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 for VIMOS

Airmass constraint:

The lack of atmospheric dispersion correction places severe limits on the time window in which observations through a MOS mask can be successfully executed. In practice the requirements are that both pre-imaging and MOS of the same field must be obtained while the field is plus/minus 2 hours from the meridian. It is the user's responsibility to make sure that the airmass constraints specified in the OBs do not conflict with this operational requirement, i.e. the airmass constraints must be set such that they correspond to +/-2hours or less from the meridian:

 

Users may request to waive this rule, but are advised to carefully consider the potentially severe and negative effect this will have on their data quality.

Additional Classification Rules for VIMOS

As stated in the Call for Proposals, any set of MOS data obtained with VIMOS in Service Mode where at least 3 of the 4 arms functioned will be considered an observation completed within constraints.

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