Change page style: 

Observing Condition Constraints in the ITC

The observing condition constraints define the poorest conditions under which a queue-scheduled observation should be executed and indicate the conditions that can be expected by classically-scheduled observers. See the detailed translation between frequency of occurrence (the %-ile bin) and physical property, which is a function of observing wavelength and zenith distance of the source.

Within the ITC, the selected constraint determines which of several input files are loaded to define the sky transmission and emission, and the image size adopted for the nominal point source. Links to further details, or to the ASCII files themselves, are available here:


  • Image quality (see the detailed translation for the meaning of the %-ile bins)
    • The delivered image quality (not simply the seeing) is affected by many parameters including atmospheric properties (e.g. coherence length and time, scale length), wind speed and direction, guide star brightness and relative distance from target and observing wavelength. The tabulated values of image quality assume zenith pointing. As a first approximation, used in the ITC, the image size increases with air mass to the 3/5 power e.g. it is 50% larger at air mass = 2. The wavefront sensor employed for tip-tilt image motion compensation, either peripheral or on-instrument, is selected in the ITC as part of the telescope configuration.
  • Sky transparency (cloud cover) (see the detailed translation for the meaning of the %-ile bins)
  • Sky transparency (water vapour) (see the detailed translation for the meaning of the %-ile bins)
    • optical (spectrum not yet available)
    • near-IR (0.9-5.6µm coverage, resolution 1nm, sampled every 0.5nm). Selected spectra and data files are available for 20%-, 50%- and 80%-ile conditions at air masses of 1, 1.5 and 2.
    • mid-IR (6-28µm coverage, resolution 1nm, sampled every 0.5nm). Selected spectra and data files are available for 20%-, 50%-, 80%-ile and "any" conditions at an air mass of 1.5.
  • Sky background (see the detailed translation for the meaning of the %-ile bins)
    • optical: spectrum not yet available
    • near-IR: 0.9-5.6 µm coverage, resolution 2cm-1. An example spectrum and data file are available
    • mid-IR: the water vapour column and airmass are taken into account by the ITC in calculating the mid-IR sky background, and it need not be explicitly defined here. The sky background should be set to "any" for mid-IR observations in the PIT and OT, except perhaps in cases where optically-dark skies are needed to guide on a faint guide star for a high-priority target.
  • Air mass
    • This is the typical air mass (i.e. 1 / cos[zenith distance]) expected during the observation.


Last update September 15, 2006; R. Mason