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, 'seeing')
  • the allowable sky transparency
  • for Adaptive Optics instruments (currently CRIRES, NACO and SINFONI), the Strehl ratio on the reference star.
  • for instruments observing in 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 (i.e. for time critical events, multi-epoch monitoring)
  • the allowable local sidereal time range (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, seeing (i.e. image quality), moon constraints, as well as Strehl ratio for Adaptive Optics mode observations.

Note: the seeing 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), i.e. at the observed wavelength, for most of the VLT instruments (i.e. it is the image quality).

Additional Observing Constraints and Classification Rules for VLTI

Sky transparency

The AO system MACAO at the UTs and the tip-tilt system STRAP 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 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. Any AMBER triplet or MIDI 2-telescope baseline can be used that is included in the scheduled quadruplet. Obviously, all OBs of a SCI-CAL-SCI concatenation must have indicated the same baseline.

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 FINITO) 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.

We recommend to use LST ranges of at least 3 hours whenever possible. Otherwise it might be difficult to schedule the observation. To obtain observations at different projected baseline lengths and angles, we recommend to make use of different baseline configurations within the assigned AT quadruplet as far as possible rather than restricting the LST intervals. 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 MIDI


MIDI observations are executed by using the MACAO (multi application curvature adaptive optics) systems at all UTs, and using the tip-tilt corrector system called STRAP at all ATs. With the successful use of MACAO at the UTs and STRAP at the ATs, the image quality in terms of FWHM and positional stability on the MIDI detector remains very good, virtually independent of the regular (optical) seeing value.

MACAO/STRAP guiding is mandatory in service mode observations. ESO assures that the MACAO systems are used at the UTs and the STRAP systems at the ATs, and that in both cases the adaptive optics loop is closed during the entire observation. Otherwise, the OB will be considered for re-execution.

Sky transparency

Observations in the thermal IR regime may require clear or photometric conditions, depending on the spectroscopic quality required. However, MIDI observations can even be done with thin cirrus.

Sky transparency constraints are mostly relevant for MIDI in HIGH_SENS mode, where the photometry measurements are performed after the fringe exposure. Variability of water vapor quantity in THN condition may therefore affect the visibility accuracy obtained. In SCI_PHOT mode however (where fringe and photometry measurements are performed simultaneously), effect of THN condition should be strongly reduced. It is therefore recommended to set the constraint to THN in SCI_PHOT (to increase the probability to have the OB executed).

CLR sky conditions may also be advisable for Coude guide stars with aV magnitude close to the limit.

Pointing Restrictions

The feasible HA range for different configurations with MIDI is shown in the plots of the VLTI  pointing restrictions in the declination/hour angle space.

Instrument selector

On this page: