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Observing Overheads

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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 will required multiple acquisitions as well as re-acquisitions to ensure accurate centering in the slit. Both acquisition and reacquisition times are summarized in Table 1. One should assume one full acquisition for each ~2 hours of science. 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, one should assume a reacquisition every ~45 minutes of science when estimating overheads.

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 typically dominated by telescope motions (nominally about 7 seconds/motion, for small offsets) and by array readout. The readout overheads are listed in Table 2. The lower noise readout modes have larger overheads per coadd because the detector is read out multiple times, but since the exposure times are usually longer, the fractional overhead is about the same. The time to write each file is ~8.5 seconds. 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.)

Coadding (summing multiple exposures in a single FITS file) reduces the overhead of writing files to disk. For very short exposures one should generally use coadds to accumulate the data until it is time to offset. For very long sequences and/or very short exposures, 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.

Table 2: GNIRS readout overheads.

Read Mode / Coadd / File
Very Bright Object 0.18s 8.56s
Bright Object 0.70s 8.56s
Faint Object 11.14s 8.56s
Very Faint Object 22.30s 8.56s

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