This 2020README.txt file contains minimal information to help users access the CatWISE2020 Catalog, and was written on 2020 May 4. The web page https://catwise.github.io contains the latest updates to CatWISE information. The CatWISE2020 Catalog is available via the Infrared Science Archive (https://irsa.ipac.caltech.edu) in the WISE/NEOWISE Enhanced and Contributed Products area, and at https://portal.nersc.gov/project/cosmo/data/CatWISE/2020) The CatWISE2020 Catalog consists of 1,890,715,640 sources over the entire sky selected from WISE and NEOWISE survey data at 3.4 and 4.6 microns (W1 and W2) collected from 2010 Jan. 7 to 2018 Dec. 13. This dataset includes six times as many exposures and spans over sixteen times as large a time baseline as the AllWISE catalog. CatWISE adapts AllWISE software to measure the sources in co-added images created from six-month subsets of these data, each representing one coverage of the inertial sky, or epoch. CatWISE2020 includes the measured motion of sources over the 8 year span of the data, typically in 12 epochs. The scatter with respect to Spitzer photometry at faint magnitudes in fields out of the Galactic plane and at low ecliptic latitude (corresponding to lower WISE coverage depth) is similar to the CatWISE Preliminary Catalog, reaching 0.217 mag (equivalent to SNR 5) at approximately W1=17.7 and W2=16.5, vs. W1=16.9 and W2=15.9 for AllWISE. The 90% completeness depth for CatWISE2020 is at roughly W1=17.7 and W2=17.5, about 1.7 mag deeper than in the Preliminary Catalog. From comparison to Gaia, the motions are over a dozen times more accurate than those from AllWISE. The primary differences between the CatWISE2020 and CatWISE Preliminary Catalogs are: 1) The detection list for CatWISE2020 uses an updated version of the unWISE Catalog (Schlafly et al., ApJS, 240, 30, 2019) available at https://faun.rc.fas.harvard.edu/unwise/neo5/band-merged The unWISE Catalog was generated using "crowdsource" software, while the Preliminary Catalog used the "MDET" software (the detection software used for AllWISE; Marsh and Jarrett, PASA, 29, 269, 2012) to generate the detection list. The crowdsource software is much better at recovering source detections in high-density regions such as the Galactic plane. The identification number for the source detection (unwise_objid) is included in the CatWISE2020 output. 2) The CatWISE2020 Catalog is based on data obtained in the W1 and W2 bands with WISE and NEOWISE through December 2018, adding two years to the data sets used for the Preliminary Catalog source measurements. This results in more accurate motion measurements. These two factors result in roughly twice as many sources in CatWISE2020. 3) While both catalogs use the WPHOT point spread function fitting software to measure positions, motions, and photometry for detected sources, no attempt was made to improve the fit by adding new sources to the detection list in CatWISE2020. For the Preliminary Catalog, up to 3 iterations of adding sources were made. 4) Both catalogs measured photometry separately in data from survey scans with ascending ecliptic latitude vs. descending latitude, but the final aperture values are the result of averaging fluxes in the CatWISE2020 Catalog, while in the Preliminary Catalog, the aperture magnitudes were averaged. 5) A more accurate algorithm was used for calculating Galactic coordinates in the CatWISE2020 Catalog, so these coordinates are provided in the IRSA release, while they are hidden in the IRSA release for the Preliminary Catalog. 6) Coordinates in the CatWISE2020 Catalog are in the ICRS system at epoch MJD=57170, while in the Preliminary Catalog they are in J2000 at epoch MJD=56700. The CatWISE2020 Catalog on NERSC is separated into 18,240 gzipped ascii files in IPAC table format, one for each of the same number of spatially overlapping tiles, using the same equatorial coordinate grid as for WISE and unWISE. The 52 tiles nearest the ecliptic poles used slightly different processing, and their file names contain the string "opt0_2020" and do not have an underscore ("_") separating the date and time stamp in the name. All other file names contain the string "opt1_2019" and have an underscore separating the date and time stamp in the name. Two of the 52 tiles with "opt0" processing in the CatWISE2020 Catalog (0816m667 and 2597p666) were processed with "opt1" in the Preliminary Catalog. All file names begin with an 8 character string of form: rrrrsddd where rrrr is the decimal right ascension and sddd is the signed declination of the tile center to 0.1 degrees, in J2000, with s being "p" for positive declination or "m" for negative declination. The WISE tile lookup service at https://irsa.ipac.caltech.edu/applications/WISETiles/ provides a useful tool for identifying the tile(s) containing a coordinate. For example, the brown dwarf Gliese 570D, at RA 14h57m16.2s, or 224.3175 in decimal form, and declination -21d22m16s, will be found in the file with the prefix 2243m213. The files are organized on NERSC into 359 directories, one for each decimal degree of right ascension from 0 to 358 (there are no tiles beginning with 359). For example the file containing Gliese 570D is in https://portal.nersc.gov/project/cosmo/data/CatWISE/2020/224. Each catalog file contains typically 80,000 rows, one per source, with information similar to that in the AllWISE catalog (http://wise2.ipac.caltech.edu/docs/release/allwise/expsup/index.html). On NERSC, there are 187 formatted columns of information about each source, with the column format and description specified in the file 2020cwcat.sis20200318.txt at https://portal.nersc.gov/project/cosmo/data/CatWISE/. On IRSA, two columns (w1fitr and w2fitr) are hidden as they do not contain useful information. Clicking on a column name in the query form on IRSA provides the column format and description. Additional information about most of the columns is available via the AllWISE Explanatory Supplement. CatWISE2020 Catalog sources are required to: 1) be "primary" in their tile (i.e. be from the tile where that source is furthest from the tile edge) and 2) have W1 SNR >= 5 with no identified artifacts (a value of 0 for the "ab_flags" in the W1 part (left character) of column 178) or have W2 SNR >= 5 with no identified artifacts (a value of 0 for the "ab_flags" in the W2 part (right character) of column 178). Sources that do not meet these criteria go into the reject file for their tile, so there are also 18,240 gzipped ascii reject files in IPAC table format, in the same decimal RA directory as their corresponding catalog file. Reject files typically contain 11,000 sources, although near the Galactic center this can reach over 120,000 due to the large number of artifacts. Reject files have one additional column (for a total of 188) indicating if the source is "primary" in its tile. The column format and description are specified in the file 2020cwrej.sis20200318.txt on NERSC at the same location as the catalog file specification. Catalog and reject file names for a given tile generally differ only in having the letters "cat" or "rej" as appropriate. Additional information about the "primary" designation and the artifact flags is given in Eisenhardt et al., ApJS 247, 69, 2020.