Turbulence Category at Phase 1 for any instrument: At Phase 1, the classical seeing constraint is replaced by the choice of the Turbulence Category for all instruments.In order to harmonize this new scheme between all non-AO and AO instruments, a single Turbulence Category is introduced at Phase 1 for all instruments. These additions are important constraints for instruments using adaptive optics (AO) and need to be taken into account when scheduling observations to ensure that the science goals can be achieved. This constraint generalizes the classical seeing constraint used so far, and takes into account the coherence time and the fraction of turbulence in the ground layer, for instruments that need it, in addition to the classical seeing. Starting with Phase 1 of ESO Period 105 (Sept 2019), we introduce a new constraint, the atmospheric turbulence constraint.It is therefore a quantity measured at the (requested) airmass and wavelength of observation.
HORIZON DEFINITION ASTRONOMY FULL
Image Quality (IQ) ,defined as the full width at half maximum (FWHM) of long-exposure stellar images, is a property of the images obtained in the focal plane of an instrument mounted on a telescope observing through the atmosphere.The seeing values in the table and figure below are measured in the V band at zenith. As a preface, s eeing is an inherent property of the atmospheric turbulence, which is independent of the telescope that is observing through the atmosphere outside the dome.Turbulence Category, Image Quality, and their usage at Phase 1 and Phase 2 are defined as follows:.The impact of the PWV can be evaluated with Exposure Time Calculators (ETCs). These defaults should be fine for all non-IR instruments and in any event can be lowered if required for the science. Acceptable upper limits for the PWV must be specified for all instruments in the new p1 proposal submission system, when defining the corresponding observing run. The default values are 10mm for VISIR, 20mm for KMOS, and 30mm for all other Paranal instruments.Selecting larger lunar distances may drastically reduce the gray/dark time periods in which the observations can be carried out. Observations at shorter wavelengths (B- to R-band) in grey and dark time are also not affected very much by the presence of the moon if the distance is >~50-60 deg, and often the sky is darker at 60-70 deg away from the moon than at 120 deg away, when the moon is low above the horizon (see Fig 5 of Patat 2004, Messenger Issue 118)įor observations in optical wavelengths it is advised to select moon distance up to 60-70 degrees. Hence, defining a moon distance constraint larger than 50 deg for I-band observations are unnecessarily limiting the time available for the execution. Measurements performed on Paranal in I-band during bright time have shown that the sky brightness is approximately constant for moon distances >~50deg. Important note on the moon distance constraint for optical wavelengths:Īt the dry, low water vapour site of Paranal during clear or photometric nights we expect that the night sky brightness does not show a strong dependence of the lunar distance (Rayleigh scattering I ~ I 0 × (1 + cos 2Θ),
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