The very brightest stars in the near-infrared sky saturate the 2MASS detectors so heavily that they may not be unambiguously detected during scan processing (cf. IV.7). These objects will therefore not naturally be passed into the 2MASS PSC. However, users should be aware of their presence because of their effect on surrounding regions. Therefore, 18,122 very bright star "placeholders" for stars brighter than approximately magnitude 5 in all three 2MASS bands have been added to the Catalog.
There is no existing all-sky catalog of very bright near-infrared sources. A compilation of very bright stars was made to assist with artifact identification. This list was built by combining actual bright star measurements from the Two Micron Sky Survey and the Catalog of Infrared Observations (Gezari, Pitts, & Schmitz 2000), with estimated J, H and Ks magnitudes for other bright stars using the PPM, IRAS and MSX source catalogs as "seeds." The brightest of these sources were used as the "placeholders" for the PSC. Because the preliminary processing of 2MASS data provides no quantitative brightness information for these stars, the PSC entries for the "placeholder" include only approximate positions and 2MASS coordinate designations. They are flagged in the PSC as having rd_flg="888".
The 2MASS extended source pipeline processing attempts to detect and characterize galaxies as large as ~5´ in diameter. A "seed" catalog of known galaxies with optical diameters larger than 1´ (as given in NED) is used as a part of the extended source processing. Known galaxies for which more than 75% of the object is fully contained within one Atlas Image are processed accordingly. Objects which are either too large, or too close to the edge of an Image are not processed. As with the bright stars, it is desirable that users be aware of the presence of such large galaxies. Therefore, "placeholders" for the 37 large galaxies in the area of the 2MASS Second Incremental Release that were not processed have been added to the Catalog. The only information provided for these objects is position, usually taken from NED, the equivalent 2MASS coordinate designation, and the source identification.
See the Table of Large Galaxy Placeholders in the XSC.
[Last Update: 14 February 2000, by R. Cutri]