A guide to IRCS imaging exposure times
[Note:
The thermal background level at K (K'), L', and M' are two or three times
higher than those on Cassgrain focus before 2005 (See
Figure) due to high emissivity of image rotator on IR Nasmyth
focus. Currently our telescope division is trying to solve this problem.
With new AO system coming , it is not necessary to worry about the high
thermal background because the new AO system will not use the image rotator.]
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' | 13.4 | 76 000 | 20 | 5 |
| K | 13.0 | 98 800 | 20 | 5 |
| L' | 3.8 | 203 000 000 | - | - |
| M' | 1.0 | 400 000 000 | - | - |
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 |