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 MIDI
MIDI observations are executed by using the MACAO (multi application curvature adaptive optics) systems at all UTs, and using the tip-tilt corrector system called STRAP at all ATs. With the successful use of MACAO at the UTs and STRAP at the ATs, the image quality in terms of FWHM and positional stability on the MIDI detector remains very good, virtually independent of the regular (optical) seeing value.
MACAO/STRAP guiding is mandatory in service mode observations. ESO assures that the MACAO systems are used at the UTs and the STRAP systems at the ATs, and that in both cases the adaptive optics loop is closed during the entire observation. Otherwise, the OB will be considered for re-execution.
Observations in the thermal IR regime may require clear or photometric conditions, depending on the spectroscopic quality required. However, MIDI observations can even be done with thin cirrus.
Sky transparency constraints are mostly relevant for MIDI in HIGH_SENS mode, where the photometry measurements are performed after the fringe exposure. Variability of water vapor quantity in THN condition may therefore affect the visibility accuracy obtained. In SCI_PHOT mode however (where fringe and photometry measurements are performed simultaneously), effect of THN condition should be strongly reduced. It is therefore recommended to set the constraint to THN in SCI_PHOT (to increase the probability to have the OB executed).
CLR sky conditions may also be advisable for Coude guide stars with aV magnitude close to the limit.
The feasible HA range for different configurations with MIDI is shown in the plots of the VLTI pointing restrictions in the declination/hour angle space.