Photometry and astrometry with JWST -- I. NIRCam Point Spread Functions and the first JWST colour-magnitude diagrams of a globular cluster

D. Nardiello, L. R. Bedin, A. Burgasser, M. Salaris, S. Cassisi, M. Griggio, M. Scalco

Submitted on 14 September 2022


As the James Webb Space Telescope (JWST) has become fully operational, early-release data are now available to begin building the tools and calibrations for precision point-source photometry and astrometry in crowded cluster environments. Here, we present our independent reduction of NIRCam imaging of the metal-poor globular cluster M92, which were collected under Director's Discretionary Early Release Science programme ERS-1334. We derived empirical models of the Point Spread Function (PSF) for filters F090W, F150W, F277W, and F444W, and find that these PSFs: (i) are generally under-sampled (FWHM~2 pixel) in F150W and F444W and severely under-sampled (FWHM~1 pixel) in F090W and F277W; (ii) have significant variation across the field of view, up to ~15-20 %; and (iii) have temporal variations of ~3-4 % across multi-epoch exposures. We deployed our PSFs to determine the photometric precision of NIRCam for stars in the crowded, central regions of M92, measured to be at the ~0.01 mag level. We use these data to construct the first JWST colour-magnitude diagrams of a globular cluster. Employing existing stellar models, we find that the data reach almost the bottom of the M92 main sequence (~0.1 M), and reveal 24 white dwarf candidate members of M92 in the brightest portion of the white dwarf cooling sequence. The latter are confirmed through a cross-match with archival HST UV and optical data. We also detect the presence of multiple stellar populations along the low-mass main sequence of M92.


Comment: 14 pages, 11 figures, 2 tables. Accepted for publication in MNRAS on September 13, 2022. PSF models, catalogs and stacked images are publicly available at

Subjects: Astrophysics - Solar and Stellar Astrophysics; Astrophysics - Astrophysics of Galaxies; Astrophysics - Instrumentation and Methods for Astrophysics