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: 'Image Quality' measure as FWHM at observed wavelength and airmass
  • the Turbulence category (TC) for SPHERE, MUSE, ERIS and VLTI instruments that use full-AO. This combines probability of realisation of seeing and coherence time.
  • the allowable sky transparency
  • the allowable maximum Precipitable Water Vapour (PWV) should be provided for all instruments. The default value is set to 30 mm and should be fine for all non-IR instruments. All instruments include PWV in the ETC calculations that can be used to evaluate the impact of different PWV values on data.
  • the allowable twilight constraint that defines the earliest time in minutes with respect to the end of the evening 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)

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 already 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 (as requested). 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.

Additional Observing Constraints for KMOS

Image quality:

The image quality is measured on the reconstructed IFU images of the reference stars during acquisition and on reconstructed IFU images of point sources in the science exposures (if available), or on the guide probe otherwise (if all science targets are extended sources).

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 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 KMOS observations in H and K gratings, the Moon illumination (FLI) can be entirely relaxed in most cases by selecting FLI=1. For observations with the  YJ grating,  grey time and moon ditances up to 90 degree are acceptable. Faint observations in IZ band should be performed in dark time. 

Water vapour:

The precipitable water vapour (PWV) is a constraint that can be set in the Constraint Set of p2. It can take values from 0.5 to 20.0 mm. The default value for KMOS is set to 20.0 mm. The K and HK bands are affected by strong telluric lines independent of the PWV value. Extremely low PWV values (< 1 mm) open new windows in atmospheric transparency in IZ (850 nm < lambda< 930 nm and lambda > 1000 nm), YJ (lambda < 1100 nm and 1170 nm < lambda< 1240 nm), H (1510 nm < lambda < 1540 nm and 1550 nm < lambda< 1575 nm) which allow the detection of additional emission/absorption lines. If such a low PWV value is needed please give a scientific justification in the Readme file. The KMOS ETC can be used to simulate the influence of different PWV values, and see Sect. 3.4.8 in the KMOS user manual for further details and a figure illustrating the PWV influence.


The twilight constraint in the Constraint Set of p2 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. The constraint can take values between -30 (= OB can be executed 30 minutes before the end of twilight) and +15 (OB should start not earlier than 15 minutes after twilight). In particular, KMOS K-band observations can already be taken before the end of the astronomical twilight.

Additional Classification Rules for KMOS

Starting from P110, the classification rules for KMOS will not depend on the “rotator optimisation” option.

For Mosaic mode, if more than 2 arms are not available in mosaic mode the user will be asked whether the observations still make sense.

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