11 July 2016 Investigating the effect of atmospheric turbulence on mid-IR data quality with VISIR
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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, Mario E. van den Ancker, Daniel Asmus, Daniel Asmus, Christian Hummel, Christian Hummel, Hans-Ulrich Käufl, Hans-Ulrich Käufl, Florian Kerber, Florian Kerber, Alain Smette, Alain Smette, Julian Taylor, Julian Taylor, Konrad Tristram, Konrad Tristram, Jakob Vinther, Jakob Vinther, Burkhard Wolff, 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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