[Paper Review] The SDSS Coadd: 275 deg^2 of Deep SDSS Imaging on Stripe 82

J. Annis, M. Soares-Santos+18 more|arXiv (Cornell University)|Nov 28, 2011

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy56 references107 citations

TL;DR

This paper presents the construction and characterization of a deep coadded imaging survey of 275 deg² in Stripe 82 using repeated SDSS observations, achieving a depth of r ≈ 23.5 for galaxies and r ≈ 24.3 for stars. By coadding 20+ individual scans with S/N-weighted stacking, PSF modeling, and photometric calibration, the resulting catalog provides a high-fidelity, homogeneous photometric dataset with 1.1″ median r-band seeing and photometric precision of 0.5% in g, r, i and 1% in u, z, enabling precision photometry and cosmological studies.

ABSTRACT

We present details of the construction and characterization of the coaddition of the Sloan Digital Sky Survey Stripe 82 \ugriz\ imaging data. This survey consists of 275 deg$^2$ of repeated scanning by the SDSS camera of $2.5\arcdeg$ of $δ$ over $-50\arcdeg \le α\le 60\arcdeg$ centered on the Celestial Equator. Each piece of sky has $\sim 20$ runs contributing and thus reaches $\sim2$ magnitudes fainter than the SDSS single pass data, i.e. to $r\sim 23.5$ for galaxies. We discuss the image processing of the coaddition, the modeling of the PSF, the calibration, and the production of standard SDSS catalogs. The data have $r$-band median seeing of 1.1\arcsec, and are calibrated to $\le 1%$. Star color-color, number counts, and psf size vs modelled size plots show the modelling of the PSF is good enough for precision 5-band photometry. Structure in the psf-model vs magnitude plot show minor psf mis-modelling that leads to a region where stars are being mis-classified as galaxies, and this is verified using VVDS spectroscopy. As this is a wide area deep survey there are a variety of uses for the data, including galactic structure, photometric redshift computation, cluster finding and cross wavelength measurements, weak lensing cluster mass calibrations, and cosmic shear measurements.

Motivation & Objective

• To produce a deep, coadded imaging survey of Stripe 82 using repeated SDSS observations to extend photometric depth beyond single-pass SDSS data.
• To develop and validate a robust image coaddition pipeline that preserves photometric accuracy and astrometric fidelity across multiple epochs.
• To calibrate the coadded data to better than 1% photometric precision and model the PSF accurately for precision 5-band photometry.
• To identify and mitigate systematic issues in star-galaxy classification arising from PSF modeling imperfections.
• To produce a publicly accessible, homogeneous, high-fidelity photometric catalog for use in cosmology, galaxy evolution, and weak lensing studies.

Proposed method

• Coadded 20+ individual SDSS imaging runs in ugriz filters using S/N-weighted stacking, incorporating inverse variance maps and seeing corrections.
• Applied a relative photometric calibration scheme to align calibrated and uncalibrated data across different epochs.
• Modelled the PSF by coadding individual PSFs from each exposure, enabling accurate photometry and shape measurements.
• Used the SDSS PHOTO pipeline to produce source catalogs from coadded images, including photometry, astrometry, and morphological parameters.
• Validated photometric accuracy using a high-quality star catalog (Ivezić et al. 2007) and cross-checked with color-color diagrams and number counts.
• Identified systematic misclassification regions via PSF-model vs magnitude plots and verified with VVDS spectroscopy.

Experimental results

Research questions

• RQ1 How can repeated SDSS imaging in Stripe 82 be coadded to achieve deeper photometric depth while preserving photometric and astrometric accuracy?
• RQ2 To what extent does PSF modeling in the coadd process support precision 5-band photometry for stellar and galactic sources?
• RQ3 What systematic errors in star-galaxy classification arise from PSF mis-modeling, and how can they be detected and corrected?
• RQ4 How well do the coadded data match expected galactic structure models in terms of star number counts and color distributions?
• RQ5 What is the photometric precision and depth of the final coadded catalog, and how does it compare to single-pass SDSS data?

Key findings

• The coadded imaging reaches a 50% completeness limit of r ≈ 23.5 for galaxies and r ≈ 24.3 for stars, representing a 2-magnitude improvement over single-pass SDSS.
• The r-band median seeing is 1.1″, improved from 1.4″ in single-pass data, with band-to-band seeing scaling consistent with Kolmogorov turbulence models.
• Photometric precision is 0.5% in g, r, i and 1% in u, z, as validated against the Ivezić et al. (2007) star catalog.
• Color-color diagrams of stars show a sharp, narrow stellar locus, confirming high-fidelity photometry and accurate PSF modeling.
• Minor PSF mis-modeling leads to a region in PSF-model vs magnitude space where stars are misclassified as galaxies, confirmed via VVDS spectroscopy.
• The final catalog contains 13 million galaxies and is the largest homogeneous precision photometric catalog complete to r=23.5, publicly available through SDSS DR7.