A guide to IRCS echelle spectroscopy exposure times 

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.

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: LM

Due 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 detector

We recommand the signal level less than 6000 ADU (=22,800 e-) for a single exposure to achieve less than 1% non-linearity.

Echelle Exposure Time Calculator

Please use Echelle-ETC to estimate total exposure time for the IRCS Echelle mode.


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