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 and Classification Rules for ERIS

Image Quality/Turbulence Category (TCAT)

The constraints and classification for ERIS observing blocks (OBs) depends on what AO mode is being used.

For NGS, LGS, and LGS-SE observations:

  • The TCAT specified at p1 corresponds to a specific threshold of the V-band seeing at zenith and coherence time.
  • The same TCAT is used at p2 (with the image quality left at the default value).
  • The OB is graded by comparing the DIMM-measured seeing and coherence time to the thresholds of the requested TCAT.

For no-AO observations:

  • The TCAT specified at p1 corresponds to a specific V-band seeing at zenith threshold.
  • The image quality specified at p2 is the predicted image quality (FWHM) given the requested TCAT, airmass, and central wavelength. The ETC calculates this value for all no-AO simulations.
  • The OB is graded by comparing the requested image quality to the realized FWHM in the IFS/NIX images at the requested wavelength (or approximated from the DIMM if no obvious point source is present) over the course of the OB.

The maximum permissible turbulence category for each AO mode are:

  • NGS, LGS: 85%
  • LGS-SE: 30%
  • noAO: Any

Sky Transparency

The permissible sky transparency categories for each AO mode are:

  • NGS, LGS, LGS-SE: photometric, clear, thin cirrus
  • noAO: photometric, clear, thin cirrus, thick cirrus

Observations using faint NGS (>10th magnitude) or TT stars (>16th magnitude) are not recommended under thin cirrus conditions.


Observations utilising NGS must be performed at airmass less than 1.9. Observations utilising LGS or LGS-SE must be performed at airmass less than 1.9. You should use the ETC to select an appropriate airmass constraint for your observations, considering the airmass of your target at culmination.

Lunar Constraints

You should not over-specify the Moon constraints as this reduces the chances of the observation being executed. While the Moon does not directly affect infrared observations, it can cause issues when using faint NGS/TT stars.


LGS observations using a tip-tilt star with a Gaia RP magnitude fainter than 16th magnitude are particularly affected. These OBs must have an moon illumination (FLI) constraint of 0.5 or less to ensure the background in the low order wavefront sensor from scattered moonlight does not overwhelm the faint guide star. Consult the user manual for more information about suitable lunar constraints as a function of guide star magnitude.

For most other ERIS observations the moon illumination constraint can be entirely relaxed by selecting FLI=1. Only for the shortest wavelength filters (<1.2um) there is an increased contribution from the sky background, potentially degrading the sensitivity of your observations.

Twilight Constraints

Observations of bright objects can be carried out in twilight, up to 45 minutes before the end of astronomical twilight. You should use the twilight constraint in p2 to specify the number of minutes before the end of astronomical twilight that an OB can be started (e.g., -30 means the OB can be started at the earliest 30 minutes before the end of astronomical twilight).

PWV Constraints

The sky transmission and emission in the L and M-band vary strongly as a function of precipitable water vapour (PWV). You should use the ETC to select an appropriate PWV constraint for OBs with observations at these wavelengths.

LST Constraints

Specifying LST constraints may be desirable for observations using pupil tracking. The length of the LST interval must at the very least exceed the duration of the OB. Relaxing the interval further will make scheduling your observation easier. You should use at least the duration of the OB plus an additional 30 minutes. There are no limits on the maximum duration of the LST interval.

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