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 (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 (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)
  • 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 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 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. 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.

Special Note for UT4 OB Classification Rules

Ellipticity was detected in some HAWK-I, MUSE and SINFONI observations from 07 May 2017 onwards when pointing away from the wind. The problem is under investigation and not yet understood.  In the interrim there is an additional criterion imposed during OB classification, related to elongation, defined as 100*(1-B/A)%, where A and B are the FWHM on the major and minor axes, respectively.

  • For HAWK-I:
    • A. If elongation < 10% for most stars
    • B. If 10% < elongation < 20% for most stars
    • C. If 20% > elongation for most stars 
  • For MUSE:
    • If there are stellar objects in the reconstructed cube FoV, adopt HAWK criteria.
    • If there are no stellar objects in the reconstructed cube FoV, use the SGS (slow guidance sensor) with criteria as above, but relaxed to 15% and 25% to account for the SGS distortions
    • If there are no stellar objects in the FoV or SGS the classification is based only on the average FWHM on the auto-guider.
  • For SINFONI:
    • For LGS/NGS no special ellipticity criteria are applied.
    • For NoAO the HAWK-I criteria are adopted only if
      • the target is a point source
      • the FWHM can be reliably measured (>100 ADU peak counts)
      • the PSF is resolved (FWHM > 4 pixels)

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.

Baseline choice

The choice of the baseline configuration to be used must be indicated in the baseline field of the constraint set in each OB (this applies for UT as well as for AT baselines). Alternate baselines may be mentioned in the dedicated instrument comment field of the OB (labeled "Information on alternate baselines", located below the user comment field in P2PP3). AT baselines are scheduled as quadruplets.  Please choose the Phase1-assigned quadruplet. Obviously, all OBs of a concatenation must have indicated the same baseline configuration.

Intervals of local sidereal time (LST)

Constraints on the local sidereal time (LST) at which the OB has to be executed must be specified in the "Sidereal Time" section of each OB. Desired constraints on the projected baseline length and azimuth angle, or the airmass must be translated into LST intervals. Furthermore, the specified LST interval must be limited to the time range when the target is above 30 degrees altitude (above 40 degrees for GRAVITY) and when the observation is not unfeasible due to the delay line restrictions or shadowing effects. The desired and feasible LST interval can be calculated using the visibility calculator VisCalc or the GRAVITY ETC.

We recommend to use LST ranges of at least 3 hours whenever possible. Otherwise it might be difficult to schedule the observation.   The minimum accepted LST range is 1.5 hours. In case that a desired observation shall be done only once, but can be executed at several alternative non-continguous LST ranges (for instance either some hours before meridian or some hours after meridian), more than one LST intervals can be specified for an OB. The time separation between these intervals must be strictly larger than 1 hour.  Please be reminded that multiple OBs are needed if your target shall be observed more than once (for instance at different LST intervals).

The LST range of the calibrator OBs must each cover the complete LST range of the science target and must begin 30 minutes earlier (if CAL precedes SCI) or end 30 minutes later (if CAL follows SCI). In other words, if the SCI LST ranges from time1 to time2, the preceding CAL must cover time1 - 30 minutes until time2, and the following calibrator must cover time1 until time2 + 30 minutes. This ensures that calibrator OBs are available at any time when the science target observation can be attempted. Optionally, any CAL OB may cover time1 - 30 minutes until time 2 + 30 minutes. The following figure provides an illustration:


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. 

VLTI OBs must make use of concatenation containers, which means that they can not make use of relative time link containers (containers of containers are not yet offfered). Relative time links should, as far as possible, be translated into absolute time constraints using the information on the block-wise schedule of baseline configurations. Please contact the User Support Department for assistance on the best strategy. 

Additional Observing Constraints and Classification Rules for PIONIER

Sky transparency

Observations in the IR regime may require clear sky conditiona depending on the brightness of the target. For the required sky transparency for different object magnitudes and instrument modes, please see the latest information on the PIONIER instrument webpage.


The specified seeing constraint is compared to the optical seeing delivered by the DIMM. The exact relationship between seeing, MACAO/STRAP performance, IRIS performance, and final data quality is not yet known in detail. However, with the use of MACAO/STRAP and IRIS, the injection of flux into the PIONIER fibers remains effective, even for moderate conditions. For the required seeing for different object magnitudes and instrument modes, please see the PIONIER instrument  webpage.

Pointing Restrictions

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

Intervals of local sidereal time (LST)

The 30 min before and after SCI extensions of the CAL LST intervals is reduced to 15 min for PIONIER. PIONIER observations can also use a


sequence without asking for a waiver.

In this case, both SCI OBs need to be identical, in particular need to observe the same target and have the same LST intervals. CAL1 and CAL3 require LST intervals as in the regular case above. CAL2 needs LST intervals that equal those of either SCI, CAL1, or CAL2.


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