EUROPEAN SOUTHERN OBSERVATORY La Silla - Science Operation Department

We present a method to correct the chromatic and airmass dependent extinction for N-band spectra taken with TIMMI2. Usually, the target and calibrator star have to be observed at similar airmass in order to obtain reliable spectrophotometric fluxes. Our method allows to correct the atmospheric extinction and substantially improves the spectrophotometric flux calibration, when the standard stars were observed at a very different airmass than the targets. Hundreds of standard star measurements in several passbands (N1, N8.9, N10.4, N11.9) were used to derive mid-IR extinction coefficients. We demonstrate that applying our correction of the differential extinction to test data results in a spectrophotometric accuracy up to 2% within the literature flux.


Contents:

• The influence of atmospheric extinction in the mid-IR
• Data analysis
• Results
• Differential extinction as function of airmass
• N-band extinction coefficients
• Application to spectroscopic data
• References