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 CRIRES, NACO and SINFONI, the Strehl ratio on the reference star.
  • 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 (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 NACO

Fields Filled In for the User

Users should note that the seeing, airmass and Strehl Constraint Set (as well as the target Right Ascension and Declination) fields are automatically filled when the NAOS-PS-generated configuration file is loaded. Users are allowed only to edit the Strehl field, however they must never increase the value above that provided in the AO configuration file.

Moon constraints

The Moon does not affect IR observations with CONICA. However, the moon may affect the quality of the adaptive optics correction, if the source used for wavefront sensing is fainter than V=16. In these cases, reducing the FLI  constraint to approximately 0.7 and increasing the distance to the Moon to approximately 50 degrees is generally adequate. Even here, it is important not to over-specify the constraints, as this reduces the chances of the Observing Block being executed. For wavefront sensing in the IR, these recommendations can be ignored.

LST ranges and targets to be tracked through the meridian

There are a two basic rules that must be abided by for such cases:

  • For targets that are to be tracked through the meridian the OB must contain an LST time interval. This makes it straightforward to schedule at the telescope.
  • For any OB that has an LST time interval specified the length of that interval must exceed the total execution time for the OB itself.

Further considerations:

  • Like all other such constraints the LST constraint will be used to judge the success or failure of the execution of the OB.  The LST constraint will be judged, again like all other constraints, in such a way that it can be fully met or mostly met.
  • For UT4 it has been decided that the limit for the LST being mostly met is if the OB starts/ends within 20 minutes of the designated starting/ending point of the LST window.  Of course OBs which are executed fully within the LST window constraint will be judged as having fully met that constraint.
  • Aside from the influence that the 20 minutes has on the degree to which the LST constraint is met, it also has an influence on the ease with which the OB can be selected for observation in the first place.  That is, the process of selecting the suite of OBs that are "observable" at any point in time begins with filtering all OBs in the system to determine those that are "sufficiently up."  For the LST consideration, "sufficiently up" translates into "if all other constraints were met, would we mostly meet the LST constraint if we were to do this OB now."  Thus, an increase of 20 minutes on top of the window provided in the OB allows more OBs to make it through the filtering process.
  • Finally, note that very wide LST ranges, while easing the schedulability of the OB, do potentially compromise the symmetry with which the OB is observed with respect to the time of meridian transit.

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