Exoplanets & Stellar Populations Group

Ground-based NIR photometry and astrometry

Ground-based astrometry with wide field imagers. V. Application to near-infrared detectors: HAWK-I@VLT/ESO
(Libralato et al., 2014 A&A, 563, 80)

High-precision astrometry requires accurate point-spread function modeling and accurate geometric-distortion corrections. Libralato et al. 2014 demonstrates that it is possible to achieve both requirements with data collected at the high acuity wide-field K-band imager (HAWK-I), a wide-field imager installed at the Nasmyth focus of UT4/VLT ESO 8 m telescope. Their final astrometric precision reaches ~3 mas per coordinate for a well-exposed star in a single image with a systematic error less than 0.1 mas. They constructed calibrated astro-photometric catalogs and atlases of seven fields: the Baade’s window, NGC 6656, NGC 6121, NGC 6822, NGC 6388, NGC 104, and the James Webb Space Telescope calibration field (in the Large Magellanic Cloud).
We make these catalogs and images electronically available to the community.

High-precision astrometry with VVV. I. An independent reduction pipeline for VIRCAM@VISTA
(Libralato et al., 2015, MNRAS, 450, 1664)

We present a new reduction pipeline for the VIRCAM@VISTA detector and describe the method developed to obtain high-precision astrometry with the VISTA Variables in the Vía Láctea (VVV) data set. We derive an accurate geometric-distortion correction using as calibration field the globular cluster NGC 5139, and showed that we are able to reach a relative astrometric precision of about 8 mas per coordinate per exposure for well-measured stars over a field of view of more than 1 deg². This geometric-distortion correction is made available to the community. As a test bed, we chose a field centred around the globular cluster NGC 6656 from the VVV archive and computed proper motions for the stars within. With 45 epochs spread over four years, we show that we are able to achieve a precision of 1.4 mas yr^-1 and to isolate each population observed in the field (cluster, Bulge and Disc) using proper motions. We used proper-motion-selected field stars to measure the motion difference between Galactic disc and bulge stars. Our proper-motion measurements are consistent with UCAC4 and PPMXL, though our errors are much smaller. Models have still difficulties in reproducing the observations in this highly reddened Galactic regions.
The Fortran routine code is available here.