VLTI/AMBER Closure Phase Calibration Reference Data Set

Besides the visibility, the closure phase is one of fundamental observables provided by long-baseline interferometric instruments such as the AMBER instrument at the VLT Interferometer.  The closure phase is essential to measure asymmetries in the source brightness distribution or to reconstruct aperture synthesis images.

In order to use the closure phase information for scientific studies, the sign of the closure phase is of particular importance and defines the orientation of the source on the sky.  Since, for most applications, a 180-degree uncertainty on the source orientation has strong implications on the scientific interpretation, much attention should be given to a proper and unambiguous closure phase sign calibration.

In principle, the closure phase sign can be derived from conventions (see Memo by Le Bouquin & Merand). However, this requires very careful treatment of the sign of various quantities both in the data reduction and user data modeling software and offers no redundancies to independently confirm or cross-check the derived orientation in the final modeling results.

Therefore, for the development and application of  data reduction and user modeling software, we recommend to follow not only all conventions, but also to confirm the closure phase sign using a reference data set with well-defined properties.

For this task, we offer on this website a VLTI/AMBER data set, which was recorded on the well-studied binary system Theta1 Ori C ( Kraus et al. 2009).  The data was recorded on one VLTI UT triplet (UT1-UT3-UT4) and three AT triplets (A0-D0-H0, A0-K0-G1, D0-H0-G1) and shows both in visibility and closure phase the expected clear wavelength-differential signature resulting from the presence of the binary companion.  Below, we provide tar-archives containing the AMBER-raw data files or pre-reduced OIFITS-files, which can be used to determine or verify the closure phase calibration in any user software.

At the time of the AMBER observations, the Theta1 Ori C companion (C2) was located WEST of the near-infrared bright primary component (C1), with the brightness ratio of 1:3. Please consult Table 3 in the A&A paper mentioned above to find the precise binary separation and position angles at the time of the various AMBER observations.

The VLTI/AMBER raw data sets on Theta1 Ori C and the corresponding calibrators can be downloaded here:

Furthermore, we provide OI-FITS files which were extracted from the AMBER raw files using the amdlib 2.1 data reduction software. Please note that these OI-FITS files can NOT be used to calibrate closure phases derived with the amdlib 3 software, since the amdlib 3 release uses a different closure phase convention!

Please address any questions or comments to skraus(at)astro.ex.ac.uk