Appendix 1. Introduction to the 2MASS Extended Mission and Ancillary Data Products
4. Cautionary Notes
All 2MASS survey, 6x and calibration observations were made with the
same facilities using essentially the same observing mode, and the raw
imaging data acquired from these observations were all processed with
variants of the same software pipeline.
As a result, the reduced image and extracted source data from
all 2MASS observations share similar characteristics and limitations.
However, the 2MASS Extended
Mission Ancillary Products are fundamentally different by design
than the highly reliable and uniform All-Sky Release Catalogs and
Image Atlas. Because of these differences, the Extended Mission products
should not be used the same way as the All-Sky Release PSC and XSC.
Instead, the Ancillary Products should be used as a resource to learn
more about individual objects where detailed follow-up is possible, or
to conduct studies over small regions of the sky that do not require
the high degrees of uniformity and reliability that characterize the
Some of the key differences between the Extended Mission and All-Sky
Release products about which you should be aware are:
- The Survey Point and Extended Source Reject Tables
(PSRT and XSRT) and 6x and Calibration Point and Extended Source
Working Databases (PSWDB and XSWDB) have not been filtered to have
the same high degree of reliability as the All-Sky Release PSC and XSC.
Instead, they contain all extractions made during pipeline
data reduction which include valid detections of astronomical sources
as well as a large number of spurious extractions of faint noise
excursions, image artifacts and transient events. Therefore,
the Survey PSRT and XSRT and 6x and Calibration PSWDBs and XSWDBs
should not be used to conduct statistical studies of source
populations, for example.
Exceptions to this are the 6x-PSC and 6x-XSC
which are subsets of the full 6x-PSWDB and 6x-XSWDB drawn using similar
reliability standards as those used to construct the All-Sky Release
PSC and XSC.
In general, though, extra care should be taken to verify the authenticity of
source detections drawn from the Extended Mission source tables.
The Extended Mission Source Tables and Image Atlases do not share the
same uniformity of sky coverage
as the All-Sky Release Catalogs and Atlas. Whereas the All-Sky Release
Atlas and Catalogs contain only one unique detection of sources
observed more than once during survey operations, and images from only
one unique scan of a survey tile, the Extended Mission Source Tables and
Atlases may contain one or more independent detections of sources
and images in regions observed multiple times. The most extreme
example of this are the Calibration WDBs and Atlas in which there
may be anywhere from one to nearly 3700 independent measurements and
images of objects available in selected regions of the sky.
- Perhaps most importantly, the 2MASS Extended Mission products have
not received the same level of scrutiny and validation as the All-Sky
Release Atlas and Catalogs. Consequently, the general properties and
limitations of the Ancillary Products are not as thoroughly documented.
as those of the All-Sky Release products. The 2MASS Extended Mission
should be used with caution. Please familiarize yourself with the
description of each data set and product, and the Cautionary Notes
accompanying each before using them.
Because of the common heritage of the 2MASS Extended Mission
and All-Sky Release data products, users are strongly encouraged to
first review Section I.6 of the Explanatory
Supplement that contains detailed Cautionary Notes pertaining
to the 2MASS All-Sky Release products. Sections A1.4a-d that
follow below contain high-level summaries of key limitations and
features of the Survey Reject Tables and Full Image Atlas,
the 6x Working Databases, Catalogs and Image Atlas, and the
Calibration Working Databases and Image Atlas.
However, these are not complete descriptions of all limitations
and features for each
product, and they do not include summaries for the
Merged Source Information Tables
or the Combined Calibration Scan Images
and Source Lists. Please see the Appendices that describe
each of the Extended Mission data products for comprehensive listings
of the Cautionary Notes:
[Last Updated: 2006 December 20; by R. Cutri]
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