Guide for IRCS imaging exposure time estimation

Background-limited operation


In 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.
Filter Background Min. BLIP time (s)
(mag/arcsec2) (e-/s/arcsec2) 20 mas 52 mas
J 14.7 41 300 20 5
H 13.7 100 000 9 3
K' 12.9 107 000 8 3
K 12.4 165 000 6 3
L' 2.6 520 000 000 - -
M' -0.1 740 000 000 - -

Non-linearity of the camera detector


To 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 magnitudes


The 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
Filter Saturation magnitude Max exp
time (s)
1 s 5 s 10 s 30 s 60 s 120 s 300 s
J 10.7 12.4 13.2 14.4 15.2 16.0 17.2 1000
H 10.6 12.3 13.1 14.4 15.4 16.6 - 200
K' 10.0 11.7 12.5 13.7 14.6 15.5 17.4 420
K 9.9 11.6 12.4 13.7 14.5 15.5 18.4 330

20 mas pixel scale
Filter Saturation magnitude Max exp
time (s)
1 s 5 s 10 s 30 s 60 s 120 s 300 s
J 11.3 13.0 13.8 15.0 15.7 16.5 17.5 6000
H 11.7 13.4 14.2 15.4 16.2 17.0 18.1 1300
K' 11.1 12.8 13.6 14.8 15.6 16.3 17.4 2800
K 11.0 12.7 13.5 14.7 15.5 16.2 17.3 2100
L'   0.62
M'   0.28

Shortest exposure times with sub-array sizes


Sub-Array Size Readout Mode
Normal (sec) Thermal (sec)
10242 (Full Array) 0.41 0.119
8962 0.33 0.096
7682 0.26 0.076
6402 0.20 0.058
5122 0.14 0.042
3842 0.10 0.029
2562 0.06 0.018
1282 0.029 0.009
642 0.017 0.006

Overheads for thermal-infrared observations


The short integration times required at L' and M'-bands result in substantial overheads.
Observing mode Total required time
w/o AO[total integration time] × 3
w/ AO [total integration time]× 3 + 15-20min(1)

(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.




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