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 MUSE P109

Image Quality

For WFM observations (NOAO and AO) the OB Image Quality (IQ) refers to the FWHM of the MUSE image at 700nm. The IQ value to be used in the OB is provided by the ETC by using as input parameter the Turbulence category requested in the proposal. Note that observations in NFM are not classified based on the IQ.

Turbulence category

Observations in NFM are classified according to seeing and coherence time, therefore users are requested to define in the OBs the Turbulence category requested in the proposal. Note that WFM (AO and NOAO) OBs should not use the Turbulence category constraint, but just IQ (see above). NFM observations can be performed only when Turbulence category is 10%, 20%, 30% or 50%, or only 10% if the NGS is 18.5<J<19 mag.

Sky transparency

WFM-AO and NFM observations cannot be performed in thick conditions.


Because of the physical limit imposed by the AOF system, WFM-AO and NFM observations can be performed only when the target airmass is up to 1.9 (i.e., airmass <2). However, in general NFM observations at high airmass are highly not recommended, and if the NGS is 18.5<J<19 then the target airmass should be lower than 1.3.

Twilight constraint

There is a constraint in p2 called twilight constraint that indicates if an OB can be executed during twilight. This constraint is used to define the earliest time with respect to the end of the astronomical twilight when the execution of the OB can be started. While the relation between the time difference from the evening twilight end and sun elevation varies during the year, for Paranal due to its low latitude this difference is small. Therefore the constraint is given in minutes as a difference in time with respect to the end of astronomical twilight (i.e. the time when the solar elevation is -18 degrees). The default value of twilight constraint is 0. A negative number means that it is allowed to start the observation before the end of the astronomical twilight. The twilight constraint can take values between -45 and 0 minutes.

Precipitable Water Vapour

Telluric absorption features of O2 and H2O are stronger at the (very-near-infra) red end of the MUSE spectrum, albeit not nearly as problematic as in the JHK NIR and Mid-IR bands. Although for O2 not much can be done, the user can exercise some level of control over the amount of H2O absorption by using the precipitable water vapour, PWV, constraint when creating the OBs at Phase 2. The median value at Paranal is 2.5 mm of H2O, and any user intending to request a value below the median will need to request a waiver with a proper scientific justification. The effect in the transmission in the affected waivelength range can be checked by using the MUSE ETC. In general, it is strongly recommended to keep the default PWV value, unless scientifically justified.

Special Note for UT4 OB Classification Rules

Ellipticity was detected in some MUSE (and HAWK-I) 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.

  • If there are stellar objects in the reconstructed cube FoV, the following criteria apply:
    • A. If elongation < 10% for most stars
    • B. If 10% < elongation < 20% for most stars
    • C. If 20% > elongation for most stars
  • 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.

Instrument selector

On this page: