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.
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.
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is intended to be a "living document", i.e., it is modified at frequent intervals, such that a hardcopy soon becomes outdated.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:
| 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 |
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.
It is anticipated for later in 2002, likely sometime in the early (northern) autumn.
Last update: 2002 Apr 8, 2mass@ipac.caltech.edu.
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