IV. 2MASS Data Processing

4. Point Source Detection and Photometry

d. Very Bright Stars

Stars brighter than approximately magnitude=4 to 5 will saturate in even the 51-ms R1 exposures. The photometry algorithms are currently unable to evaluate useful magnitudes for these objects, so the default magnitudes are set to -99.999 in the Second Incremental Release PSC for any saturated bands. The corresponding "rd_flg" value for those bands is "3".

Very bright stars, 2-3 magnitudes brighter than the R1 saturation level, may not be unambiguously detected during the point source characterization. Placeholders for these very bright stars are included in the 2MASS Second Incremental Data Release PSC, with positions taken from external catalogs such as the Hipparcos, ACT, or PPM catalogs, when available. The decision to include a placeholder is based on photometric estimates for objects from the Catalog of Infrared Observations (Gezari, Pitts, & Schmitz 2000) and the Two Micron Sky Survey, or near-infrared magnitudes deduced from visual and/or IRAS 12 micron photometry and some knowledge of the stellar spectral type. These placeholders are almost certainly incomplete in the Galactic plane. Because many of the brightest near infrared stars are variables, and because of large uncertainties in the interpolated magnitudes for the placeholders, there are instances of a placeholder being in the PSC when 2MASS has not detected a bright star at the location.

The "placeholder" entries in the PSC have "rd_flg" values of "888". The positions of these entries should be used only with extreme caution, and are provided for informational purposes only. There are 18,122 very bright star placeholder entries in the 2MASS Second Incremental Release PSC. The brightest star in the release area is probably Ceti (Mira; 2MASSI J0219208-025841), with K~-4.5 mag.

[Last Updated: 2000 June 20; by R. Cutri & T. Evans]

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