A Calibrated Measurement of the Near-IR Continuum Sky Brightness Using Magellan/FIRE

Abstract

We characterize the near-IR sky background from 308 observations with the FIRE spectrograph at Magellan. A subset of 105 observations selected to minimize lunar and thermal effects gives a continuous, median spectrum from 0.83 to 2.5 microns which we present in electronic form. The data are used to characterize the broadband continuum emission between atmospheric OH features and correlate its properties with observing conditions such as lunar angle and time of night. We find that the moon contributes significantly to the inter-line continuum in the Y and J bands whereas the observed H band continuum is dominated by the blended Lorentzian wings of multiple OH line profiles even at R=6000. Lunar effects may be mitigated in Y and J through careful scheduling of observations, but the most ambitious near-IR programs will benefit from allocation during dark observing time if those observations are not limited by read noise. In Y and J our measured continuum exceeds space-based average estimates of the Zodiacal light, but it is not readily identified with known terrestrial foregrounds. If further measurements confirm such a fundamental background, it would impact requirements for OH-suppressed instruments operating in this regime.