A guide to IRCS echelle spectroscopy exposure times
[Note: The sensitivities at 2.2 - 5.5 um is worse than before 2005 (See Figure). The main reason is two or three times higher thermal background level 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 do not use this image rotator.]
Near-infrared wavelengths: JHK
At high spectroscopic resolution (R > 5000), it is possible to work between the OH lines which dominate the broad-band background flux from the night sky. The brightness of the night-sky continuum is not well-determined, but measurements suggest a value of ~ 1000 photons/s/m2/arcsec2/um may be typical.The small pixels and high resolving power of the IRCS echelle result in a background flux between the OH lines which is less than the detector dark current. Observations at JHK (i.e., where the thermal background is unimportant) are therefore always detector-limited and sky subtraction is not required. Exposures longer than a few minutes will have an appreciable dark current, however, and a dark frame will be required.
Note that while increasing the exposure time will not degrade the scientific quality of your data, provided you are well away from OH lines (which will saturate), there is always a risk of instrument and/or software failure resulting in the loss of the most recent exposure. We therefore caution against exposures longer than 900 seconds.