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 VIRCAM

  • It is recommended to specify the weakest possible Constraint Set values.
  • The minimum seeing for VIRCAM is 0.6 arcsec, due to instrumental PSF of 0.51 arcsec.
  • The recommended airmass is up to ~1.7 (if applicable) due to implemented short term scheduling optimization algorithm.
  • There is a new constraint in P2PP3 called TWILIGHT constraint.This constraint can be used to define the earliest time with respect to the end of the astronomical twilight when the execution of the OB can be started. While the relation between the time difference from the evening twilight end and sun elevation varies during the year, for Paranal due to its low latitude this difference is small.Therefore the constraint is given in minutes as a difference in time with respect to the end of astronomical twilight (i.e. the time when the solar elevation is -18 degrees). The default value of twilight constraint is 0. A negative number means that it is allowed to start the observation before the end of the astronomical twilight, a positive number means start the observation at least that many minutes after the end of the astronomical twilight.
    The recommended values for the twilight constraint for deep exposures are given in the VISTA/VIRCAM User's Manual (section 7.3).

Additional Classification Rules for VIRCAM

The following new QC rules are implemented in order to improve the completion rate of all surveys, and avoid repeating several times the same OB due to variable seeing during the observation, that affects only a small fraction of images in a tile:

Seeing condition classification rules:

  • C QC grade and status Executed (must be repeated):
    • Average image quality (IQ) over one tile is >10% out of constraint
    • At least one pawprint IQ is >20% out of constraint
  • B QC grade and status Completed:
    • At least one pawprint IQ is between 0% and 20% out of constraints _and_ average IQ is within 10% of the constraints
    • The average seeing is between 0% and 10% out of constraints
  • A QC grade and status Completed:
    • The average IQ _and_ every single pawprint IQ is within constraint

Image ellipticity classification rule:

  • C QC grade and status Executed (must be repeated):
    • there are >10 chips over a full tile with ellipticity >0.2
  • B QC grade and status Completed:
    • there are >4 and <10 chips over a full tile with ellipticity >0.2
    • or ellipticity up to 0.25 if FWHM*(1+ell) is ≤ 0.7
  • A QC grade and status Completed:
    • - there are <=4 chips over a full tile with ellipticity >0.2

OB classification is done on Paranal by the observers and is based onon-the-fly pipeline that reduces all the data as they are observed. It should be noticed that this data reduction uses master calibration files that are updated regularly, but may not be the best ones for a particular observation.

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