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 VISIR

Quality Control of OBs executed in service mode will be mainly based on airmass and seeing constraints as well as PWV measurements and sensitivity estimates obtained from standard star observations during the observing night.

Image quality:

The image quality constraint, as defined in the OB, is compared to the full width at half maximum (FWHM) of the science data in imaging and spectroscopy modes. Note that this image quality constraint directly relates to the achieved spatial resolution of the science data in the infrared, and is not the optical seeing at zenith (which is specified in the Phase 1 proposal). However, the image quality constraint specified in the OB cannot be more stringent than the image quality corresponding to the optical seeing specified at Phase 1. The VISIR Exposure Time Calculator can be used to estimate the image quality corresponding to a given instrument setup and optical seeing.

In cases where the image quality cannot be measured directly (e.g. for faint or spatially extended sources), the image quality will be assessed from standard star data obtained close in time to the science observations. In case multiple filters are present within an OB, the image quality constraint will be assessed using the shortest-wavelength filter. A measured image quality of 0.3"(~10μm) and 0.6"(~20μm) will always satisfy the image quality constraint.

Atmospheric Transparency:

Atmospheric classification like PHO/CLR/THN/THK are not necessarily related to the photometric stability and sensitivity in the mid-infrared. The main guideline to classify a specific OB will be therefore based on photometric observations of mid-infrared standard stars, frequently monitored during the night. The sensitivities guaranteed by the observatory are listed in the corresponding section of the VISIR instrument page. The OB will be considered fully within constraints regarding sensitivity if an observation of a standard star at zenith in the same filter as the service OB results in an equal or better limiting flux as published ("grade A"). If the limiting flux obtained with standard stars exceeds the published sensitivities in the same filter by not more than 10%, the OB will be considered as "grade B", while the OB will be classified as "grade C" in all other cases. An analog classification scheme will be applied for spectroscopic observations.

Precipatable Water Vapour:

The opacity in the mid-infrared strongly depends on the precipitable water vapour (PWV) in the earth's atmosphere. On Paranal, the PWV contents of the atmosphere is continuously monitored by a dedicated radiometer. PWV can be set as an observing constraint in the constraint set in P2PP for VISIR observations conducted in service-mode. It can take values ranging from 0.5 to 20.0 mm. The median PWV value on Paranal is 2.1 mm, with strong seasonal variations. The VISIR ETC can be used to simulate the influence of different precipitable water vapour (PWV) values. Typical values for the PWV constraint may be up to 10 mm for observations in the N-band (which are typically not very sensitive to PWV) and 1.5 mm for observations in Q.

Moon Constraints:

Moon constraints are usually irrelevant in the mid-infrared and are not taken into account for the classification of the OB. Telescope guiding and active optics can, however, under certain circumstances be compromised for moon distances <30 degrees.

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