The first version of the Swift UVOT Serendipitous Source Catalog (UVOTSSC) provides positions and magnitudes, as well as errors and upper limits of confirmed sources, for observations taken from the start of operations in 2005 until October 1st of 2010. The first version of the UVOTSSC has been produced by processing the image data obtained from the Swift Ultraviolet and Optical Telescope (UVOT). The data processing was performed at the Mullard Space Science Laboratory (MSSL, University College London, U.K.) using Swift FTOOLS from NASA's High Energy Astrophysics Software package (HEASoft-6.11), with some customizing of the UVOT packages in order to get more complete source detection and to properly apply quality flags to those sources that were detected within the UVOT image artifacts. The total number of observations with 17'x17' images used for version 1 of the catalog is 23,059, giving 6,200,016 sources in total, of which 2,027,265 have multiple entries in the source table because they have been detected in more than one observation. Some sources were only observed in one filter. The total number of entries in the source table is 13,860,568. The S/N ratio for all sources exceeds 5 in at least one UVOT filter, the rest of the filters having a S/N greater than 3. One Swift ObsID can consist of one or more images, which for this catalog have been summed, yielding the quoted total exposure times. The original UVOT images can be found in the on-line archives at MAST, and in the Swift archives at <a href="http://swift.ac.uk/">http://swift.ac.uk/</a> and at the HEASARC (<a href="http://heasarc.gsfc.nasa.gov/W3Browse/all/swiftmastr.html">http://heasarc.gsfc.nasa.gov/W3Browse/all/swiftmastr.html</a>), using the ObsID as the search key. For higher temporal resolution, the original images need to be used because the catalog data herein are summed over all of the individual images within an ObsID. The upper limits per filter for the summed images are constructed for each ObsID because the sensitivity hardly varies over the detector. Usually the images within one ObsID share the same pointing, however, whereas the quoted upper limits always apply for sources near the pointing direction given, if the images had small offsets in pointing they may not apply to sources near the edge of the summed image, which is typically about 8 arcminutes from the quoted pointing direction. U, B, V, UVW2, UVM2 and UVW1 refer to the filter bandpasses defined in the UVOT Filterwheel section of the MSSL documentation at <a href="http://www.mssl.ucl.ac.uk/www_astro/uvot/uvot_instrument/filterwheel/filterwheel.html">http://www.mssl.ucl.ac.uk/www_astro/uvot/uvot_instrument/filterwheel/filterwheel.html</a>. This HEASARC table contains version 1.1 of the Swift UVOT table of observations in which the sources in the source table were detected and contains the details of 23,059 Swift UVOT observations. The HEASARC has changed the names of many of the parameters from those given in the original table. In such cases, we have listed the original names in parentheses at the end of the parameter descriptions given below. There is a related table which lists the 13,860,568 source detections that is available at the HEASARC as the SWUVOTSSC table. This table was created by the HEASARC in May 2017 based upon the <a href="https://cdsarc.cds.unistra.fr/ftp/cats/II/339">CDS Catalog II/339</a> file summary.dat. This is a service provided by NASA HEASARC .
