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 (seeing)
- the allowable sky transparency
- for Adaptive Optics instruments (currently CRIRES, NACO and SINFONI), the Strehl ratio on the reference star.
- for VLTI instruments, the local sidereal time range and the availability of the desired baseline
The Observing Constraints are specified by the user at Phase II 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 II constraints that are more strict than those specified in the original proposal. Users can however relax the constraints during the submission of their Phase II material. The values in the OB constraint sets that are selected (and approved) during Phase II 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, 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.
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 sensitivity estimates obtained from standard star observations during the observing night.
Optical seeing will be used to classify the seeing constraint requested in the observing proposal for a specific OB. The resulting image quality in the mid-infrared image is better than the optical seeing. As a rule of thumb, optical seeing between 0.6-0.8" will result in diffraction limited images in the N and Q band. A measured image quality of 0.3"(~10μm) and 0.6"(~20μm) will always satisfy the seeing constraint.
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 to be fully within constraints, regarding sensitivity, if an observation of a standard star at zenith in the same filter as the service OB results in a equal or better limiting flux as published ("grade A"). If the limiting flux obtained with standard stars exceeds less then 10% these published sensitivities in the same filter, 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.
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 <30deg.