I. Introduction

i. Origin and Quality of Photometry

Photometry of point sources is performed several different ways during 2MASS data processing, and the "default magnitudes" listed for each source in the PSC can have different origins depending on source brightness and environment. The source and quality of the "default magnitude" fields in the First Release PSC are summarized by the "rd_flg", "bl_flg" and "cc_flg." It is essential that users refer to these flags when interpreting photometry for any source in the Catalog. Each one of these flags has three characters, each corresponding to one band; the first character is the J-band value, the second is the H value, and the third is the K_s value.

Faint Stars

The majority of sources in the 2MASS PSC have default magnitudes obtained using profile-fit photometry performed simultaneously on the combination of all six individual 1.3-s "Read 2-Read 1" (R2-R1) exposures. These sources are indicated with a rd_flg value of "2" in the appropriate bands. Occasionally, the profile-fitting photometry routines will fail for sources in crowded environments, or that lie in regions with complex backgrounds. If a valid aperture photometry magnitude is available it will be listed in the default magnitude field in the appropriate band. However, such magnitudes are highly uncertain. These objects have a rd_flg value of "4" in the affected bands.

Bright Stars

Sources brighter than 7-8 magnitudes will saturate in the 1.3-s R2-R1 exposures. These objects have default magnitudes from aperture photometry performed on the 51-ms Read1 frames. Such sources have rd_flg=1 in the appropriate band.

Saturated Read1

Stars brighter than approximately 5th magnitude will saturate in even the 51-ms exposures. The default magnitude assigned for these objects is -99.999, and the rd_flg value for the appropriate band is "3".

Bright Star Filler Entries

The very brightest stars in the near-infrared sky saturate the detectors so heavily that they may not be unambiguously detected during processing. Placeholders have been included in the 2MASS First Release PSC to account for these very bright stars. The default magnitudes for these objects is always -99.999, and the rd_flg value is "8" in all three bands.

The following table summarizes the rd_flg values and their significance with respect to the origin of the "default magnitudes" in the PSC records. The "rd_flg" is a 3-character string, where the first character refers to the J-band, the second to H and the third to K_s.

Blended Sources

The blend flag (bl_flg) included with each point source record indicates the number of components fit simultaneously in profile fit photometry (rd_flg="2"). This provides a measure of source density and possible confusion

Bright Star Artifacts

• Diffraction Spikes

  Diffraction spikes from stars can trigger false detections or alter the flux for a real source. For the brightest stars, sources along the diffraction spikes have been deleted from the PSC. Surviving sources which are possibly contaminated by diffraction spikes have a "D" entry in their confusion flag (cc_flg).




• Scattered Light

  Light scattered within the telescope and camera optics can trigger false detections in the vicinity of extremely bright stars. For the First Incremental Release catalog sources lying within a magnitude-dependent circular regions around bright stars have been removed from the catalog if they are below a specified brightness threshold relative to the bright star. Objects which remain in the PSC but which are close to bright stars have a cc_flg value of "C".




• Latent Images

  NICMOS3 detector material produces a latent image after exposure to a bright source which fades over a timescale of order 10 seconds. This persistence effect produces a trail of spurious sources in the wake of a bright star at exactly the 2MASS frame spacing. Since the position of these artifacts are well known they are flagged in the catalogs. Since a real source could lie under one of these afterimages, persistence is flagged with a "probability of persistence" and the catalog has been selected to include only sources with <50% persistence probability. Sources with persistence probability between 10%-50% are indicated by a "P" value in the cc_flg in the appropriate band.

The CC_FLG also encodes several other conditions that indicate challenges to point source photometry, such as confusion during bandmerging, and during the duplicate source rectification procedure. The following table summarizes the possible values in the CC_FLG.

Several measures of photometric uncertainty are provided for each 2MASS "default magnitude". The first, "msig" is the pure measurement error returned by the respective photometry algorithm. "Msigcom" is the root-sum-square combination of "msig" with the uncertainty in the nightly photometric zero-point offset, the estimated flat-fielding residual (0.005 mags), and the R1 photometric normalization uncertainty, for bright sources. Finally, the scan signal-to-noise ratio, "snr", is the ratio of the source brightness to a characteristic "point source noise" derived from the mode of the distributions of "msig" for all sources in a tile.

The point source extraction threshold algorithm that responds to confusion noise in high source density regions also responded to elevated noise due to low frequency structure in the image backgrounds due to OH airglow. This results in an effective loss in sensitivity in the H-band relative to J and K_s in severe airglow conditions, leading to a number of J and K_s detected sources.

2MASS positions are tied to the Hipparcos Tycho reference frame on a tile-by-tile basis. Typically there are an abundant number of Tycho stars in a tile and the astrometric solution yields positions accurate to <0.2" for bright sources. A small fraction of tiles contain few Tycho stars and the positional solution may random walk to an amplitude of as much as 1". The astrometric uncertainties account for this random walk.

There is also known to be a ~0.09" bias in the reconstructed declinations of the bright Read1-only-measured stars relative to fainter stars. No measurable bias exists in right ascension.

[Last Updated: 1999 May 5; by R. Cutri and M. Skrutskie]