Calibration Plan
Standard Calibrations
The most commonly used observing configurations of each instrument are calibrated by means of the Observatory's Calibration Plan. Calibration Plans for each instrument and mode are described in detail in their corresponding User Manual, and their execution times are not charged to the Service Mode programmes.
Additional Calibrations
Please check carefully in the User Manual whether or not the instrument configurations that you plan to use are included in the instrument's Calibration Plan. If this is not the case, you must provide additional OBs for calibration appropriate for each mode not included in the Calibration Plan. Their execution time is then charged to your time allocation.
User-supplied OBs for calibration are only executed once. You should assume that your science OBs will be executed completely independently of each other, possibly on different nights, and take into account that no user-supplied OB (for a science or a calibration target) will be executed more than once. You should submit enough OBs to cover that situation. For example, if you need to observe and flux-calibrate six targets in a filter that is not supported by the calibration plan of the instrument, you need to provide six OBs for photometric calibration to allow for the case in which each target is observed on a different night. This must be done even if the same calibration star can be used for all the science targets.
Please indicate in the User Comments fields of science OBs when a user-provided calibration must be executed contiguously.
'Concatenation containers' should be used within p2 to link science and calibration observations that must be executed contiguously.
Please note that there are two separate kinds of calibrations in operational terms:
- Calibrations that observe a reference celestial source, like a spectrophotometric standard star or a nearly-featureless star for the measurement of telluric lines. The OBs prepared for this kind of calibration are generically considered as normal OBs (to be prepared within an "OB" in p2), and their construction is the same as for other science OBs. However, depending of the instrument they use either templates whose name contains the indicator calib, or flags in the templates that indicate that the observation is intended for calibration.
- Calibrations with no celestial source associated, like flat fields taken with a flat field lamp or dark exposures. OBs for this kind of calibration should be prepared within the "CB" (Calibration Block) in p2, as no acquisition template needs to be specified in this case. Calibrations to be executed in daytime are the most common examples in this category.
VLTI Calibration Plan
VLTI OBs must be submitted as concatenations of calibrator star OBs (CAL) and science target OBs (SCI), except for astrometric or GRAVITY-wide observations with GRAVITY that may not require a calibrator.
The selection of the calibration star should be based on a reasonable compromise between different characteristics such as brightness (comparable to the target), distance on sky (close to the target), diameter (stable diameter known with a good precision, ideally unresolved for the baseline and wavelength), spectral type, or other characteristics (e.g. suitability as spectrophotometric standard). More information on the calibration strategy is provided in the user manuals of the instruments.
Please be reminded that all calibration star data are public as soon as they are listed in the archive.
The use of nested containers for VLTI observations is offered, that include groups of (SCI/CAL) concatenations or time links of (SCI/CAL) concatenations. In case of imaging observations, the use of groups of concatenations is mandatory to define the set of concatenation that contribute to the same image (the same uv plane).
GRAVITY Calibration Plan
GRAVITY observations are recommended with the standard calibration sequence of SCI-CAL with a total execution of 1h (30 min SCI OB followed by a 30 min CAL OB).
Deviations from this standard are now allowed within the 1h time limit of a concatenation.
GRAVITY-wide observations do not allow an absolute calibration, so that SCI stand-alone observations are recommended. Astrometric observations may also be conducted with SCI stand-alone OBs.
Longer sequences such as a CAL-SCI-CAL sequence can only be used after an approved waiver request.