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
    • Seeing is defined as the instrument FWHM in arcsec, at the wavelength of observation, on the focal plane of the instrument's detector, i.e. after the image has been taken through the entire telescope and instrument. It is not the instantaneous seeing outside the dome, but rather the image quality. Please see exceptions for AO, VLTI, and IFU instruments in the instrument-specific field.
    • Seeing = 'n' (no constraint) corresponds to Seeing = 2.0 or greater  (1.4 for NACO and MIDI)
  • 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 < FLI1.0 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 CRIRES and any APEX instrument.
    • The PWV constraint must be specified using the \Target macro in the proposal form as shown in the ESOFORM User Manual.

Please note that observing conditions requested at Phase 1 cannot be altered at Phase 2 (see Service Mode Policies).

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