A guide to IRCS echelle spectroscopy exposure times
Near-infrared wavelengths: JHKAt 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.
If OH lines should be subtracted clearly for your science, the longest exposuretimes we recommand are 900 s for J-band, 200 s for H-band 300 s for K-band.
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.
Thermal-infrared wavelengths: LMDue to the high thermal background, observations become background-limited in about 30 seconds at L', and about 4 seconds at M'. A point source with L=1 saturates in about 1 second.
Non-linearity of the spectrograph detectorWe recommand the signal level less than 6000 ADU (=22,800 e-) for a single exposure to achieve less than 1% non-linearity.
Targets without AO guide stars:There is no autoguider available for the instruments on Infrared-Nasmyth focus of Subaru telescope. IRCS, one of IR-Nasmith instruments of Subaru telescope, also relies on AO188 for guiding. Therefore, targets without suitable AO188 guide stars, either, NGS, or TTGS, relies on the telescope tracking, although it is plausible to monitor tracking errors with slit-viewer images from the camera detector and to adjust target acquisition on slits. Any exposure longer 300 seconds on targets without suitable AO guide stars are strongly recommended against.
Echelle Exposure Time CalculatorPlease use Echelle-ETC to estimate total exposure time for the IRCS Echelle mode.