Observing Constraints and Classification Rules

General Observing Constraints

Every requested observation has multiple observing constraints. The 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: 'Image Quality' measure as FWHM at observed wavelength and airmass
  • the Turbulence category (TC) for SPHERE, MUSE, ERIS and VLTI instruments that use full-AO. This combines probability of realisation of seeing and coherence time.
  • the allowable sky transparency
  • the allowable maximum Precipitable Water Vapour (PWV) should be provided for all instruments. The default value is set to 30 mm and should be fine for all non-IR instruments. All instruments include PWV in the ETC calculations that can be used to evaluate the impact of different PWV values on data.
  • the allowable twilight constraint that defines the earliest time in minutes with respect to the end of the evening astronomical twilight when the execution of the OB can be started (see the note below).
  • 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)

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 determine 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.

Note about the twilight constraint: this observing constraint has been introduced to allow specifying start of observation with respect to the start of the night: e.g. to delay start of observations for faint targets until the sky gets darker, or allow starting observations for very bright targets already during the twilight. The original motivation for this constraint is related to sky brightness in near-IR that is affected by excitation of OH lines, and is not affected by other constraints (e.g. moon distance/phase). It does not apply to astronomical twilight at the end of the night (i.e. sunrise). 

General Classification Rules

Quality Control of OBs executed in Service Mode will be based on the specified constraints in the OB for airmass, atmospheric transparency, image quality/seeing, moon constraints, twilight constraint, as well as Strehl ratio for Adaptive Optics mode observations (as requested). 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 was executed 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 and Classification Rules for VLTI

Sky transparency

The AO systems MACAO or CIAO at the UTs and the tip-tilt system NAOMI at the ATs can be used only if the sky conditions are better than THICK. A calibration of the photometric spectrum of VLTI instruments is not precise to the level of PHO conditions of the VLT instruments. As a result, a sky transparency better than CLR should not be requested.

Moon constraint

The sky background introduced by the moon does not significantly affect the data quality for near and mid-infrared interferometry. There are some moon restrictions due to the guiding of the telescopes of up to a required distance of 20 deg between science target and the moon. For service mode observations, ESO Science Operations takes into account these restrictions. As a result, a moon constraint is not part of the constraint set of VLTI instruments.


The airmass constraint is active for VLTI OBs with defaults of 1.6 for GRAVITY and 2.0 for PIONIER and MATISSE. These values shall normally not be changed.

Baseline configuration

The choice of the baseline configuration must be indicated in each OB in the baseline field of the acquisition templates of any of the VLTI instruments.  We use broad baseline designations small, medium, large, astrometric, UTs, see for more details http://www.eso.org/sci/facilities/paranal/telescopes/vlti/configuration/P105.html . Baseline configurations can be specified as a keyword list, where the first entry denotes the primary choice, and other entries denote alternative choices. For example 'small, medium' means that the small configuration is the primary choice, and the medium configuration can be used as an alternative.

Choices of baseline configurations (including alternative configurations if present) need to be consistent across different OBs within the same concatenation.

Type of VLTI observations

There is a keyword in the acquisition templates of all VLTI instruments, where we ask to tag the observation with one or more of the types snapshot, time-series, imaging, astrometry, as already specified in the Phase 1 proposal, see https://www.eso.org/sci/facilities/paranal/cfp/cfp105/recent-changes.html . Valid combinations of VLTI types are imaging and time-series, astrometry and time-series, and astrometry and snapshot.

Intervals of local sidereal time (LST)

The LST constraint shall only be used in cases where there is a strong scientific reason for this constraint. The observability will be calculated on Paranal, and shall not any longer be encoded into the LST constraint when preparing OBs. Still, the observer needs to make sure that the target coordinates are observable with the specified baseline configurations, and that SCI and CAL OBs in a concatenation can be observed back-to-back.

Time constraints

Absolute time constraints at which an OB shall be executed must be entered in the time constraint section of the individual OB. Please be aware that there are only a limited number of service mode nights available per period and that baseline configuration are scheduled blockwise. It is thus important to check that the desired baseline configuration is indeed available in service mode during the specified time interval.

Additional Observing Constraints and Classification Rules for MATISSE

Sky transparency

MATISSE observations can be carried out under CLR or THN skies, depending on mode and target flux (see MATISSE instrument webpage.

Transparency category

The required turbulence catgory for different object magnitudes and instrument modes, can be found on the MATISSE instrument  webpage.

Pointing Restrictions

The feasible HA range for the AT  configurations offered with MATISSE is shown in the plots at VLTI Configurations page (click on the links named after the 4-telescope configurations offered).

Intervals of local sidereal time (LST)

If LST intervals are defined, the LST range of CAL OBs shall exceed those of SCI OBs by exactly 30 min.

MATISSE observations are recommended with the standard CAL-SCI sequence. However, deviations from this standard are now allowed within the 1h limit per concatenation.   

Longer sequences of 1.5h can be used after approved waiver request only.


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