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 All-Sky Catalogs.

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:

Appendix 1. Introduction to the 2MASS Extended Mission and Ancillary Data Products

4. Cautionary Notes

A1.4.a. General Cautionary Notes for the Extended Mission Ancillary Data Products

A1.4.b. Cautionary Notes for the Extended Mission Point Source Reject Tables, Working Databases and 6x Catalog

A1.4.c. Cautionary Notes for the Extended Mission Extended Source Reject Tables, Working Databases and 6x Catalog

A1.4.d. Cautionary Notes for the Extended Mission Atlas Images