A guide to IRCS imaging exposure times

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 15.7 18 000 80 13

H 13.9 83 000 12 4

K' 14.1 40 000 30 5

K 13.7 52 000 20 5

L' 5.0 70 000 000 - -

M' 1.8 200 000 000 - -

Non-linearity of the camera detector

To achieve less than 1% non-linearity, we recommand a maximum signal value 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.
[non-linearity of camera detector]

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, and 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)

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

L' ⁽¹⁾ 0.20

(1) 512x512 pixel subarray.

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
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'M' result in substantial overheads.


Observing mode	Total required time
w/o AO	[total itime] × 3
w/ AO	[total itime] × 3 + [15-20min] per target for AO parameter adjustment⁽¹⁾

(1) If targets have similar magnitudes within 1-2 mag, the AO parameter adjustment can be performed just a few times during the observation in a stable seeing condition.