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 (mag/arcsec²)	Background (e^-/s/arcsec²)	Min. BLIP time (s) 20 mas	Min. BLIP time (s) 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 Maunakea. 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)
Filter	1 s	5 s	10 s	30 s	60 s	120 s	300 s	Max exp time (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)
Filter	1 s	5 s	10 s	30 s	60 s	120 s	300 s	Max exp time (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
Sub-Array Size	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.