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' )
  • 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 SINFONI

Users must enter the seeing/FWHM (noAO mode) or the seeing together with the on-axis K-band Strehl ratio (natural guide star (NGS) or laser guide star (LGS) mode) for all observation blocks. Further information can be found in the Observing Conditions webpage.

Adaptive optics observations (NGS/LGS mode):

As mentioned above, for closed loop AO observations we request the users to enter the seeing/FWHM as indicator if the respective observation block can be scheduled at the current observation condition. In addition we request for all closed loop AO observations to enter the on-axis K-band Strehl ratio in the constraint set. The latter is compared with the real time AO performance estimator which derives this value from the wavefront errors. The Strehl ratio entered in the constraint set cannot be higher than predicted by the ETC for the given brightness of the natural guide star. This can be tested with the "Verify" button in P2PP. The off-axis performance on the target in the user selected bands can be estimated with the SINFONI exposure time calculator.

Seeing limited observations (noAO mode):

For seeing limited observations the FWHM shall be specified for the respective bands of the science observations in the "seeing" field of the constraints set. The constraints sets and definitions are the same as for all other seeing limited VLT instruments (the input field for the Strehl ratio should remain unchanged).

Lunar constraints

Do not overspecify the Moon constraints! The Moon does not directly affect infrared observations, but it does affect the quality of the active and adaptive optics corrections, in particular if the reference stars are too faint. It is recommended not to observe objects when they are closer than 30 degrees from the Moon. For SINFONI, the Moon illumination (FLI) can be entirely relaxed in most cases by selecting FLI=1. If the source used for AO correction is fainter than R=15, it is recommended to increase the Moon constraint by selecting FLI=0.7and a Moon distance of about 50 degrees. For this, it is necessary to request Lunar Phase = 'g' in the proposal (see Service Mode Policies).


Observations in LGS mode should not have an airmass constraint larger than 1.5

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