VI. Analysis of the Release Catalogs
1. Comparison of Achieved Performance of the All-Sky Release Catalogs
with Level 1 Science Specification
The 2MASS project has prepared an All-Sky Release, very similar in
characteristics to previous Incremental Releases. This Release
increases the integrated sky coverage in the public domain from 47% to
virtually 100%, with improved processing of all sources. The All-Sky
Release data products include a Point Source Catalog containing
positions and brightness information
(see II.2a)
for over 470 million objects, an Extended Source
Catalog containing positions, magnitudes and basic shape information (see II.3a)
for nearly 1.6 million resolved sources, and an Image Atlas containing
nearly 5 million J, H and Ks images covering the survey area.
In constructing this document, the 2MASS team has drawn upon
and summarized both internal and Explanatory Supplement documentation.
To provide the most complete information, many of the links
here point to the original detailed analyses. In all cases, however,
the 2MASS team has arranged this page so that all primary
information is available in summary in the first level below this front page.
The 2MASS All-Sky Release includes improved
processing of data from the area contained in the Incremental
Data Releases, and is intended to supercede these earlier releases. This
summary outlines the major improvements in
the final data processing pipeline.
The 47% of the sky delivered in the Second Incremental Release has been in the
public domain since 2000 March and has led to hundreds of
publications -- with
most now coming from outside the 2MASS collaboration. This rigorous
practical testing of the preliminary product has yielded only minor comments on the data's shortcomings. All evidence
points to the All-Sky Release being of better quality than its predecessor.
For reference, this
link
documents the reports associated with the development and validation of the final pipeline.
The release
preview page
supplies the details of sky coverage and source counts.
This Explanatory Supplement is available as web pages, as
was the case for the previous Releases. Section I.6
is of particular interest. It contains a compilation of caveats that will
be of interest to users of 2MASS data products. The relevant subsections
are:
Below is a summary of science team analysis of the release
characteristics in the context of the Survey's Level 1 specifications.
2MASS photometry has a dynamic range of >20
magnitudes, owing to extraction software that addressed three
different exposure regimes. The Survey's nominal frame exposure time
was 1.3 sec, obtained by differencing two reads of the NICMOS3
array separated by that time interval. Both reads were independently
recorded and saved for future processing. The first read (often
referred to as "Read_1" throughout the Supplement) occurred 51 msec
after array reset and represents an effective 51 msec exposure
image. The second read ("Read_2") occurred 1.351 sec after reset.
The point source database includes a rd_flg flag which
distinguishes between different techniques for populating the
"default magnitude" column for sources,
depending on their level of saturation (all XSC photometry derives
from the 1.3 sec Read_2-Read_1 difference). Since different
techniques can be used in different wavelength bands for a given
source, the rd_flg is a
three-character flag with an independent value for each band revealing
the source of the default magnitude (i.e., j_m,
h_m, or k_m) column in the Point Source release database.
- rd_flg=2: This photometry
derives from PSF profile-fitting applied to the 1.3 sec
integration time Read_2-Read_1 exposure difference and
represents the bulk of the default photometry in the 2MASS
Point Source release. Documentation in this Supplement may cite
either "Read_2-Read_1" or "Read_2" photometry in referring to
source extractions from the 1.3 sec exposures. Although
the rd_flg=2 default magnitude is based on the PSF-fitting
algorithm, aperture photometry is also provided in separate
columns, because aperture magnitudes can be
superior for bright stars and
in circumstances where the PSF fit is poor. Typically,
rd_flg=2 sources have magnitudes ranging from about 9
to the faint detection limit at around 16.
The saturation threshold is band-specific, with
Ks-band tending to saturate for sources nearly one-half to
one magnitude brighter than for the J and H bands.
- rd_flg=1: This photometry
derives from aperture photometry operating on the 51
msec image obtained immediately following array reset.
The primary objective was to extract reliable flux for
sources saturated in the 1.3 sec Read_2-Read_1 exposure.
Documentation in this Supplement may cite "Read_1" photometry in
referring to unsaturated source extractions from the 51
msec exposure. Magnitudes for "Read_1" extractions
typically range from magnitude 4 to 9.
- rd_flg=3: This photometry
derives from estimating source flux from the wings of a stellar
image completely saturated in even the 51 msec Read_1
exposure. 2MASS successfully photometered sources as bright as
magnitude -4 using this technique. In keeping with the terms
above, photometry extracted with this method is often referred
to as "Read_3" photometry, when, in reality, there was no third
readout of the system.
- The photometry of sources is carried out in each band (JHKs)
independently. A process called "band merging" combines the bands
into a single database row with common coordinates. The band
merging process may give inconsistent results when close pairs
are inconsistently deblended by the software in each band.
This sort of confused band merge is indicated by rd_flg=6.
Band merging can occasionally fail outright, in which case
two (or even three) distinct sources with rd_flg=1,2 or 3
in one or two bands lies spatially adjacent to another
source with rd_flg=1,2, or 3 in two or one band.
This Release includes
reliable sources fainter than the Level 1
Specification 10-
flux
limits (as was also the case for the Second Incremental Release).
In order to distinguish between "Catalog" sources and
the fainter content of the Release Point Source database, the 2MASS project
has created a new column/flag -- photometric quality, ph_qual.
This flag contains three characters, one for each band. The
ph_qual designation, as the name suggests, represents the
photometric uncertainty in the measurement -- with "A" representing
measurements with photometric uncertainty consistent with
the Level 1 Catalog specifications. "B","C","D", and "E"
represent successively poorer measurements. "Catalog" sources include
- Sources
receiving photometric quality of "A" (SNR>10 and <0.109)
in any one of the three wavelength bands.
- All "Read_1" sources (i.e., saturated in the 1.3 s
"Read_2-Read_1"
exposure) successfully extracted by the pipeline (ph_qual = A, B, C, D,
or E) and not identified as artifacts. Read_1 sources which do not receive
an "A" in any of the three ph_qual entries technically do not meet the
Level 1 requirements individually. The 2MASS Science Team concluded it was
more important to include these 14173 bright reliable sources at the "Catalog"
level for the sake of completeness.
- All "Read_3" sources which completely saturated even the 51 ms
"Read_1" exposure. The uncertainties for Read_3 extractions are substantially
larger than the Level 1 uncertainty requirements for bright sources, however
the Level 1 specifications place no requirements on these thoroughly saturated
sources. Once again, the Science Team opted to include these sources at the
"Catalog" level, since they were highly reliable and improve completeness at
the brightest magnitudes.
[Last Updated: 2003 March 21; by M. Skrutskie]
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