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Observing Overheads

GNIRS observations incur overheads during the acquisition of the target and during the data taking.


The table below summarizes the acquisition times that one should normally assume in determining total time required to acquire an astronomical source into a GNIRS slit. These acquisition times include the initial telescope slew, acquisition imaging, and instrument reconfiguration. The Altair-NGS and non-AO spectroscopy acquisition times are mean values based on actual records from the first ~year of GNIRS operations at Gemini North, assuming that acquisitions of targets requiring sky subtraction (H > 16 mag) take on average about 4 minutes longer than acquisitions of bright targets. The LGS and imaging times are estimates, as few observations have been taken in these modes to date.

With all acquisitions, the actual time depends on the complexity of the acquisition (very faint targets requiring long exposures will clearly take longer to acquire than bright stars) and the quality of the Phase II preparation (e.g., finding charts, helpful notes). The program is always charged the actual time used (including errors in the OT definition, but not including telescope- or instrument-related problems). "Blind" acquisitions using a reference source (for H > 20 mag targets) take the same time as a normal acquisition.

Long spectroscopic observations also require re-acquisitions to ensure accurate centering in the slit. Both acquisition and reacquisition times are summarized in Table 1. The time between reacquisitions depends on slit width and location of the target in the sky. As the PI cannot know ahead in what part of the sky the target will be observed, we suggest assuming reacquisition every 45 minutes in estimating the overhead.

Table 1: GNIRS acquisition overheads.

Mode GNIRS + PWFS GNIRS + Altair NGS GNIRS + Altair LGS GNIRS Imaging
Approx. Acquisition Time 12 min 14 min 25 min 12 min
Approx. Re-Acquisition Time 6 min 6 min 6 min N/A

Overheads during data-taking - Phase I

Overheads during an observing sequence are usually dominated by telescope motions (nominally about 10 seconds/motion, for small offsets) and by array readout. The readout overhead is equal to the minimum exposure time given in the OT, and is incurred per coadd. The lower noise readout modes have longer overheads because multiple reads are involved, but since the exposure times are usually longer, the fractional overhead is still low. The time to write the file is of order 2 seconds and is usually consumed in the ~10 seconds for telescope motion. For most observations one can assume an overhead (after acquisition) of 10%. M band observations are an exception to this: at medium resolution assume an overhead of 40% because of the short exposures; at high resolution assume 20%. If it is necessary to observe at the (not recommended) R=1700 with the short camera in M band, assume 110% overhead (which is why it is not recommended.)

Readout overhead for different readmodes is given on the science detector page. For most observations these are likely to be negligible, with the exception of very short exposure times, such as one uses in the M-band, or very long sequences.  For an 0.5 sec observation, the total time per exposure is ~0.7seconds, nearly a 50% overhead.  The readout overhead is the same for each coadd, however increasing coadds reduces the overhead writing to disk, as well as the total number of files. For very short exposures one should generally use coadds to accumulate the data in one file until it is time to dither on the sky. For long sequences and/or very short exposures, the PIs should calculate the readout overhead and take it into account in their total time allocation.

Overheads during data-taking - Phase II

The OT currently adds a fixed overhead per observation for acquisition, depending on the type of observation. Programs are charged for the actual time taken to acquire the target. The OT also assumes a fixed 10 sec overhead for every telescope move and instrument config change in the observation sequence, which is approximately correct. Overheads for readout and coadding are also included in the OT time estimates.

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