Instruments
IRCS
Imaging
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Guide for IRCS imaging exposure time estimationBackground-limited operationIn order to achieve the maximum sensitivity in a given integration time, individual exposures should be long enough for the photon shot noise from the sky background to dominate over the array read noise. In practice, background-limited performance (BLIP) is considered to be when the sky counts exceed the square of the read noise by a factor of 3. Observations at L' and M' are background-limited even in the shortest exposure time. The following tables
assume use of the infrared secondary mirror.
Non-linearity of the camera detectorTo achieve less than 1% non-linearity, the maximum signal counts should be less than 4000 ADU (= 22,400 e-) for a single exposure. For L' and M' bands imaging, the signal level of 7000 ADU (= 39,200 e-) can be allowed with less than 3% non-linearity because of very high background level. Please see here for the detailed information of non-linearity. Saturation magnitudesThe time it takes for a source to saturate is obviously dependent on the seeing, which can be extremely good on Mauna Kea. The following exposures times should therefore be considered as a guide, which are based on 0.3'' seeing for the 52 mas pixel scale, and Adaptive Optics for the 20 mas pixel scale. These information are based on no-AO data. 52 mas pixel scale
20 mas pixel scale
Shortest exposure times with sub-array sizes
Overheads for thermal-infrared observationsThe short integration times required at L' and M'-bands result in substantial overheads.
(1) The time needed for AO parameter adjustment. If the targets have similar magnitudes within 1-2 mag, the AO parameter adjustment can be performed within a few times during the observation in a stable seeing condition. |