Ground-based astrometry with wide field imagers. V. Application to near-infrared detectors: HAWK-I@VLT/ESO

TL;DR

This paper demonstrates high-precision ground-based near-infrared astrometry using HAWK-I at VLT, achieving ~3 mas accuracy, constructing detailed catalogs, and validating methods with archival data and multiple stellar fields.

Contribution

It introduces a method for precise astrometry with wide-field NIR imagers, producing calibrated catalogs and demonstrating effective cluster membership separation over 8-year baselines.

Findings

Achieved ~3 mas per coordinate precision with systematic errors below 0.1 mas.

Created detailed astro-photometric catalogs for seven fields, including the LMC.

Validated approach by combining archival optical data with HAWK-I observations.

Abstract

High-precision astrometry requires accurate point-spread function modeling and accurate geometric-distortion corrections. This paper 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 8m telescope. Our 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. We 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. Furthermore, as a demonstration of the efficacy of our approach, we combined archival material taken with the optical wide-field imager at the MPI/ESO 2.2m with HAWK-I observations. We showed that we are able to achieve an excellent separation between cluster members and field objects for NGC 6656 and NGC 6121 with a time base-line of about 8 years. Using both HST and HAWK-I data, we also study the radial distribution of the SGB populations in NGC 6656 and conclude that the radial trend is flat within our uncertainty. We also provide membership probabilities for most of the stars in NGC 6656 and NGC 6121 catalogs and estimate membership for the published variable stars in these two fields.