I. Introduction
8. Referencing 2MASS
a. 2MASS Source Naming Convention
Source designations for objects in 2MASS Catalogs conform to the recommendations outlined in International Astronomical Union's Specifications concerning designations for astronomical radiation sources outside the solar system. Source designations should be given as:
2MASxy Jhhmmss[.]ss±ddmmss[.]s
The brackets and periods are not explicitly in the name, and are shown above only to illustrate that the last digits before the declination sign are tenths and hundredths of RA seconds and the final digit in the designation is in tenths of DEC arcseconds. The decimal parts are truncated rather than rounded, per the IAU recommendation. The coordinate sequence is the sexagesimal J2000-equinox RA and declination, hence the "J" designator.
Source designations in the All-Sky Release contain one additional digit of precision in RA and DEC than did those in the 2MASS Incremental and Sampler Releases.
CAUTION: Coordinates extracted from the designation column are less accurate than those given in the right ascension and declination columns of the source Catalogs. The use of designation coordinates will result in a maximum (unsigned) error of 0.0996" in declination and a maximum (unsigned degree) error of 0.1488" in right ascension.
The x and y in the acronym correspond to characters that
specify the Catalog and data release from which the
object was taken, respectively. The acronyms are as follows:
| Acronym | Catalog/Release |
| 2MASS | For objects in the All-Sky Point Source Catalog |
| 2MASX | For objects in the All-Sky Extended Source Catalog |
| 2MASSI | For objects in the Incremental Release Point Source Catalogs |
| 2MASXI | For objects in the Incremental Release Extended Source Catalogs |
| 2MASSs | For objects in the 2MASS Sampler Point Source Catalog |
| 2MASXs | For objects in the 2MASS Sampler Extended Source Catalog |
| 2MASSW | For objects in the Working Point Source Databases (pre-release) |
| 2MASXW | For objects in the Working Extended Source Databases (pre-release) |
The acronym designation and the equinox indicator "J" are not carried explicitly for entries in the Catalogs to save space. The coordinate sequence is tabulated for each source and is included in the designation field for each source in the Catalogs.
For example, the complete designations for two sources in the 2MASS All-Sky Release PSC and XSC are:
2MASS J18365633+3847012 (= 
Lyr)
and
2MASX J12103265+3924207 (= NGC 4151).
[Last Update: 2006 June 5, by R. Cutri]
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