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).

Details of the classification rules are available on the PSO QC0 OB grading page.

Additional Observing Constraints and Classification Rules for HAWK-I

Lunar constraints

Do not overspecify the Moon constraints as this reduces the chances of the Observing Block being executed! The Moon does not directly affect infrared observations. For most HAWK-I observations the Moon illumination (FLI) can be entirely relaxed by selecting FLI=1. Only for the shortest wavelength filters (<1.2mic) there is an increased contribution from the sky background, potentially degrading the sensitivity of your observations.

Twilight constraints

Because HAWK-I is an infrared imager, observations of bright objects may be carried out in twilight, in particular if observed at lambda>2.0mic. 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.

Airmass constraints (AO observations only)

In order for the AO-module to function well, the airmass constraint for AO OBs must not be > 1.9.

PWV constraints

The level of precipitable water vapour (PWV) can have an impact on the atmospheric transmission of near-IR light. Figure 10 of the User Manual shows the effect of PWV on different near-IR bands. The PWV has about a 5% to 15% impact on the total transmission depending on the filter band and PWV value. Please refer to Figure 10 and HAWK-I ETC for more details.

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