The Two Micron All Sky Survey at IPAC
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The Two Micron All Sky Survey at IPAC
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The Two Micron All Sky Survey (2MASS) project is designed to close the gap
between our current technical capability and our knowledge of the
near-infrared sky. In addition to providing a context for the
interpretation of results obtained at infrared and other wavelengths, 2MASS
is providing direct answers to immediate questions on the large-scale
structure of the Milky Way and the Local Universe. The optimal use of the
next generation of infrared space missions, such as HST/NICMOS, the
Space Infrared Telescope Facility (SIRTF), and the Next Generation Space
Telescope (NGST), as well as powerful ground-based facilities, such as Keck I,
Keck II, and Gemini, require a new census with vastly improved sensitivity
and astrometric accuracy than that previously available.
To achieve these goals, 2MASS has uniformly scanned the entire sky in three
near-infrared bands to detect and characterize point sources brighter than
about 1 mJy in each band, with signal-to-noise ratio (SNR) greater than 10,
using a pixel size of 2.0". This has achieved an 80,000-fold improvement in
sensitivity relative to earlier surveys.
2MASS used two highly-automated 1.3-m telescopes, one at Mt. Hopkins, AZ,
and one at CTIO, Chile. Each telescope was equipped with a three-channel
camera, each channel consisting of a 256×256 array of HgCdTe detectors,
capable of observing the sky simultaneously at J (1.25 microns), H (1.65
microns), and Ks (2.17 microns).
The 2MASS arrays imaged the sky while the telescopes scan smoothly in
declination at a rate of ~1´ per second. The 2MASS data "tiles"
are 6° long in the declination direction and one camera frame (8.5´)
wide. While the entire telescope scanned in the declination direction, the
telescope's secondary mirror tilted opposite the scan direction to momentarily
freeze the focal plane image. At the end of a 1.3-s exposure the secondary
flew back to its start position and froze a new piece slightly displaced
from the previous frame. The dead time between frames used for secondary
flyback and array reset was less than 0.1-s. The camera field-of-view shifted
by ~1/6 of a frame in declination from frame-to-frame. The camera images each
consisted of six pointings on the sky for a total integration time of 7.8 s,
with sub-pixel "dithering", which improves the ultimate spatial resolution of
the final Atlas Images. When accounting for dead
time and the time to point the telescope and initiate a scan, the 2MASS
observing system integrated on the sky approximately 84% of each night.
The northern 2MASS facility began routine operations in 1997 June, and the
southern facility in 1998 March.
Survey operations were complete for both hemispheres on 2001 February 15.
.
Analyses of the data from the entire sky which
has been fully processed show that they easily meet and often exceed the
Level 1 Science Requirements
for the Survey.
The University of Massachusetts (UMass) was responsible for the overall
management of the project, and for developing the infrared cameras and on-site
computing systems at both facilities. The Infrared Processing and Analysis
Center (IPAC) is responsible for all data processing through the Production
Pipeline, and construction and distribution of the data products. The 2MASS
project involves the participation of members of the
Science Team from several different institutions. The 2MASS
project is funding by the National Aeronautics and Space Administration
(NASA) and the National Science Foundation (NSF).
The immediate scientific benefits from the 2MASS survey include:
2MASS is currently producing the following data products for the entire sky:
2MASS has released data products on the approximate schedule:
Each night of released data consists of about 250,000 point sources,
2000 galaxies, and 5000 images, or, equivalently, about 13.8 GB of data per
facility.
The Catalogs alone for the Second Release consist of more than 65 GB of
uncompressed data.
The completed and fully-processed Survey will consist of about 2 TB of
catalogs and compressed images.
The following are the Survey's Level 1 requirements. These are nominal levels
only. The actual performance achieved in many cases surpassed these
requirements as described in
Section VI.1 of
the on-line Explanatory Supplement.
(For unconfused sources outside of the Galactic Plane (|b|>10°), and
outside of any confusion-limited areas of the sky outside of the Galactic
Plane.)
(For unconfused sources outside of the Galactic Plane (|b|>10°), and
outside of any confusion-limited areas of the sky outside of the Galactic
Plane.)
The sky coverage will be >>95% for galactic latitude |b|>10°
and ~95% for |b|<10°. The overall coverage will have no gaps
> 200 square degrees.
For example three-color composite
Atlas Images showing many beautiful and interesting objects
observed so far, see the 2MASS Picture of the Week and
the 2MASS
Image Gallery.
Last Update: 2006 February 1
Return to the 2MASS Home Page
A Brief Explanation of 2MASS
Introduction
It has been nearly 30 years since the last large-area near-infrared survey of
the sky was carried out. The Two Micron Sky Survey (TMSS; Neugebauer &
Leighton 1969) scanned 70% of the sky and detected ~5,700 celestial sources of
infrared radiation. Since that time there has been a revolution in the
development of infrared detector technology. New, large format, sensitive
array detectors can now detect astronomical objects over 100
million times fainter than those detected in the TMSS.
Scientific Objectives and Benefits
2MASS Data Products
2MASS Data Releases
2MASS Level 1 Requirements
Magnitude Limits Band Wavelength (µm) Point Sources (SNR=10)
Extended Sources J 1.25 15.8 15.0 H 1.65 15.1 14.3 Ks 2.17 14.3 13.5
Note: At SNR=10, sigma(mag) = 2.5 / ln 10 = 0.109. Also, the feasibility of
the point source requirement at H is uncertain, due to lack of knowledge of
background variations.
Galactic Latitude Range Parameter >|30|° |20|-|30|° |10|-|20|° <|10|° Differential Completeness Point Sources 0.99 -- --
-- Extended Sources 0.90 --
-- -- Differential Reliability Point Sources 0.9995 0.9995 0.9995 0.9995 Extended Sources 0.99 0.99 0.80 --
Note: Differential Completeness (Reliability) is the completeness
(reliability) of sources falling in the last half-magnitude bin above the
completeness limit. Also, the completeness for extended sources applies only
to galaxies that can be reliably distinguished from point sources, with
"shape" parameter values 0.5" greater than those of the mean point sources.
Photometric precision
unconfused point sources
(for sources with SNR >> 20) 5% unconfused extended sources
(for isophotal magnitude at 20 mag/sq. arcsec.) 10% (H < 13.8)
Photometric spatial uniformity point sources 4% extended sources 10% Brightest measurable stars photometric bias (for Ks>4) <2% repeatability 5% for Ks=8 10% for 4<Ks<8 Position Reconstruction Error 0.5"