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, 'seeing')
- the allowable sky transparency
- for Adaptive Optics instruments (currently CRIRES, NACO and SINFONI), the Strehl ratio on the reference star.
- for instruments observing in 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
- the allowable absolute time window (i.e. for time critical events, multi-epoch monitoring)
- the allowable local sidereal time range (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.
General Classification Rules
Quality Control of OBs executed in Service Mode will be based on the user's specified constraints for airmass, atmospheric transparency, seeing (i.e. image quality), moon constraints, as well as Strehl ratio for Adaptive Optics mode observations.
Note: the seeing 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), i.e. at the observed wavelength, for most of the VLT instruments (i.e. it is the image quality).
Additional Observing Constraints and Classification Rules for ISAAC
Do not overspecify the Moon constraints! The Moon does not directly affect infrared observations. For most ISAAC 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, degrading the sensitivity of your observations. 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 Observing Block being executed.