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 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	-	-

Note: Thermal background has increased with the installation of the new AO3k DM (up to a factor of 2) and NBS (about 10%). We are working on reducing the background coming from the AO3k DM. Numbers in the table will be updated as soon as measurements become available.

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.

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 and the seeing condition is stable, the AO parameter adjustment would be required once in several hours.