A guide to IRCS imaging exposure times
[Note: The thermal background level was recovered to low level one before 2005 (See Figure) since the image rotator was recoated and reinstalled in July, 2008. The sensitivities in tables are revised based on new low thermal background levels.]
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 | - | - |
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 | |||||||
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 | |||||||
Overheads for thermal-infrared observations
The short integration times required at L'M' result in substantial overheads.| Observing mode | Total required time |
|---|---|
| w/o Adaptive Optics | [total itime] × 3 |