Observing conditions: definitions

The following definitions of the observing conditions are common between Phase 1 preparation and Phase 2 preparation, as well as between Service Mode and Visitor Mode observations. These basic definitions are listed below. Please use the instrument selector on the right to see instrument-specific information at the bottom of the page. There are additional Service Mode Policies and Service Mode Guidelines regarding the observing conditions.

Observing conditions are defined as follow:

  • Sky Transparency
    • Photometric: No visible clouds, transparency variations under 2%, only assessable by the analysis of photometric standard stars.
    • Clear: Less than 10% of the sky (above 30 degrees elevation) covered in clouds, transparency variations under 10%.
    • Thin cirrus: transparency variations above 10%.
  • Seeing (Phase 1) and Image quality (Phase2) constraints 
    • Seeing and Image Quality: Starting from Period 96, we differentiate between seeing constraints at Phase 1 and image quality constraints at Phase 2. This represents a significant change in the way users have to request seeing constraints at Phase 1.
      The article by Martinez et al. entitled "On the Difference between Seeing and Image Quality" in Volume 141 of the ESO Messenger describes the meaning of these two quantities.
    • Seeing (Phase 1): Seeing is an inherent property of the atmospheric turbulence, which is independent of the telescope that is observing through the atmosphere outside the dome. Starting from Period 96, the seeing information to be provided  in Box 3 of a proposal is the seeing in the V band at zenith. This change now ensures that the scheduling tool uniformly takes the seeing into account.
    • Seeing = 'n' (no Phase 1 seeing constraint) corresponds to Seeing = 2.0  for most instruments (except 1.4 for NACO and SPHERE, and 1.5 for AMBER and PIONIER).
    • Image Quality (Phase 2): The Image Quality, 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. It is therefore a quantity measured at the (requested) airmass and wavelength of observation. It has been observed that the image quality in a large telescope is always better than the seeing at the same wavelength and airmass, owing to the finite outer scale of the turbulence, as opposed to the commonly used Kolmogorov theory that considers an infinite outer scale.
    • In Phase 2, it is expected that the Image Quality is defined in the constraint set of the OB. Please see exceptions for AO, VLTI, and IFU instruments in the instrument-specific field. 
    • With the help of the Exposure Time Calculators (ETCs) the requested Phase 1 V-band seeing at zenith can be transformed into the desired image quality at the observed wavelength and airmass for each instrument.
    • Note that the same Phase 2 image quality at different bands may correspond to different seeing constraints to be requested at Phase 1. In this case, please request the most stringent Phase 1 seeing constraint.
  • Moon
    • Moon illumination (fraction of lunar illumination, FLI) is defined as the fraction of the lunar disk that is illuminated at local (Chile) civil midnight, where 1.0 is fully illuminated. By definition, moon illumination equals 0 when the moon is below the local horizon.
    • Lunar Phase = 'd' (dark time) corresponds to FLI < 0.4.
    • Lunar Phase = 'g' (grey time) corresponds to 0.4 ≤ FLI ≤ 0.7 and Moon Minimum Angular Distance = 90 or less.
    • Lunar Phase = 'n' (no constraint) corresponds to 0.7 < FLI ≤ 1.0 and Moon Minimum Angular Distance = 60 or less. Exceptions to this statement are applicable for infrared observations. If this is the case for your observations, please refer to the instrument-specific service mode rules for observing constraints for full details.
    • Lunar illumination does not have a noticeable influence on the feasibility of infrared observations.
    • Important note on the moon distance constraint for optical wavelengths:

      At 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 ~ I0 × (1 + cos2Θ),

      Measurements performed on Paranal in I-band during bright time have shown that the sky brightness is approximately constant for moon distances >~50deg. Hence, defining a moon distance constraint larger than 50 deg for I-band observations are unnecessarily limiting the time available for the execution. 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)

      For observations in optical wavelengths it is advised to select moon distance up to 60-70 degrees. Selecting larger lunar distances may drastically reduce the gray/dark time periods in which the observations can be carried out.

  • Precipitable Water Vapour (PWV)
    • Acceptable upper limits for the PWV must be specified for all proposals requiring VISIR, CRIRES and any APEX instrument.
    • The PWV constraint must be specified in the new p1 proposal submission system, when defining the corresponding observing run. 

Please note that observing conditions requested at Phase 2 should match those at Phase 1 (see Service Mode Policies).

The two figures below show the percent chance of success for the realization of combined sky transparency (clear sky or better left; thin clouds or better on the right) conditions and different seeing values, across the year at Paranal. The black line through the middle represents the 50% chance of success. See also the Messenger article "Fifteen Years of Service Mode Operations: Closing the Loop with the Community" by F. Primas et al. (2015).

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