J-PLUS: Turning Off the Bright Stars

TL;DR

This work tackles the extended PSF problem in wide-field imaging by building a per-exposure, non-parametric PSF for the J-PLUS DR3 survey. It decomposes the PSF into central, middle, and outer components derived from carefully selected stars, then merges them without fixed radii and projects the result to 2D for subtraction. The method yields a significant improvement in sky subtraction and recovers about $10\%$ more survey area, enabling studies of low-surface-brightness features near bright stars and around nearby galaxies. The approach is implemented in open-source software within the Maneage framework, with potential applicability to J-PAS and other deep, multi-filter surveys, and is designed to be fully reproducible.

Abstract

Photometric surveys require precise point spread function (PSF) characterization, as it varies across filters and is crucial for accurate photometry and low surface brightness (LSB) studies. However, the small PSF size provided by default pipelines suits only barely resolved objects, making it difficult to analyze regions near bright stars (rendering those regions unusable). These components are then combined to generate a final PSF for each exposure and filter, spanning 15 mag arcsec-2 in surface brightness and 4 arcmin in radius in the broad bands. In narrow-band filters, the J-PLUS PSF exhibits two rings, whereas in broad-band filters, only one ring is observed. Additionally, the position of the ring shifts with filter wavelength: as the filters become redder, the ring radius increases. We find that there is no significant variation in the extended PSF observed as a function of time (within 2.5h) or position in the field of view. The radial profile of NGC 4212 (which is close to a star) is also studied before/after PSF-subtraction. We developed a novel method to determine the central coordinates of saturated stars, and classify stars without using Gaia magnitudes. Additionally, mirror reflections are automatically detected and masked. Furthermore, in combining different stars and various components of the PSF, we avoided the use of a fixed radius by introducing a new method that does not depend on radial measurements. Accurate characterization of the extended PSF and its subtraction improves sky subtraction, increases the effective area of the survey by about 10%, and enables the study of extended large LSB features in wide area surveys like J-PLUS. Our pipeline is published as free software (GNU GPLv3) an can be customized to other surveys such as J-PAS, where its impact will be even greater due to its depth. This paper is fully reproducible and produced from Commit 4860c70.

Figures (12)

• Figure 1: Flow chart of extended the PSF construction for a J-PLUS tile. All images are scaled to the color-bar at the bottom. The top row shows the coadded image of three exposures which are shown in second row and the second row shows the three exposures that were used to build it. The third row shows the exposure(s) used for the various parts of the PSF in the second exposure of the second row: the single exposure itself for the center and middle parts and all exposures $\pm2.5$ hours before/after the exposure for the outer part. The marked green and red stars are used for creating central (Sect. \ref{['sec:createcenter']}) and middle parts (Sect. \ref{['sec:createmiddle']}), respectively. The stars under pink crosses (on the right) are used to create the outer PSF (Sect. \ref{['sec:constructout']}). The fourth row presents a selection of the star stamps, with other sources masked, while the fifth row displays the coadded stamps. These are subsequently combined to construct the "Unified PSF", shown in the final row. Its 1D radial profile up to $9.17$ arcmin (in radius) is then projected into the circular "Projected PSF". The surface brightness of the united PSF (from the central pixel to outer) changes about $15$ mag arcsec$^{-2}$ (See Sect. \ref{['sec:finalpsf']}).
• Figure 2: Radial profiles of the coadds used for different PSF components (fifth row of Fig. \ref{['fig:flowchart']}). The blue, green, and red profiles correspond to the central, middle, and outer regions, covering radii of $0.46$, $0.92$ and $9.17$ arcmin (in radius), respectively. Vertical lines indicate the points where these regions are joined.
• Figure 3: Finding the center of saturated stars from the bleeding pixels (black in bottom-left panel). After collapsing the pixels along X and Y axis, the peak of the distribution shows the center coordinate of the saturated star.
• Figure 4: Sorting the brightest stars in an image by magnitude using flux at a certain radius. Colored radial profiles correspond to stars in the top of the image (same same color border). Stars are sorted based on flux within the vertical gray region (which is just beyond the saturated radius of the brightest star).
• Figure 5: Flowchart of identifying and masking internal reflections (ghosts). The star's image (panel a) is divided into quadrants by azimuthal angle (0-90,90-180,180-270,270-360 degrees). The radial profile of different angles are found (in blue, red, green, and purple) and shown in panel b. The quadrant which has less signal-to-noise ratio (in this case the violet one) is chosen to create a 2D circular projection of that profile (panel c). The 2D projection is subtracted from the input, highlighting all problematic signal (including ghosts and wings of fainter stars) in panel d. We detect all of those low signal-to-noise with NoiseChisel in panel e and then mask them in panel f. The pixels that belong to spikes are re-inserted, as described in the text.