Observing Constraints and Classification Rules

General Observing Constraints

Every requested observation has multiple observing constraints. The 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, 'image quality' ) or (for SPHERE) the atmospheric conditions (combination of seeing and coherence time)
  • the allowable sky transparency
  • for CRIRES, NACO and SINFONI, the Strehl ratio on the reference star (as applicable).
  • 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 (see the note below).
  • the allowable absolute time window for the start of the observation (i.e. for time critical events, multi-epoch monitoring)
  • the allowable local sidereal time range for the entire observation (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 determine 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.

Note about the twilight constraint: this observing constraint has been introduced to allow specifying start of observation with respect to the start of the night: e.g. to delay start of observations for faint targets until the sky gets darker, or allow starting observations for very bright targets during the twilight. The original motivation for this constraint is related to sky brightness in near-IR that is affected by excitation of OH lines, and is not affected by other constraints (e.g. moon distance/phase). It does not apply to astronomical twilight at the end of the night (i.e. sunrise). 

General Classification Rules

Quality Control of OBs executed in Service Mode will be based on the specified constraints in the OB for airmass, atmospheric transparency, image quality/seeing, moon constraints, twilight constraint, as well as Strehl ratio for Adaptive Optics mode observations. If all constraints are fullfilled the OB will get assigned Quality Control grade "A", while the "B" quality control is assigned if some constraint is up to 10% violated. The observations with quality control grades A or B are completed, while those with quality control grade "C" (out of constraints) will be re-scheduled and may be repeated. In exceptional cases an OB may get status completed with quality grade "D", meaning that it was executed out of constraints but will not be repeated.

Note: for most instruments the image quality 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). For the instruments where the image quality cannot be directly measured (AO, VLTI, fibre instrument), it is either not used for classification or is obtained from the wavefront sensor of the active optics of the telescope.

Special Note for UT4 OB Classification Rules

Ellipticity was detected in some HAWK-I, MUSE and SINFONI observations from 07 May 2017 onwards when pointing away from the wind. The problem is under investigation and not yet understood.  In the interrim there is an additional criterion imposed during OB classification, related to elongation, defined as 100*(1-B/A)%, where A and B are the FWHM on the major and minor axes, respectively.

  • For HAWK-I:
    • A. If elongation < 10% for most stars
    • B. If 10% < elongation < 20% for most stars
    • C. If 20% > elongation for most stars 
  • For MUSE:
    • If there are stellar objects in the reconstructed cube FoV, adopt HAWK criteria.
    • If there are no stellar objects in the reconstructed cube FoV, use the SGS (slow guidance sensor) with criteria as above, but relaxed to 15% and 25% to account for the SGS distortions
    • If there are no stellar objects in the FoV or SGS the classification is based only on the average FWHM on the auto-guider.
  • For SINFONI:
    • For LGS/NGS no special ellipticity criteria are applied.
    • For NoAO the HAWK-I criteria are adopted only if
      • the target is a point source
      • the FWHM can be reliably measured (>100 ADU peak counts)
      • the PSF is resolved (FWHM > 4 pixels)

Additional Observing Constraints and Classification Rules for NACO


Users should note that in both the proposal (Phase 1) and the NAOS Preparation Software (hence in the Constraint Sets of the OBs at Phase 2) one specifies the seeing at V and at zenith.  This distinguishes NACO from other non-AO instruments for which the image quality is used at Phase 2.

Fields Filled In for the User

Users should note that the seeing (see above), 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 NACO 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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