1. the extended source catalog is just that. the processor tried to identify all sources that were not well-fit by just a single psf. therefore the database from which the xsc was selected includes objects made out of multiple stars that are close together, such as double and triple stars; artifacts around bright stars due to non-flat background around such bright stars; meteor and plane streaks; as well as true extended objects.

THE EXTENDED SOURCE CATALOG IS NOT ONLY A GALAXY CATALOG.

2. we have attempted to reject objects that are not truly extended stars by the following:

- excluding regions around bright stars;
- attempting to identify meteor and plane streaks in the database;
- discriminating against objects made of multiple stars based on a few parameters measured for each source.

the algorithms we have used so far are not perfect, and SOME OF THESE OBJECTS REMAIN. in particular:

- 2mass cannot obtain accurate magnitude measures for very bright stars due to saturation, and hence the excluded region, which depends on magnitude, is incorrect for many of these very bright stars. in many cases, we will only be able to do an accurate job of identifying such sources when 2mass has completed surveying the entire sky, in order to identify bright sources from other scans which cause artifacts in a given scan.

- a prototype streak finder in the database relied heavily on finding multiple sources along a line from a single scan. streaks that produce few sources cannot be found with that algorithm and hence remain in the catalog. streaks in high source density areas are harder to identify, and hence more probably remain there.

- some combinations of multiple stars have absolutely identical parameters to those of true galaxies, and hence cannot be excluded based on any current 2mass measurement without also tossing true extended sources.

3. THE XSC IS REALLY TWO CATALOGS IN ONE. In order to get into the catalog, a source in the extended source database first had to pass the filters given in #2 above. then _only spatial extent parameters_ were used to select sources for entry into the database. the spatial extent parameters used to select sources are combined into an "E" flag by a decision tree algorithm, which attempted to separate true extended sources from sets of multiple stars.

every source in the xsc passed that decision tree "E" flag criteria. no guarantee is offered to the user as to how many of these sources are truly extended sources as opposed to multiple stars, artifacts, etc. the goal here was just completeness.

the second catalog within the xsc are "galaxy-like" sources. the decision tree used not only spatial extent parameters, but also source colors, to create a "G" flag for each source. source color is a very powerful tool in order to select galaxies from groups of stars. we attempted to meet stringent reliability standards, which vary as a function of source density, for sources with "G" <= 1.4, henceforth called "G" sources.

THEREFORE IT IS EXTREMELY IMPORTANT TO PAY ATTENTION TO THE "g" AND "e" FLAGS IN ANY ANALYSIS OF THE XSC.

4. the "G" sources attempted to meet the following reliability standards: 99% for source density < xx, 90% for yy < source density < xx, and no reliability standard at all for source density > yy.

the goal was to have a high reliability catalog outside the galactic plane. inside the galactic plane, the goal was shifted to completeness at the expense of reliability. HENCE THE USER SHOULD EXPECT TO FIND ARTIFACTS EVEN AMONG THE "g" SOURCES IN THE GALACTIC PLANE. thus it clearly is important to check the "density" parameter in order to know the characteristics of the portion of the catalog that you are analyzing.

spot checks of this first-release catalog show that we have come close to meeting our reliability goals, but probably have not quite achieved it. we will continue to refine our algorithms to try to fully meet those goals by the time of the final reprocessing for the entire 2mass dataset.