Observing Constraints and Classification Rules

General Observing Constraints

Every requested observation has multiple observing constraints. Typical observing constraints are:

  • the allowable brightest lunar phase
  • the allowable smallest moon-to-object angular separation
  • the allowable maximum airmass
  • the allowable maximum image size (i.e. FWHM at observed wavelength, 'image quality' )
  • the allowable sky transparency
  • for CRIRES, NACO and SINFONI, the Strehl ratio on the reference star (as applicable).
  • for instruments observing in the mid-IR (CRIRES and VISIR), the allowable maximum Precipitable Water Vapour (PWV)
  • the allowable twilight constraint that defines the earliest time in minutes with respect to the end of the astronomical twilight when the execution of the OB can be started
  • the allowable absolute time window for the start of the observation (i.e. for time critical events, multi-epoch monitoring)
  • the allowable local sidereal time range for the entire observation (e.g. for ADI observation)
  • for VLTI instruments, the availability of the desired baseline

The Observing Constraints are specified by the user at Phase 2 for each Observation Block. Since the execution conditions required by each programme are an important ingredient in the process of building up the Long Term Schedule of an observing semester, and thus determines which programmes can or cannot be scheduled, users are not allowed to specify at Phase 2 constraints that are more strict than those specified in the original proposal. Users can however relax the constraints during the submission of their Phase 2 material. The values in the OB constraint sets that are selected (and approved) during Phase 2 preparation (and review) cannot be changed later during the observing period.

General Classification Rules

Quality Control of OBs executed in Service Mode will be based on the user's specified constraints for airmass, atmospheric transparency, image quality/seeing, moon constraints, as well as Strehl ratio for Adaptive Optics mode observations. If all constraints are fullfilled the OB will get assigned Quality Control grade "A", while the "B" quality control is assigned if some constraint is up to 10% violated. The observations with quality control grades A or B are completed, while those with quality control grade "C" (out of constraints) will be re-scheduled and may be repeated. In exceptional cases an OB may get status completed with quality grade "D", meaning that it is taken out of constraints but will not be repeated.

Note: for most instruments the image quality constraint as defined in the OB is judged against the full width at half maximum (FWHM) of a point source in the resulting image (or spectral image). For the instruments where the image quality cannot be directly measured (AO, VLTI, fibre instrument), it is either not used for classification or is obtained from the wavefront sensor of the active optics of the telescope.

Additional Observing Constraints for VIMOS

Airmass constraint:

The lack of atmospheric dispersion correction places severe limits on the time window in which observations through a MOS mask can be successfully executed. In practice the requirements are that both pre-imaging and MOS of the same field must be obtained while the field is plus/minus 2 hours from the meridian. This holds for all targets that are taken with the default rotator offset angle of 90 deg. An exception are targets with declinations in the range -45<DEC<-5 deg that are taken with a rotator offset angle of 0 deg. Those can be observed up to plus/minus 3 hours from meridian. It is the user's responsibility to make sure that the airmass constraints specified in the OBs do not conflict with these operational requirements, i.e. the airmass constraints must be set such that they correspond to +/-2 hours (pink shaded area in the graph below) or +/-3 hours (green shaded area in the graph below) or less from the meridian:


Users may request to waive this rule, but are advised to carefully consider the potentially severe and negative effect this will have on their data quality.

These rules are also valid for PILMOS observations.


The twilight constraint can be used to define the earliest time with respect to the end of the astronomical twilight when the execution of the OB can be started. While the relation between the time difference from the evening twilight end and sun elevation varies during the year, for Paranal due to its low latitude this difference is small. Therefore the constraint is given in minutes as a difference in time with respect to the end of astronomical twilight (i.e. the time when the solar elevation is -18 degrees). The default value of twilight constraint is 0. A negative number means that it is allowed to start the observation before the end of the astronomical twilight, a positive number means start the observation at least that many minutes after the end of the astronomical twilight. The twilight constraint can take values between -30 and +30 minutes. In particular, R-, I- and z-band observations can already be taken before the end of the astronomical twilight.

Additional Classification Rules for VIMOS

As stated in the Call for Proposals, any set of MOS data obtained with VIMOS in Service Mode where at least 3 of the 4 arms functioned will be considered an observation completed within constraints.

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