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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 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.

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.

Table 1: NIRI acquisition overheads.

Action NIRI + PWFS NIRI + Altair NGS NIRI + Altair LGS
Imaging Acquisition 6 min 10 min 15 min

Table 2: NIRI overheads for each image.

Array Size Low Background Medium Background High Background
1024 x 1024 5.0s + 11.16s * Ncoadds 5.09s + 0.71s * Ncoadds 5.11s + 0.20s * Ncoadds
768 x 768 5.12s + 6.48s * Ncoadds 5.14s + 0.41s * Ncoadds 5.14s + 0.11s * Ncoadds
512 x 512 5.11s + 3.10s * Ncoadds 4.99s + 0.21s * Ncoadds 5.12s + 0.08s * Ncoadds
256 x 256 5.12s + 0.98s * Ncoadds 5.17s + 0.08s * Ncoadds 5.17s + 0.04s * Ncoadds

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.

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