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, '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 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.

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 OH lines excitation, 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 is taken 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 CRIRES

Moon constraints:

Moon does not affect observations obtained by CRIRES. However, as the wavefront sensor is sensitive in the R-band, restricted moon constraints are only meaningful for AO observations with a natural guide star (NGS) fainter than R = 15 mag. In these cases, reducing the FLI constraint to approximately 0.7 and increasing the distance to the Moon to approximately 50 degrees is generally adequate. Even here, it is important not to over-specify the constraints, as this reduces the chances of the Observation Block to be executed. And, it is necessary to request Lunar Phase = 'g' in the proposal (see Service Mode Policies).

Note, however, that telescope guiding and active optics can, under certain circumstances, be compromised for moon distances < 30 deg.

Seeing: FWHM and on-axis Strehl ratios for adaptive optics observations using a Natural Guide Star:

For closed loop AO observations of point sources, the users must specify in the constraint set the desired on-axis Strehl ratio, i.e. the ratio of the maximal intensity of the PSF to the maximal intensity expected for the theoretical PSF with no turbulence. This is the main parameter regarding atmospheric turbulence. It should be set for the K band; this value is produced by the CRIRES ETC.

In addition, all closed loop AO observations should enter also the FWHM of the seeing, although this constraint is less relevant. The constraint on the seeing refers to the image quality measured by the DIMM or by the guide probe, as they are the most relevant for the MACAO system. It is used as a guide, together with the coherence time, in order to estimate the achievable Strehl ratio.

If the AO star is not the science target, it is worth noting that the isoplanetic angle depends on (airmass)-8/5, and on the wavelength as λ6/5. On the other hand, the Strehl ratio, SR, decreases with the angular distance to the AO star θ as SR ∝exp(-(θ/θ0)5/3).

Image quality: FWHM for seeing limited observations, no adaptive optics (NoAO):

For NoAO observations, only the image quality at the wavelength of observations is relevant. The user should therefore specify the FWHM for the respective bands of the science observations. The constraints set and definitions are the same as for all other seeing limited VLTinstruments. The input field for the Strehl ratio should remain unchanged.

The classification of each observation will be based either (a) on the measured spatial profile of the spectrum if the object is a point source, or (b) on FWHM of a star seen on the Slit Viewer if there are more stars available, or (c) on the image quality measured on the guide probe otherwise.

Atmospheric transparency

Photometric conditions (PHO) should be asked only for spectrophotometry. Observations in AO mode with an AO star fainter than about R = 15 should preferably require CLR conditions. For all other cases, thin clouds conditions (THN) usually suffice.

Water vapour

From P90 onwards the precipitable water vapour (PWV) is a new constraint that should be set in the Constraint Set of P2PP3. It can take values ranging from 0.5 to 20.0 mm. Water vapour affects observations in particular beyond 2000 nm. Typically, values of 2-4 mm are reasonable choices in this case. The CRIRES ETC can be used to simulate the influence of different precipitable water vapour (PWV) values.


From P90 onwards, there is a new constraint in P2PP3 called TWILIGHT constraint.This constraint 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. 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, a positive number means start the observation at least that many minutes after the end of the astronomical twilight. The twilight constraint can take values between -30 and +15 minutes. In particular, K-, L- and M-band observations can already be taken before the end of the astronomical twilight. Test have shown that even the J-band observations are not much influenced by the twilight.

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