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 (seeing)
  • the allowable sky transparency
  • for Adaptive Optics instruments (currently CRIRES, NACO and SINFONI), the Strehl ratio on the reference star.
  • for VLTI instruments, the local sidereal time range and 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, 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.

Additional Observing Constraints and Classification Rules for VLTI

Sky transparency

The AO system MACAO at the UTs 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). Alternative baselines may be mentioned in the ReadMe file. AT baselines are scheduled as quadruplets. Any AMBER triplet or MIDI 2-telescope baseline can be used that is included in the scheduled quadruplet.

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 science OB. Desired constraints on the projected length, azimuth angle of the baseline, or the airmass must be translated into LST intervals, for example by using the ESO Visibility Calculator VisCalc. Furthermore, the specified LST interval must be limited to the time range when the target is above 30 degrees altitude and when the observation is not unfeasible due to the delay line restrictions or shadowing effects. The feasible HA range can be checked with VisCalc.

We recommend to use LST ranges of at least 3 hours. Otherwise it might be difficult to schedule the observation. The minimum accepted LST range is 1 hour. In case that a desired observation shall be done only once, but can be executed at several alternative non-contiguous LST ranges (for instance either some hours before meridian or some hours after meridian), up to five alternative LST intervals can be specified for one OB (science OB and corresponding calibrator OB). The time separation between these intervals must be at least 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).

Additional Observing Constraints and Classification Rules for AMBER

Sky transparency

Observations in the IR regime may require clear depending on the spectroscopic quality wished. Also for targets close to the brightness limit of AMBER, it is recommended to request clear sky conditions. For the required sky transparency for different object magnitudes and instrument modes, please see the latest information on the AMBER instrument webpage.

Seeing

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 AMBER fibers remains effective, even for moderate conditions. For the required seeing for different object magnitudes and instrument modes, please see the AMBER instrument  webpage.

Pointing Restrictions

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

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