General Observing Constraints

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Every requested observation has multiple observing constraints. The observing constraints are:

• the allowable brightest lunar phase (to be consistent with Phase 1 constraint)

• 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 (to be consistent with Phase 1 constraint according to turbulence category and airmass; it can be estimated through the ETC)

• the T category for SPHERE, MUSE and VLTI instruments that use full-AO. This combines probability of realization of seeing and coherence time.

• the allowable sky transparency (to be consistent with Phase 1 constraint)

• 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)

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The Observing Constraints are specified by the user at Phase 2 for each Observation Block. Since the execution conditions required by each program are an important ingredient in the process of building up the Long Term Schedule of an observing semester, and thus determine which programs 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 (Phase 2 golden rules). 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.

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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).