The 2MASS All-Sky Data Release Frequently Asked Questions (FAQ) Page

I. General Information

• "How do I reference 2MASS and its data products in my papers?"
• "Is it true that NASA will not fund archival research on the 2MASS databases, because they are considered ground-based data?"

II. Accessing 2MASS Data Products

• "When is the All-Sky Data Release expected to occur?"

• "How do I obtain a DVD-ROM version of the Release?"
• "How much does the DVD-ROM cost?"
• "In the past, I received a DVD-ROM containing the 2MASS Second Incremental Release. Unfortunately, I am not able to read it. What hardware supports the 2MASS DVD-ROM?"
• "How can I ftp the Release Catalogs?"
• "I can't find TIFF or other full-resolution images. Are they on the site?"
• "I would like to use an Atlas Image or Image Mosaic from a 2MASS image gallery in a publication. How do I obtain permission to use the Image?"
• "When will there be an Atlas Image DVD-ROM made, or will there ever be one?"
• "Can the Explanatory Supplement be downloaded as a single file?"
• "Are the uncompressed versions of the Atlas Images from the Release available somehow?"
• "I've tried to run a table upload using IRSA's GATOR tool, using a table named '2mass.tbl', but my query crashed with an error message?"

III. 2MASS Atlas Images

• "I would like to measure upper magnitude limits in J, H, and K_s at a particular position, using the online Atlas Images. Is this possible, and what are the limitations?"
• "What are the pixel units on the Atlas Images, and how do I convert these units into magnitudes?"

IV. 2MASS Point Source Catalog and Databases

• "How can I convert 2MASS magnitudes into Janskys?"
• "Is it possible to obtain the observation date of an object in the All Sky Data Release?"

V. 2MASS Extended Source Catalog and Databases

• "I find it sometimes difficult to match the result of a coordinate query of the Extended Source Catalog (XSC), because the field id_cat is only 9 characters long, and thus, longer galaxy names, such as ESO, MCG, etc., are truncated. Why is this?"
• "For the Extended Source Catalog, the column entry for the mean surface brightness at the half-light radius doesn't seem right. What is meant by this parameter, [band]_mnsurfb_eff?"

• "When do you plan to have the Statistical Analysis Tool on IRSA available to analyze the All-Sky Data Release?"

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• "How do I reference 2MASS and its data products in my papers?">

  The primary journal reference for 2MASS and its image and catalog data products is:

  The Two Micron All Sky Survey (2MASS)
  M.F. Skrutskie, R.M. Cutri, R. Stiening, M.D. Weinberg, S. Schneider, J.M. Carpenter, C. Beichman, R. Capps, T. Chester, J. Elias, J. Huchra, J. Liebert, C. Lonsdale, D.G. Monet, S. Price, P. Seitzer, T. Jarrett, J.D. Kirkpatrick, J. Gizis, E. Howard, T. Evans, J. Fowler, L. Fullmer, R. Hurt, R. Light, E.L. Kopan, K.A. Marsh, H.L. McCallon, R. Tam, S. Van Dyk, and S. Wheelock, 2006, AJ, 131, 1163.
  (Bibliographic Code: 2006AJ....131.1163S)

  Please include the following standard acknowledgment in any published material that makes use of 2MASS data products:

  "This publication makes use of data products from the Two Micron All Sky Survey, which is 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."

• "Is it true that NASA will not fund archival research on the 2MASS databases, because they are considered ground-based data?"

  This was the case up until 2002. 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. This will also be true for future NRA's.

• "How do I obtain a DVD-ROM version of the Release?"

  To order a 2MASS All-Sky Data Release DVD-ROM set, please email your request to the 2MASS Helpdesk, and include your full name and mailing address. Please note: The DVD-ROM contains Catalog data only, and no Atlas Images or movies. It is intended for use by professional astronomers only. N.B.: The format of the Catalog files is bar-delimited ascii, suitable for reading into database packages, such as Postgres, Informix or Oracle. The user should consult the All-Sky DVD Release Updates for the most recent information on the DVD-ROMs.

• "How much does the DVD-ROM cost?"

  The DVD-ROM is free of charge.

• "In the past, I received a DVD-ROM containing the 2MASS Second Incremental Release. Unfortunately, I am not able to read it. What hardware supports the 2MASS DVD-ROM?"

  We know of the following drives which will support our past DVD-ROMs:

  • Creative Labs DVD6240E
  • Creative Labs 6X DVD 6630E
  • Dell badged 4X DVD-ROM
  • portable Sony DVD
  • Pioneer DVD
  • Pioneer DVD-U02
  • Pioneer DVD-103S trayless
  • Toshiba DVD-ROM SD-C2202 1D13
  • Toshiba DVD SD-C2102
  • Samsung DVD-ROM
  • Teac DV516B
  • LG DRD-8160B

• "How can I ftp the Release Catalogs?"

  Electronic access to the All-Sky Release Catalogs via ftp is described in Section I.4.b of the on-line 2MASS Explanatory Supplement.

• "I can't find TIFF or other full-resolution images. Are they on the site?"

  Yes, full-resolution JPG renditions of Atlas Images and Atlas Image mosaics can be found on the 2MASS Image Galleries.

• "I would like to use an Atlas Image or Image Mosaic from a 2MASS image gallery in a publication. How do I obtain permission to use the Image?"

  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."

• "When will there be an Atlas Image DVD-ROM made, or will there ever be one?"

  Given the sheer volume of Image data (~10 Terabytes), the production of a DVD-ROM of this sort is prohibitive. The full-fidelity Atlas Images can be accessed on-line using the 2MASS Image Services of the Infrared Science Archive (IRSA).

• "Can the Explanatory Supplement be downloaded as a single file?"

  The Explanatory Supplement for the All-Sky Release is currently intended to be a "living document", i.e., it will be modified at frequent intervals, such that a hardcopy soon becomes outdated. For the All-Sky Data Release, we will offer the Explanatory Supplement as a single document on the DVD-ROM version of the Release. However, users should continue to refer to the online version for up-to-date information.

• "Are the uncompressed versions of the Atlas Images from the Release available somehow?"

  The full-fidelity (non-lossy-compressed) Atlas Images are available via the on-line 2MASS Image Services of the Infrared Science Archive (IRSA).

• "I would like to measure upper magnitude limits in J, H, and K_s at a particular position, using the online Atlas Images. Is this possible, and what are the limitations?"

  First a caveat: the 2MASS Atlas Images that are currently available from the IRSA web services (irsa.ipac.caltech.edu) are lossy-compressed "Quicklook" 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 do not recommend attempting to measure brightness upper limits from the Quicklook images.

  An alternative to estimating upper limits for non-detected sources would be to examine the properties of the 2MASS Point Source Catalog (PSC) entries in the vicinity of the position of interest. You can do this with a simple position-radius search using the IRSA GATOR tool. 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="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. You should be able to measure a characteristic upper limit for each band from the sources around your position. You can then quote these as approximate upper limits for your sources.

• "What are the pixel units on the Atlas Images, and how do I convert these units into magnitudes?"

  The pixel values for the Atlas Images are in data-number units ("DN"). They are not normalized by area - which is approximately 1 arcsec²/pix. To convert a measurement on the Atlas Images to a calibrated mag, you need the zero point magnitude given in the image header. Look for the keyword "MAGZP". It is then straightforward to compute the calibrated mag:

  mag = MAGZP - 2.5 log₁₀ (S)

  where the S is the integrated, background-subtracted flux in "DN" measured in some region. You do not need to divide by exposure time because that has been taken into account in the calculation of the zero point values.

  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 K_s bands respectively.

  Because the pixels in the Atlas Images were generated by combining and upsampling pixels in the individual 2MASS exposures, adjacent Atlas Image pixels are correlated. This correlation must be taken into account when estimating photometric uncertainties in measurements on the Atlas Images. Please see section VI.8.a of the Explanatory Supplement for guidelines on how to do this.

  A word of warning: You should not use the compressed, QuickLook images, available from the Quicklook Image 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.

• "I find it sometimes difficult to match the result of a coordinate query of the Extended Source Catalog (XSC), because the field id_cat is only 9 characters long, and thus, longer galaxy names, such as ESO, MCG, etc., are truncated. Why is this?"

  (The following information applied to the Second Incremental Data Release Catalogs. It may be now out-of-date for the All-Sky Data Release.)

  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, 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.

• "How can I convert 2MASS magnitudes into Janskys?"

  2MASS catalog and database magnitudes can be converted to flux densities (Janskys) using the following flux for zero-magnitude zero point conversion values:

  Band	Lambda (µm)	Bandwidth (µm)	Fnu - 0 mag (Jy)	Flambda - 0 mag (W cm-2 µm-1)
  J	1.235 ± 0.006	0.162 ± 0.001	1594 ± 27.8	3.129E-13 ± 5.464E-15
  H	1.662 ± 0.009	0.251 ± 0.002	1024 ± 20.0	1.133E-13 ± 2.212E-15
  Ks	2.159 ± 0.011	0.262 ± 0.002	666.7 ± 12.6	4.283E-14 ± 8.053E-16

  For more information see Section VI.4a of the All-Sky Data Release Explanatory Suppement.

• "When is the All-Sky Data Release expected to occur?"

  The 2MASS All-Sky Data Release took place on 25 March 2003. All Atlas Image and Catalog data are available on-line.

• "Is it possible to obtain the observation date of an object in the All Sky Data Release?"

  Yes. The "jdate" column in the All-Sky Release PSC and XSC gives the Julian date for that observation to an accuracy of ±30 sec. If you are using the the GATOR Catalog query engine at IRSA, "jdate" is part of the "standard" format of available Point Source Catalog columns that can be selected for your table output. Another column, "date" (which is the UT date of the morning that an observation is completed), can be selected as part of the "long form" format for available columns.

• "I've tried to run a table upload using IRSA's GATOR tool, using a table named '2mass.tbl', but my query crashed with an error message?"

  The answer to your question is very simple: the table is loaded into the IRSA database, which will not accept file names that begin with a number. Table name and format guidelines for the GATOR table upload service are described here.

• "When do you plan to have the Statistical Analysis Tool on IRSA available to analyze the All-Sky Data Release?"

  The 2MASS All-Sky Point Source Catalog Statistics Service is now available on-line through the Infrared Science Archive (IRSA).

  This tool can be used to estimate quickly the 2MASS PSC source number count and color distributions in a specified circular region of the sky without having to run a more time-consuming query on the full PSC.

• "For the Extended Source Catalog, the column entry for the mean surface brightness at the half-light radius doesn't seem right. What is meant by this parameter, [band]_mnsurfb_eff?"

  The wording in the Explanatory Supplement and in the Extended Source Catalog column descriptions for this parameter is not quite accurate. The half-light surface brightness refers to the mean flux within the half-light (effective) radius. In other words, it is the mean surface brightness, which is also known as the effective surface brightness.

Last update: 2007 June 14, 2MASS Helpdesk.

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