are advised to check the header exposures against the requested Phase II exposure times. The NICMOS ACQ ACCUM exposure times are quantized and the minimum NICMOS ACQ exposure time is different from the ACCUM mode. For example:

	T_acq = TPG_TIME + 0.228 sec

The legal TPG_TIME exposure times are given in the HST Phase II Proposal Instructions, Table 11.4, page 11-25.

Correction Procedure

The OPUS header NICMOS ACQ ACCUM exposure times can be corrected by subtracting 0.37 seconds from the exptime keyword value. The corrected time represents the actual exposure executed by the Flight Software.

If there are large differences in exposure time between the corrected value and the requested exposure in the Phase II template, check to make sure the requested time is a legal quantized exposure time. Phase II times are round down to make them correct NICMOS ACQ ACCUM exposure times.

If the large differences can not be explained by the quantization of exposure times, then contact your Contact Scientist or send email to the help desk at "help@stsci.edu".