  The Chandra Source Catalog (CSC) is a virtual X-ray astrophysics facility
that enables both detailed individual source studies and statistical studies of
large samples of X-ray sources detected in ACIS and HRC-I imaging observations
obtained by the Chandra X-ray Observatory. The catalog provides
carefully-curated, high-quality, and uniformly calibrated and analyzed
tabulated positional, spatial, photometric, spectral, and temporal source
properties, as well as science-ready X-ray data products. The latter includes
multiple types of source- and field-based FITS format products that can be used
as a basis for further research, significantly simplifying followup analysis of
scientifically meaningful source samples. We discuss in detail the algorithms
used for the CSC Release 2 Series, including CSC 2.0, which includes 317,167
unique X-ray sources on the sky identified in observations released publicly
through the end of 2014, and CSC 2.1, which adds Chandra data released through
the end of 2021 and expands the catalog to 407,806 sources. Besides adding more
recent observations, the CSC Release 2 Series includes multiple algorithmic
enhancements that provide significant improvements over earlier releases. The
compact source sensitivity limit for most observations is ~5 photons over most
of the field of view, which is ~2x fainter than Release 1, achieved by
co-adding observations and using an optimized source detection approach. A
Bayesian X-ray aperture photometry code produces robust fluxes even in crowded
fields and for low count sources. The current release, CSC 2.1, is tied to the
Gaia-CRF3 astrometric reference frame for the best sky positions for catalog
sources.
