11 July 2016 Investigating the effect of atmospheric turbulence on mid-IR data quality with VISIR
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Abstract
A comparison of the FWHM of standard stars observed with VISIR, the mid-IR imager and spectrometer at ESO's VLT, with expectations for the achieved mid-IR Image Quality based on the optical seeing and the wavelength-dependence of atmospheric turbulence, shows that for N-band data (7{12μm), VISIR realizes an image quality about 0.1" worse than expected based on the optical seeing. This difference is large compared to the median N-band image quality of 0.3-0.4" achieved by VISIR. We also note that other mid-IR groundbased imagers show similar image quality in the N-band. We attribute this difference to an under-estimate of the effect of the atmosphere in the mid-IR in the parameters adopted so far for the extrapolation of optical to mid-IR seeing. Adopting an average outer length-scale of the atmospheric turbulence above Paranal L0 = 46 m (instead of the previously used L0 = 23 m) improves the agreement between predicted and achieved image quality in the mid-IR while only having a modest effect on the predicted image quality at shorter wavelengths (although a significant amount of scatter remains, suggesting that l0 may not be constant in time). We therefore advocate adopting L0 = 46 m for the average outer length scale of atmospheric turbulence above Cerro Paranal for real-time scheduling of observations on VLT UT3 (Melipal).
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Mario E. van den Ancker, Daniel Asmus, Christian Hummel, Hans-Ulrich Käufl, Florian Kerber, Alain Smette, Julian Taylor, Konrad Tristram, Jakob Vinther, Burkhard Wolff, "Investigating the effect of atmospheric turbulence on mid-IR data quality with VISIR", Proc. SPIE 9910, Observatory Operations: Strategies, Processes, and Systems VI, 99102T (11 July 2016); doi: 10.1117/12.2233701; https://doi.org/10.1117/12.2233701
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