The Chandra Source Catalog (CSC) is the definitive catalog of X-ray sources detected by the Chandra X-ray Observatory. By combining Chandra's sub-arcsecond on-axis spatial resolution and low instrumental background with consistent data processing, the CSC delivers a wide variety of uniformly calibrated properties and science ready data products for detected sources over four decades of flux. The Chandra Source Catalog version 2.1 (CSC 2.1) was released on April 2nd 2024; the current minor release is version 2.1.1, updated on 2024 October 18. (Refer to the <a href="https://cxc.cfa.harvard.edu/csc/versions.html">version history</a> for details.) This release of the catalog includes measured properties for 407,806 unique compact and extended X-ray sources in the sky, allowing statistical analysis of large samples, as well as individual source studies in the "Master Sources" table, provided herein. The extracted properties are provided for 1,304,376 individual observation detections, identified in Chandra ACIS and HRC-I imaging observations released publicly through the end of 2021, at the <a href="https://cxc.harvard.edu/csc/about.html">Chandra X-ray Center</a>. CSC 2.1.1 includes -- as an "alpha" release -- photometric properties for 1,717 highly extended (> ~30") sources, together with surface brightness polygons for several contour levels. The sensitivity limit for compact sources in CSC 2.1.1 is ~5 net counts, achieved by using a two-stage approach that involves co-adding multiple observations of the same field prior to source detection, and then using an optimized source detection method. For each X-ray detection and source, the catalog provides a detailed set of more than 100 tabulated positional, spatial, photometric, spectral, and temporal properties (each with associated lower and upper confidence intervals and measured in multiple energy bands). The catalog Bayesian aperture photometry code produces robust photometric probability density functions (PDFs), even in crowded fields and for low count detections. Releases beyond 2 use Bayesian Blocks analysis to identify multiple observations of the same source that have similar photometric properties, and these are analyzed simultaneously to improve S/N. The energy bands used to derive many of the CSC properties are defined in Table 4 of the reference paper: ultrasoft (u: 0.2-0.5 keV), soft (s: 0.5-1.2 keV), medium (m: 1.2-2.0 keV), hard (h: 2.0-7.0) and broad (b: 0.5-7.0 keV) for the ACIS energy bands, and wide (w: 0.1-10.0 keV) for the HRC energy band. The energy bands are chosen to optimize the detectability of X-ray sources while simultaneously maximizing the discrimination between different spectral shapes on X-ray color-color diagrams. Numerous source-specific catalog properties are evaluated within defined apertures. The authors define the "PSF 90% ECF aperture" for each source to be the ellipse that encloses 90% of the total counts in a model PSF centered on the source position. Because the size of the PSF is energy-dependent, the dimensions of the PSF 90% ECF aperture vary with energy band. They define the "source region aperture" for each source to be equal to the corresponding 3-sigma source region ellipse included in the merged source list, scaled by a factor of 1.5. Like the PSF 90% ECF aperture, the source region aperture is also centered on the source position, but the dimensions of the aperture are independent of energy band. This database table was last updated by the HEASARC in March 2025 and is based on a download of the online version of the Chandra Source Catalog, v2.1.1, at the CXC using the CLI. Refer to <a href="http://cxc.harvard.edu/csc/">http://cxc.harvard.edu/csc/</a>; for details. This is a service provided by NASA HEASARC .