A guide to IRCS echelle spectroscopy 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.]
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.