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 (i.e. FWHM at observed wavelength, 'seeing')
  • the allowable sky transparency
  • for Adaptive Optics instruments (currently CRIRES, NACO and SINFONI), the Strehl ratio on the reference star.
  • for instruments observing in 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
  • the allowable absolute time window (i.e. for time critical events, multi-epoch monitoring)
  • the allowable local sidereal time range (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 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 (i.e. image quality), 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 (i.e. it is the image quality).

Additional Observing Constraints for KMOS


The image quality (seeing constraint) 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, the Moon illumination (FLI) can be entirely relaxed in most cases by selecting FLI=1. Exceptions are observations with the IZ and YJ gratings, for which grey time and moon ditances up to 90 degree are acceptable.

Water vapour:

The precipitable water vapour (PWV) is a constraint that can be set in the Constraint Set of P2PP3. It can take values from 0.5 to 20.0 mm. The default value for KMOS is set to 20.0 mm since water vapour does not affect much spectra with KMOS resolution. If, however, a PWV value below 10.0 mm is needed a waiver can be requested. The KMOS ETC can be used to simulate the influence of different PWV values.


The twilight constraint in the Constraint Set of P2PP3 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

In case that one or more of the pick-off arms of KMOS cannot be used because of technical failures or the observations of one ore more IFUs fail for other reasons, the following classification rules apply:

  • If <10% of priority 1 sources are lost the observation is classified with QC grade B
  • If >10% of priority 1 sources are lost the observation is classified with QC grade C
  • If <25% of sources of any priority (but at most 10% of priority 1 sources) are lost the observation is classified with QC grade B
  • If >25% of sources of any priority are lost the observation is classified with QC grade C

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