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Abstract

Dual-field interferometric observations with VLTI/GRAVITY sometimes require
the use of a "binary calibrator", a binary star whose individual components
remain unresolved by the interferometer, with a separation between 400 and 2000
mas for observations with the Units Telescopes (UTs), or 1200 to 3000 mas for
the Auxiliary Telescopes (ATs). The separation vector also needs to be
predictable to within 10 mas for proper pointing of the instrument. Up until
now, no list of properly vetted calibrators was available for dual-field
observations with VLTI/GRAVITY on the UTs. Our objective is to compile such a
list, and make it available to the community. We identify a list of candidates
from the Washington Double Star (WDS) catalogue, all with appropriate
separations and brightness, scattered over the Southern sky. We observe them as
part of a dedicated calibration programme, and determine whether these objects
are true binaries (excluding higher multiplicities resolved interferometrically
but unseen by imaging), and extract measurements of the separation vectors. We
combine these new measurements with those available in the WDS to determine
updated orbital parameters for all our vetted calibrators. We compile a list of
13 vetted binary calibrators for observations with VLTI/GRAVITY on the UTs, and
provide orbital estimates and astrometric predictions for each of them. We show
that our list guarantees that there are always at least two binary calibrators
at airmass < 2 in the sky over the Paranal observatory, at any point in time.
Any Principal Investigator wishing to use the dual-field mode of VLTI/GRAVITY
with the UTs can now refer to this list to select an appropriate calibrator. We
encourage the use of "whereistheplanet" to predict the astrometry of these
calibrators, which seamlessly integrates with "p2Gravity" for VLTI/GRAVITY
dual-field observing material preparation.