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 CRIRES

Moon constraints:

Moonlight does not noticeably increase the background in any of the CRIRES modes, so in general there is no need to request dark (i.e., FLI < 0.4) or grey (i.e., 0.4 ≤ FLI ≤ 0.7) time. However, it is recommended not to observe targets closer than 30 degrees to the Moon to avoid problems linked to the telescope guiding or active optics system.

The Moon may also affect the quality of the AO correction if the source used for wavefront sensing is fainter than R=14 mag. In this case, reducing the FLI constraint to approximately 0.7 and increasing the minimum distance to the Moon to approximately 50 degrees is generally adequate. Please note that for these cases it will be necessary to request Lunar Illumination = 0.7 in the Phase 1 proposal (see Service Mode Policies).

Turbulence Categories and Image Quality

Depending upon the selected observing mode (i.e., NGS or NoAO), the available Turbulence Categories change. In NGS mode OBs will be scheduled and classified according to the atmospheric turbulence constraint, whereas for NoAO mode OBs the OBs will be scheduled and classified according to Image Quality.

In NoAO mode each Turbulence Category corresponds to the percentage of realization of a given DIMM seeing (i.e., seeing at zenith in V-band, see Table 1). The CRIRES ETC provides the conversion between the Turbulence Category (i.e. Phase 1 constraint) and the OB Image Quality at the requested airmass and wavelength (i.e. IQ Phase 2 constraint), therefore users should refer to the ETC to compute the Image Quality corresponding to the turbulence category selected at Phase 1. At Phase 2 the turbulence category is not used for NoAO OBs and should be set to 100%.


Table 1: Turbulence Categories for CRIRES NoAO mode observations
Turbulence Category 10%
20% 30% 50% 70% 85% 100%
Seeing Threshold 0.50" 0.60" 0.70" 0.80" 1.0" 1.30" all


For CRIRES NGS observations the turbulence constraint is a combination of seeing and coherence time. Six categories are used, from 10% to 85% as shown in Table 2. It is not allowed to specify a turbulence category larger than 85% for NGS observations with CRIRES. At phase 2 the image quality constraint is not used for NGS OBs and it should be set to 2.0" in all NGS OBs.


Table 2: Turbulence Categories for CRIRES AO (NGS) observations
Turbulence Category Maximum Seeing Minimum Coherence Time


0.60" 5.2 ms
20% 0.70" 4.4 ms
30% 0.80" 4.1 ms
50% 1.00" 3.2 ms
70% 1.15" 2.2 ms
85% 1.40" 1.6 ms

If the AO star is not the science target, it is worth noting that the isoplanetic angle depnds 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).

Atmospheric transparency

Photometric conditions (PHO) should be asked only for spectrophotometry. Observations in AO mode with an AO star fainter than about R = 14 should preferably require clear (CLR) conditions. For all other cases, thin cirrus clouds conditions (THN) usually suffice. Observations in NGS mode cannot be performed in thick cirrus clouds (THK) conditions.

Precipitable Water vapour

The PWV is a mandatory Phase 1 and Phase 2 constraint for CRIRES. The median PWV value at Paranal is 2.5mm, and valid PWV values range from 0.5 to 30mm. For observations beyond 2000 nm reasonable PWV upper limits are 2 - 4 mm. Observations below 2000 mm are generally less afftected by PWV hence depending on the science goal reasonable upper limits can be > 5 mm. However, the effect in the transmission in the most affected CRIRES wavelength regions should be either checked in the User Manual (section "Telluric and PWV constraint"), or by using the CRIRES ETC.


The twilight 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. 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 -45 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. Therefore CRIRES users are in general adviced to set the twilight constraint to a negative value (e.g. -30 minutes).

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