NIRI 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 every hour to ensure accurate centering in the slit. These values are summarized in Table 1. 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 observing overheads.

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 only a few seconds.

Table 2: NIRI overheads for each image.

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.

Last update 2006 September 1; Andrew Stephens, Tom Geballe & Joe Jensen