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Overheads

Acquisition overheads associated with setting up on each new science target include time for slewing the telescope, configuring the instrument and WFS, and in the case of spectroscopic observations, centering the target in a slit.

Long spectroscopic observations also require re-acquisitions to ensure accurate centering in the slit. Both acquision and reacquisition times are summarized in Table 1. The time between reacquisitions depends on slit width and location of the target in the sky. For the widest slits and/or for targets observed near the meridian the time between acquisitions is roughly one hour. For narrow slits and/or targets at at hour angles greater than 2 or very high airmasses the time between reacquisitions is roughly every half hour. As the PI cannot know ahead in what part of the sky the target will be observed, we suggest using every 45 minutes in estimating the overhead.

Long imaging observations may be split over several nights to better accommodate them in the queue. Please allow for one acquisition for every 1.5 - 2 hours of observing when calculating the time required for your project.

In imaging mode automated dither patterns have an estimated on-source efficiency of ~75%, where 25% of the elapsed time is used for telescope offsetting, WFS re-acquisition, detector readout etc; the exact overheads vary with the size of the offset and the exposure time.


Table 1: NIRI acquisition overheads.

Action NIRI + PWFS NIRI + Altair NGS NIRI + Altair LGS
Imaging Acquisition 6 min 10 min 15 min
Spectroscopic Acquisition 13 min 20 min 25 min
Spectroscopic Re-acquisition 5 min 5 min 5 min


Table 2 gives estimates of the overheads associated with taking NIRI images in the various array geometries and read modes without telescope offsetting. These values include the time to read out the array Ncoadds times and to write the final FITS image. The overhead for telescope offsetting depends on the distance of the offset, but is typically ~5 seconds for small offsets.

Table 2: NIRI overheads for each image.

Array Size Low Background Medium Background High Background
1024 x 1024 2.78s + 11.15s * Ncoadds 2.78s + 0.70s * Ncoadds 2.78s + 0.18s * Ncoadds
768 x 768 2.50s + 6.49s * Ncoadds 2.50s + 0.41s * Ncoadds 2.50s + 0.12s * Ncoadds
512 x 512   2.38s + 0.20s * Ncoadds 2.38s + 0.07s * Ncoadds
256 x 256   2.96s + 0.07s * Ncoadds 2.96s + 0.03s * Ncoadds


All of the values listed on this page are included in OT calculations of the execution time of an observation. However, only the actual overheads incurred at the time of the observation will be charged to the program.