Variation of the sky background in near-infrared spectroscopy using X-Shooter and KMOS at the VLT

International audienceSkylines and associated background is and will become a major issue for the near-infrared instrumentation for large telescopes. Due to telescopes size in the future, the temporal variation of the sky background can become a real issue using standard techniques of sky subtraction (cross-beam switching, dithering, etc.), if we want to reach a precision below 1%. Only the understanding of the temporal behavior of sky background (in a short period of time) can help us, to better correct the sky background and minimize the final residual. Using ESO archives of two infrared IFU instruments at the VLT (X-Shooter and KMOS). We have studied the behavior of the sky background in two skyline-free region at 1.1 and 1.8 microns, the selected region were inspected visually to avoid small skylines that can affect the measures (See Flores et al., 2016). This analyze complete the study realized by Yang et al. (2012) in the optical region using FORS2 data. Preliminary analysis shows that the sky continuum background at 1.1 and 1.8 microns, in regions of 0.6x0.6 and 0.6x1.2 sq arcsec has temporal variation of 0.6 to 3% for consecutive exposure time of 900s and 600s, respectively. we also found that the variations of the bottom of the sky at 1.8 microns are 2 times more important than the variations at 1.2 microns. This result will be included in future simulations using standard sky subtraction techniques, in particular to reevaluate the performances of instruments aiming to detect ultra faint objects/ features