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Sensitivity and Overheads

For sensitivities of ALTAIR with NIRI and NIFS, use the NIRI and NIFS Integration Time Calculators, but see here for some restrictions regarding use of the NIRI ITC, including use at L'. The NIRI ITC also estimates FWHM and Strehl ratios of point sources.

The additional observing overhead due to Altair in both NGS and LGS modes has been measured to be about 5 minutes except for NIFS + LGS which takes an additional 8 minutes. These estimates come from the Observatory Statistics webpage. The overhead is generally small because if the coordinates are accurate, the guide star generally falls within or very close to the WFS field stop, and the loop can be closed in a few minutes.

The actual overhead time is not easily predictable as it depends on distance from the previous target, the field crowdedness, the faintness of the guide star, the accuracy of the guide star magnitude and the detail of the finder chart. Once the guide star is acquired, everything behaves as in regular operation with NIRI, NIFS or GNIRS, and the overhead associated with those instruments applies (see the NIRINIFS and GNIRS overheads sections).

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