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.

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 calibrations are generically considered as normal OBs (to be prepared under the OBsBlock tab in P2PP), 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 calibrations should be prepared under the CalBlock tab in P2PP, 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

You are requested to submit a calibration star OB for each science target OB. This pair of science target/calibration star OBs will be executed in a row in order to monitor the atmospheric and instrumental transfer function. To select calibrator stars, it is recommended to use the CalVin tool developed by ESO. CalVin selects suitable calibrators from a fixed list based on different user criteria. The underlying list of calibrators is based for MIDI on the list provided by the MIDI consortium (see Stecklum et al. VLTI calibrators Workshop) and for AMBER on the catalogue by Merand et al. CalVin diameters represent either measured angular diameters, or estimates based on fitting photometric data to stellar atmosphere models.

The selection of the calibration star should be based on a reasonable compromise between different characteristics such as brightness (not much fainter than the target), distance on sky (close to the target), diameter (unresolved or at least clearly smaller than the target), and maybe spectral type or other characteristics (e.g. suitability as spectrophotometric standard) which are important for the respective observing mode.

VLTI observations are normally performed as pairs of science and calibrator OBs. The LST range of the calibrator OB of the pair must begin 30 minutes before the start of the LST range of the corresponding science OB and end 30 minutes after the end of the LST range of the corresponding science OB. In case that a single calibration star cannot be used for the full LST range of the science target, it is possible to use more than one calibration star OB to cover the required LST range. Only one of these calibration star OBs will be executed depending on the time of the observation of the science target. This ensures that a calibrator OB is available at any time when the science target observation may be attempted. Paranal Science Operations reserves the right to exchange a user submitted calibrator by another suitable calibrator from CalVin if there is a need to do so.

It is also possible to request additional calibration star OBs so that a sequence of [calibrator - science target - calibrator] is realized. OBs for the additional calibration star should also be submitted. In this case, the sequence must be described in the "Calibration Requirements" field of the OB and in the "Special Calibration" Section of the ReadMe file, as well as in the user comment field of the OB. The execution time for the additional calibrator is subtracted from the allocated time of the programme.

We recommend to select calibrators from CalVin so that more detailed knowledge on these calibration stars will be acquired rapidly. However, the user has as well the option to use a calibrator which is not in the CalVin database. In this case, the following information is requested in the README file (Section "Special calibration requirements"):

• Name
• Coordinates
• V magnitude
• N magnitude
• Spectral Type
• Uniform Disk (UD) diameter at N band with error
• Source of diameter estimate.

We remind the users that all calibration star data are public as soon as they are listed in the archive.

AMBER Calibration Plan

There are no issues in addition to the VLTI-specific section.