Please see the Explanatory Supplement for the most detailed technical information on 2MASS and the data releases. Other technical documents can likely be found here.
The DVD-ROM can be ordered using the order form.
The DVD-ROM is free of charge.
FTP access to
the Release Catalogs, as well as the Release Scan Information Table, can be
found here.
Yes, full-resolution JPG renditions of Atlas Images and Atlas Image mosaics
can be found on the 2MASS
Image Galleries.
Images in the 2MASS Image Galleries are in the public domain, so you are
free to use the images that you download from these sites. However, we ask
that you please use the following acknowledgement, where possible, with the
image:
"Atlas Image [or Atlas Image mosaic] obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts
and the Infrared Processing and Analysis Center/California Institute of
Technology, funded by the National Aeronautics and Space Administration and
the National Science Foundation."
Given the sheer volume of Image data, the production of a DVD-ROM of this
sort is prohibitive. However, the full-resolution Atlas Images are
available online via
the IRSA Batch Image Server.
The Explanatory Supplement is intended to be a "living document", i.e., it
is modified at frequent intervals, such that a hardcopy soon becomes outdated.
However, for the Final Data Release, we will offer the Final Explanatory
Supplement as a single document on the DVD-ROM version of the Release.
Yes, they are available via
the IRSA Batch Image Server.
First a caveat: the 2MASS Atlas Images that are currently available
from the IRSA web services (irsa.ipac.caltech.edu) are lossy-compressed
versions of the images. The compression algorithm sacrifices
information particularly in the low order bits of the images,
so background levels are compromised. For this reason, we don't
recommend attempting to measure brightness upper limits from
the compressed images. An alternative to estimating upper limits for
non-detected sources would be to examine the properties of the 2MASS Point
Source Catalog entries in the vicinity of the x-ray position of interest.
You can do this with a simple position-radius search using
the Basic Catalog Search or Full CatScan Interface tools in
IRSA. Find sources in the vicinity of your positions that
are detected in only 1 or 2 of the 3 2MASS bands (look at
the "rd_flg" values in the PSC entries - non-detected bands
are indicated with rd_flg values of "0" in the appropriate
digit). For the bands with non-detections, the quoted magnitudes
are 97% confidence upper brightness limits that were determined
by placing an aperture down on the non-compressed Atlas Image
at the source position and measuring the signal and noise.
This is exactly what you wanted to do, but is done on the non-compressed
images. You should be able to measure a characteristic
upper limit for each band from the sources around your position.
You can quote these as approximate upper limits for your x-ray
sources. We are hoping to begin to make the non-compressed Atlas Images
available to the community within the next several months (see the above
FAQ). At that time, we will post here how best, e.g., using IRAF, to
measure brightness upper limits.
The pixel values for the Atlas Images are in data-number units ("DN").
To convert to a calibrated mag, you need the zero point
mag given in the image header. Look for the keyword "MAGZP".
It is then straightforward to compute the calibrated mag:
mag = MAGZP - 2.5 log10 (flux),
where the flux is the integrated flux in "DN".
If you are referring to the extended source "postage stamp"
images, then you need to look for the header keywords:
"JMAGZP", "HMAGZP" and "KMAGZP", for the J, H and Ks bands
respectively.
A word of warning: You should not use the compressed, or
"QuickLook" images, available from the Survey Visualization Tool on IRSA.
These have been lossy compressed, so for point sources, the accuracy
of the photometry is, at best, ~10%. You should not attempt isophotal
photometry on extended sources at all for these images. Instead, you should use
the Batch Images Server for 2MASS on the IRSA site to obtain
the full-resolution, uncompressed Atlas Images.
The NED identifiers extracted
for 2MASS extended sources are unfortunately truncated at 9 characters during
pipeline processing, so the full names are not available. We would have to
run a query against NED to recover the identifiers, which is something
that you can actually do yourself. Fortunately, NED is doing
the matching for us all.
If you go to the "By Parameters" selection under "OBJECTS"
on the main NED web site (at http://nedwww.ipac.caltech.edu/),
you will see that it is now possible to select NED holdings
according to source name prefix. Thus, you can select the 2MASS prefix,
and Submit the Query, to obtain listing for the 2MASS XSC objects in NED.
NED currently contains the 2MASS First Incremental Data Release XSC,
but they are planning to incorporate the Second Release in the
near future.
From the 2MASS Explanatory Supplement, specifically, Section
IV.5, these are very approximately:
This was the case up until last year. We are very happy to report
that with the ROSS 2002 announcement, 2MASS is now a fully supported
data set for ADP and LTSA archival research proposals and is
not treated like external ground-based data. For details,
see the ADP description (section A.1.2) or the LTSA description
(section A.1.3) at
http://research.hq.nasa.gov/code_s/nra/current/NRA-02-OSS-01/appendA1.html.
Band Lambda
Bandpass F_nu(0 mag)
(µm)
(µm)
(Jy)
J 1.235 0.162 1592 H 1.662 0.251 1024
Ks 2.159 0.262 666.8
Last update: 2002 Apr 8, 2mass@ipac.caltech.edu.
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