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 CRIRES
Moon constraints:
Moon does not affect observations obtained by CRIRES. However, as the wavefront sensor is sensitive in the R-band, restricted moon constraints are only meaningful for AO observations with a natural guide star (NGS) fainter than R = 15 mag. 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 Observation Block to be executed.
Note, however, that telescope guiding and active optics can, under certain circumstances, be compromised for moon distances < 30deg.
Seeing: FWHM and on-axis Strehl ratios for adaptive optics observations using a Natural Guide Star:
For closed loop AO observations of point sources, the users must specify in the constraint set the desired on-axis Strehl ratio, i.e. the ratio of the maximal intensity of the PSF to the maximal intensity expected for the theoretical PSF with no turbulence. This is the main parameter regarding atmospheric turbulence. It should be set for the K band; this value is produced by the CRIRES ETC.
In addition, all closed loop AO observations should enter also the FWHM of the seeing, although this constraint is less relevant. The constraint on the seeing refers to the seeing measured by the DIMM or by the guide probe, as they are the most relevant for the MACAO system. It is used as a guide, together with the coherence time, in order to estimate the achievable Strehl ratio.
If the AO star is not the science target, it is worth noting that the isoplanetic angle depends on (airmass)-8/5, and on the wavelength as λ6/5. On the other hand, the Strehl ratio, SR, decreases with the angular distance to the AO star θ as SR ∝ exp(-(θ/θ0)5/3).
Seeing: FWHM for seeing limited observations, no adaptive optics (NoAO):
For NoAO observations, only the seeing at the wavelength of observations is relevant. The user should therefore specify the FWHM for the respective bands of the science observations. The constraints set and definitions are the same as for all other seeing limited VLT instruments. The input field for the Strehl ratio should remain unchanged.
The classification of each observation will be based either (a) on the measured spatial profile of the spectrum if the object is a point source, or (b) on FWHM of a star seen on the Slit Viewer if there are more stars available, or (c) on the seeing measured on the guide probe otherwise.
Atmospheric transparency
Photometric conditions (PHO) should be asked only for spectrophotometry. Observations in AO mode with an AO star fainter than about R = 15 should preferably require CLR conditions. For all other cases, thin clouds conditions (THN) usually suffice.
Water vapour
Observations requiring relatively low values of water vapour in the atmosphere should specify this in the README file. The CRIRES ETC can be used to simulate the influence of different precipitable water vapour (PWV) values.