Appendix 1. Introduction to the 2MASS Extended Mission and Ancillary Data Products
2. Executive Summary of 2MASS Ancillary Data Products
The following sections provide a high level description of the contents of each of the Extended Mission Ancillary data sets, and the location of further detailed information about the data processing and analysis of the products. Click on the links above to jump to each of the individual data set descriptions. Table 1 contains a summary of the number of entries in each of the Working Databases (and Catalogs for comparison), the number of Atlas Images in each data set, and the number of 2MASS scans that went into the production of each product.
| Product | Survey WDB | All-Sky Catalogs | Survey "Reject Tables" | 6x WDB | 6x Catalog Subset | Calibration WDB | LMC/SMC Calibration WDB |
|---|---|---|---|---|---|---|---|
| Point Sources | [1,314,981,867]1 | 470,992,970 | 843,988,897 | 24,023,702 | 8,637,702 | 191,464,020 | 5,041,472 |
| Extended Sources | [2,590,500]1 | 1,647,5992 | 943,441 | 247,091 | 84,041 | 403,811 | 9,709 |
| Atlas Images | 4,879,128 | 4,121,439 | --- | 69,687 | 52,671 | 878,760 | 18,504 |
| Scans | 70,712 | 59,731 | --- | 1,328 | 1,031 | 73,230 | 1,542 |
Notes to Table 1
1 - The Survey Source WDBs are the combination of the All-Sky Catalogs and "Reject Tables." The WDBs are not released as single tables.
2 - The All-Sky Release XSC contains 540 Large Galaxy entries that are taken from special processing, and are not found in the Survey XSWDB.
a. Survey Extracted Source "Reject Tables" and Full Image Atlas
i. Survey Point Source "Reject Table" (PSRT)
All sources extracted during point source processing of the 70,712 2MASS survey mode scans of the sky were entered in the Survey Point Source Working Database (PSWDB). A highly uniform, reliable subset of extractions from the PSWDB in 59,731 survey scans were selected that comprise the 2MASS All-Sky Point Source Catalog (PSC).
The 2MASS Survey Point Source "Reject Table" (PSRT) contains astrometry and photometry in the three survey bandpasses for the 843,988,897 point source extractions from the PSWDB that were not included in the All-Sky PSC. Extractions in the Survey PSRT are drawn from all 70,712 survey mode scans, but have different characteristics in scans selected for inclusion in the All-Sky PSC than in scans that were not in the All-Sky Release:
- The PSRT extractions from the 59,731 scans that were included in the All-Sky Release consist of faint source detections and spurious detections of image artifacts and transient events that were below the reliability thresholds set for PSC source selection. They also include brighter, reliable source detections near scan edges that were rejected from the PSC during the multiple detection resolution process.
- The PSRT contains all point source extractions from the 10,981 survey mode scans that were not selected for inclusion in the All-Sky Release. These include reliable detections of astrophysical sources as well as spurious detections of low SNR noise events, image artifacts and transients.
The contents of the Survey PSRT can be summarized as follows:
- One or more independent detections of real sources that were scanned more than once during the Survey. Most of these fall in Tile overlap regions. However, ~14% of all Survey Tiles were scanned two or more times either for validation purposes or to obtain measurements under better atmospheric conditions. All objects in those tiles may have independent measurements in the PSRT.
- A large number of low signal-to-noise ratio (SNR) extractions that are a mixture of faint, real astrophysical sources and noise excursions. Source detection was made down to the SNR~3.5 level during survey data reduction to insure the completeness of the Catalog. This produced a large number of spurious noise detections. PSC source selection was cut at SNR>7 to insure meeting the Catalog's >99.95% reliability requirement.
- All flagged and unflagged detections of image artifacts produced by bright stars.
- Spurious detections of transients such as cosmic rays, residual meteor trails and hot pixel events.
Because the Survey PSRT contains a large number of spurious source extractions, this product should be used cautiously. Users are encouraged to familiarize themselves with the Cautionary Notes before using the PSRT, paying particular attention to the sections discussing source reliability.
As with the 2MASS All-Sky PSC, the reliable Survey PSRT extractions are predominantly detections of Milky Way stars, but include point-source processed photometry for virtually all of the resolved sources found in the Survey Extended Source WDB, as well as a large number of unresolved, distant galaxies. The brightness of the resolved sources in the PSRT will be systematically underestimated.
The general properties of
the 2MASS Survey PSRT are presented in A2.2.
The format of the PSRT table is described in
A2.3.a. Source detection and
photometry algorithms used to generate the PSRT are the same
as those used for the All-Sky PSC and are described in
IV.4.
ii. Survey Extended Source "Reject Table" (XSRT)
The 2MASS Survey Extended Source WDB (XSWDB) contains information for all candidate detections found to be resolved with respect to a single point-spread-function during processing of 70,712 survey-mode scans. A highly uniform, reliable subset of entries from the XSWDB in 59,731 survey scans were selected to comprise the 2MASS All-Sky Extended Source Catalog (XSC).
The 2MASS Survey Extended Source "Reject Table" (XSRT) contains astrometry, photometry and basic shape information for the 943,441 extended source candidates in the PSWDB that were not included in the All-Sky XSC. Extractions in the Survey XSRT are drawn from all 70,712 survey mode scans, but have different characteristics in scans selected for inclusion in the All-Sky XSC than in scans that were not in the All-Sky Release:
- The XSRT extractions from the 59,731 scans that were included in the All-Sky Release consist of detections of faint extended objects and single and multiple stars, along with spurious detections of image artifacts and transient events that were below the reliability thresholds set for XSC source selection. They also include brighter, reliable source detections near scan edges that were rejected from the XSC during the multiple detection resolution process.
- The XSRT contains all extended source extractions from the 10,981 survey mode scans that were not selected for inclusion in the All-Sky Release. These include reliable detections of extended astrophysical sources as well as close multiple stars and spurious detections of image artifacts and transients.
The contents of the Survey XSRT can be summarized as follows:
- One or more independent detections of true extended sources that were scanned more than once during the Survey. Most of these fall in Tile overlap regions. However, ~14% of all Survey Tiles were scanned two or more times either for validation purposes or to obtain measurements under better atmospheric conditions. The Extended Source WDB also contains duplicate entries for some sources that lie near the boundaries of Atlas Images (see A2.1b).
- "False" extended sources that are real objects on the sky, but not truly extended. These are a mixture of close multiple stars and faint sources, below the SNR=7 limit used to derive the XSC where the reliability of extended classification drops off. Most multiple stars were excluded from the XSC using the e_score < 1.4 OR g_score < 1.4 requirement, but they remain in the XSRT.
- Fragments of large galaxies and nebulae that span Tile and Atlas Image boundaries. While these are true resolved sources, they were generally excluded from the XSC.
- All flagged and unflagged detections of image artifacts produced by bright stars and transient events such as residual meteor trails and "bugs" on the camera windows.
Note that the Survey XSRT does not contain the Large Galaxy entries that are found in the XSC. These entries were constructed using special processing that combined images from multiple survey scans and are not products of the standard survey scan data processing.
The general properties of
the 2MASS Survey XSRT are presented in
Section A2.2. The format of the WDB table is described in
Section A2.3b. Source detection and
photometry algorithms used to generate the XSRT are the same
as those used for the All-Sky XSC and are described in
IV.5.
The 2MASS Full Survey Image Atlas contains 4,879,128 calibrated FITS images in the three survey bandpasses constructed from 70,712 scans of 8.5´ × 6° Survey Tiles. The Survey Atlas is the superset of images from which the All-Sky Release Image Atlas was drawn.
The image data from each survey scan are divided into twenty-three separate Atlas Images in each band. Twenty-two of these are 512×1024 (1"/pix) in size, and one is 512×698 (1"/pixel). The J, H and Ks images for each region of sky are resampled to the same scale and registered to facilitate 3-color investigations. Positional and photometric calibration data are carried in the FITS header for each image, which is described in II.4.a.
2MASS Full Survey Atlas Images are accessible via the
2MASS Image Services which are administered by the
Infrared Science Archive (IRSA).
These services allow interactive viewing and retrieval of one or more images
and include an inventory mode that lists all images that cover a
point or region on the sky. The inventory mode is a particularly
useful tool to determine if more than one scan, and its Atlas Images,
are available for a particular region of the sky.
b. Long Exposure ("6x") Working Databases/Catalogs and Atlas Images
i. 6x Point Source Working Database and Catalog (6x-PSWDB and 6x-PSC)
The 2MASS Long Exposure (6x) Point Source Working Database (6x-PSWDB) contains astrometry and photometry in the three survey bandpasses for 24,023,702 "sources" extracted from 1,328 scans of the sky taken with exposures six time longer than used in the primary survey. The 6x-PSWDB is the primary record of the pipeline point source processing of the long exposure data.
A 6x Point Source Catalog (6x-PSC) was drawn from the 6x PSWDB using criteria that are analogous to those that were used to draw the All-Sky PSC from the Survey Point Source WDB. The 6x-PSC is a view on the 6x-PSWDB that contains 8,637,594 sources drawn from 1,031 of the long exposure scans. As with the All-Sky Release PSC, the 6x-PSC is a more uniform and reliable representation of the near infrared sky that contains only one entry for each source (i.e. one detection of objects scanned and detected multiple times is selected for the Catalog). Unlike the All-Sky Release PSC, the 6x-PSC is not released as a separate table but as a subset of the larger 6x-PSWDB. 6x-PSC sources are identified in the 6x-PSWDB using the cat column found in the source record. 6x-PSC sources have a value of cat="1". 6x-PSWDB extractions not included in the Catalog (i.e. the 6x Point Source Reject File) have cat="0".
In addition to all detections that comprise the 6x-PSC, the full 6x-PSWDB contains:
- Multi-epoch detections of real sources that were scanned more than once during the 6x observations. The multiply-detected sources are found in both the 6x Tile overlap regions as well as in 73 6x Tiles that were scanned more than once.
- A large number of low signal-to-noise ratio (SNR) detections that are a mixture of faint, real astrophysical sources and noise excursions. Detections were made at the SNR~3 level during 6x pipeline processing, well below the SNR=7 limit used for the 6x-PSC, to insure the completeness of the Catalog.
- All flagged and unflagged detections of image artifacts produced by bright stars.
- Spurious detections of transients such as cosmic rays, residual meteor trails and hot pixel events.
Because the 6x Point Source WDB contains a large number of false source extractions, this product should be used cautiously. Users are encouraged to familiarize themselves with the Cautionary Notes before using the 6x-PSWDB paying particular attention to the sections discussing source reliability.
The general properties of
the 2MASS 6x PSWDB are presented in A3.2.
The format of the 6x-PSWDB/6x-PSC table is described in
A3.3a Source detection and
photometry algorithms used to generate the 6x-PSWDB are
described in A3.5. Scan and source
selection criteria used to identify sources that constitute the
6x-PSC are described in A3.7.
ii. 6x Extended Source Working Database and Catalog (6x-XSWDB and 6x-XSC)
The 2MASS 6x Extended Source WDB (6x-XSWDB) contains astrometry, photometry in the three survey bandpasses and basic shape information for 247,091 candidate "sources" found to be resolved with respect to a single point-spread-function during processing of 1,328 special scans of the sky taken with exposures six time longer than used in the primary survey. The 6x Extended Source WDB is the primary archive of the pipeline extended source processing of the survey data.
A 6x Extended Source Catalog (6x-XSC) was drawn from the 6x-XSWDB using criteria that are analogous to those that were used to draw the All-Sky XSC from the Survey Extended Source WDB. The 6x-XSC, which contains 84,041 sources drawn from 1,031 of the long exposure scans, is of higher reliability than the 6x-XSWDB, and is intended to contain only one entry for each source on the sky. Unlike the All-Sky Release XSC, the 6x-XSC is not released as a separate table but as a subset of the larger 6x-XSWDB. 6x-XSC sources are identified in the 6x WDB as having a cat flag value of "1". 6x-XSWDB entries not in the 6x-XSC have cat=0.
The 6x-XSWDB contains:
- One or more detections of true extended sources that were scanned one or more times during the 6x observations. The multiply-detected sources are found in both the 6x Tile overlap regions as well as in the 73 6x tiles that were scanned more than once. The 6x Extended Source WDB also contains duplicate entries for some sources that lie near the boundaries of Atlas Images (see A3.1d).
- "False" extended sources that are real objects on the sky, but not truly extended. These are a mixture of single and close multiple stars and faint sources, below the SNR=7 limit used to derive the 6x-XSC where the reliability of extended classification drops off. Most single and multiple stars in the 6x-XSWDB are excluded from the 6x-XSC by requiring that e_score < 1.4 OR g_score < 1.4 .
- Fragments of large galaxies and nebulae that span Tile and Atlas Image boundaries.
- All flagged and unflagged detections of image artifacts produced by bright stars and transient events such as residual meteor trails and "bugs" on the camera windows.
Note that the 6x-XSWDB does not contain any Large Galaxy entries that are found in the All-Sky XSC. These entries were constructed using special processing that combined images from multiple survey scans and are not products of the standard 6x scan data processing.
The general properties of
the 2MASS 6x Extended Source WDB are presented in
Section A3.2. The format of the WDB table is described in
Section A3.3b. Source detection and
photometry algorithms used to generate the Extended Source WDB are
described in A3.5.a.
Scan and source
selection criteria used to identify sources that constitute the
6x-XSC are described in A3.6.
The 2MASS 6x Image Atlas contains 69,687 calibrated FITS images in the three survey bandpasses constructed from 1,328 scans of the sky taken with exposures six time longer than used in the primary survey. Two types of 6x scans were made: 6° long (in declination) scans (dubbed "LSC") that covered regions analogous to Survey Tiles, and 1° long (in declination) scans (dubbed "LCA") that covered regions similar in size to Calibration Tiles.
The image data from each 6° LSC scan are divided into twenty-three separate Atlas Images in each band. Twenty-two of these are 512×1024 (1"/pix) in size, and one ranges in size from 512×648 to 512×670 (1"/pixel). For the 1° long LCA 6x scans, the images data are divided into four separate Atlas Images in each band. Three are 512×1024 (1"/pix) in size, and one is shorter, having sizes that range from 512×818 to 512×841 (1"/pixel). The J, H and Ks images for each region of sky are resampled to the same scale and registered to facilitate 3-color investigations. Positional and photometric calibration data are carried in the FITS header for each image, which has the same format as those of the Survey Atlas Images, and is described in II.4.a.
2MASS 6x Atlas Images are accessible via the
2MASS Image Services which are administered by the
Infrared Science Archive (IRSA).
These services allow interactive viewing and retrieval of one or more images
and include an inventory mode that lists all images that cover a
point or region on the sky. The inventory mode is a particularly
useful tool to determine if more than one scan, and its Atlas Images,
are available for a particular region of the sky.
c. Calibration Scan Working Databases and Atlas Images
i. Calibration Point Source Working Database (Cal-PSWDB) and LMC/SMC Calibration Point Source Working Database (LMC/SMC Cal-PSWDB)
The 2MASS Calibration Point Source Working Database (WDB) contains astrometry and photometry in the three survey bandpasses for 191,464,020 "sources" extracted from 73,230 1° long (in declination) scans of the 35 Calibration Fields (or Tiles). 2MASS calibration observations were made using the same scanning technique and exposure time as the main survey observations, and calibration scan data were reduced using virtually the same pipeline processing system that was used to reduce the survey data (see A4.4). The Calibration Point Source WDB is the primary record of the pipeline point source processing of the calibration data.
The LMC/SMC Calibration Point Source Working Database (WDB) contains astrometry and photometry in the three survey bandpasses for 5,041,472 "sources" extracted from 1,542 1° long (in declination) scans of five additional Calibration Fields (or Tiles) located in and around the LMC/SMC that were observed in support of the long exposure (6x) observations of those regions. The data acquisition and processing for these five fields were the same as for the primary survey calibration fields.
The Cal-PSWDB and LMC/SMC Cal-PSWDB contain:
- One or more detections of real sources that were scanned more than once during the Calibration observations. Multiply-detected objects are an important class of entry in the Cal-PSWDB because each field was observed between approximately 100 and 3700 times under photometric conditions.
- A large number of low signal-to-noise ratio (SNR) extractions that are a mixture of faint, real astrophysical sources and noise excursions. Detections were made at the SNR~3.5 level during calibration scan pipeline data reduction. The reliability of source extraction drops off systematically with decreasing SNR below SNR=7.
- All flagged and unflagged detections of image artifacts produced by bright stars.
- Spurious detections of transients such as cosmic rays, residual meteor trails and hot pixel events.
Because the Cal-PSWDBs contain a large number of spurious source detections, this product should be used cautiously. Users are encouraged to familiarize themselves with the Cautionary Notes before using the Cal-PSWDB, paying particular attention to the sections discussing source reliability.
The general properties of
the 2MASS Cal-PSWDBs are presented in A4.2.
The format of the Cal-PSWDB table is described in
A4.3.a. Source detection and
photometry algorithms used to generate the Calibration Point Source WDBs are
the same as those used for the All-Sky PSC and are described in
A4.4. Photometric and astrometric
calibration of the Calibration scan data was handled differently
than the Survey data, and is discussed in A4.4.
ii. Calibration Extended Source Working Database (Cal-XSWDB)
The 2MASS Calibration Extended Source WDB (Cal-XSWDB) contains astrometry, photometry in the three survey bandpasses and basic shape information for 403,811 candidate "sources" found to be resolved with respect to a single point-spread-function during processing of 73,230 1° long (in declination) scans of the 35 Calibration Fields (or Tiles). 2MASS calibration observations were made using the same scanning technique and exposure time as the main survey observations, and calibration scan data were reduced using virtually the same pipeline processing system that was used to reduce the survey data (see A4.4). The Calibration Extended Source WDB is the primary archive of the pipeline extended source processing of the calibration observation data.
The LMC/SMC Calibration Extended Source Working Database (WDB) contains astrometry and photometry in the three survey bandpasses for 9,709 "sources" extracted from 1,542 1° long (in declination) scans of five additional Calibration Fields (or Tiles) located in and around the LMC/SMC that were observed in support of the long exposure (6x) observations of those regions. The data acquisition and processing for these five fields were the same as for the primary survey calibration fields.
The Cal-XSWDB and LMC/SMC Cal-XSWDB contain:
- One or more detections of real resolved sources scanned during the calibration observations. Multiply-detected objects are an important class of entry in the Cal-XSWDB because each field was observed between approximately 100 and 3700 times under photometric conditions.
- "False" extended sources that are real objects on the sky, but not truly extended. These are a mixture of single and close multiple stars and faint sources. The reliability of extended source classification drops off rapidly below SNR=7.
- Fragments of large galaxies and nebulae that span Tile and Atlas Image boundaries. There are very few large galaxies covered by the calibration fields, though.
- All flagged and unflagged detections of image artifacts produced by bright stars and transient events such as residual meteor trails and "bugs" on the camera windows.
Note that the Cal-XSWDB and LMC/SMC Cal-XSWDB doe not contain the Large Galaxy entries that are found in the All-Sky XSC. These entries were constructed using special post processing that combined images from multiple survey scans and are not products of the standard calibration scan data reduction.
The general properties of
the 2MASS Cal-XSWDBs are presented in
A4.2. The format of the Cal-XSWDB tables
are described in A4.3b. Source detection
and photometry algorithms used to generate the Cal-XSWDBs are the
same as those used for the All-Sky XSC and are described in
IV.5.
iii. Calibration Scan Image Atlas
The 2MASS Calibration Scan Image Atlas contains 878,769 calibrated FITS images in the three survey bandpasses constructed from 73,230 1° long (in declination) scans of the 35 Calibration Fields (or Tiles).
The 2MASS LMC/SMC Calibration Scan Image Atlas contains 18,504 calibrated FITS images in the three survey bandpasses constructed from 1,542 1° long (in declination) scans of the five special Calibration Fields (or Tiles) located in and around the LMC/SMC that were observed in support of the long exposure (6x) observations of those regions.
The image data from each 1° Calibration scan are divided four separate Atlas Images in each band. Three are 512×1024 (1"/pix) in size, and one is shorter, having sizes that range from 512×648 to 512×673 (1"/pixel). The J, H and Ks images for each region of sky are resampled to the same scale and registered to facilitate 3-color investigations. Positional and photometric calibration data are carried in the FITS header for each image, which has the same format as the Survey Atlas Image headers, and is described in II.4.a.
2MASS Calibration Atlas Images are accessible via the
2MASS Image Services which are administered by the
Infrared Science Archive (IRSA).
These services allow interactive viewing and retrieval of one or more images
and include an inventory mode that lists all images that cover a
point or region on the sky. Because of the highly redundant nature of
the Calibration Image data, the inventory mode is not as useful
as for the Survey or 6x data since it will usually return a large number
of images covering any position in a Calibration field.
d. Merged Source Information
| Product | Survey | 6x | Calibration | LMC/SMC Calibration |
|---|---|---|---|---|
| Merged Point Source Information | 165,942,357 | 4,771,737 | 456,480 | 35,278 |
| Merged Point Source Cross-Reference | 396,697,288 | 12,267,173 | 204,143,440 | 4,250,232 |
| Merged Extended Source Information | 406,636 | 30,252 | 2,146 | 101 |
| Merged Extended Source Cross-Reference | 960,841 | 76,636 | 473,971 | 8,660 |
i. Merged Point Source Information Tables
The 2MASS Survey, 6x, Calibration and LMC/SMC Calibration Merged Point Source Information Tables contain combined position and brightness data for sources detected more than once in each of the respective data sets. All sightings of a source are identified by performing a positional autocorrelation of each Point Source WDB, and finding groups of extractions from different scans that fall within a 1.5" correlation radius. For each group of positionally associated extractions, the Merged Source Information tables contain the variance-weighted mean positions, mean and variance-weighted mean fluxes in the three survey bandpasses, various statistics on the distribution of positions and fluxes of all the sightings, and confirmation statistics summarizing the number of times a source was detected in a given WDB and the estimated number of times the mean position of the source was scanned. The source merging process is limited by possible source proper motion, confusion with intermittently resolved multiple sources and noise, and the internal consistency of 2MASS source position reconstruction.
As shown in Table 2, the Survey, 6x, Calibration and LMC/SMC Calibration Scan Merged Point Source Tables contain 165,942,357, 4,771,737, 456,480 and 35,278 entries, respectively. These are much smaller than the sizes of each of the respective Point Source WDBs given in Table 1 for several reasons:
- The multi-epoch sightings of each source have theoretically been collapsed down into one entry. This is most dramatic for the heavily observed Calibration WDBs.
- Only sources observed two or more times are included in the Merged Source Information Tables.
- The merging process for each WDB was restricted to extractions that have >50% probability of being real source detections (rel = [A-D]; see Section A5_1). This reduces the total number of candidate extractions in each WDB by as much as 50%.
The Merged Point Source Information Tables are designed to be used in conjunction with the respective WDBs as a source of additional data for selected objects, rather than as a replacement for them. For example, you cannot necessarily query the Survey Merged Point Source Information Table to find a complete list of detections down to some flux level in a particular region of the sky, even though the Survey observations covered the entire sky (okay, 99.998% of the sky). The more appropriate use is to identify objects of interest in the Catalogs or WDBs, and if the sources were detected more than once, use the Merged Source Information tables to obtain improved photometry and positions, or to investigate if the sources have moved or has varied in brightness.
Within the Survey, 6x, Calibration and LMC/SMC Calibration Merged Point Source Information Tables, each entry corresponds to an associated group of extractions and is assigned a unique identifier, gcntr. This identifier is cross-referenced in the individual Survey, 6x and Calibration Scan Point Source WDB entries. Each WDB entry belonging to a merged group of extractions also carries the number of times the source was detected (sdet) and an estimate of the number of times the merged source position was scanned within a given data set (spos).
The general WDB merging process is described in
Appendix 6. The contents and formats of the
Survey, 6x, Calibration and LMC/SMC Calibration Merged Point Source
Information Tables are
given in A6.3a.
ii. Merged Point Source Cross-Reference (Link) Tables
The Survey, 6x, Calibration and LMC/SMC Calibration Scan Merged Point Source Cross-Reference Tables contain the information linking each associated group of extractions with their individual WDB entries. Each table contains one row for each WDB extraction that has been associated with a merged source group. Each row in the tables provides the unique merged group identifier (gcntr), the unique WDB identifier (cntr/pts_key) and the number of groups with which that particular WDB extraction has been associated (ngrp). A WDB extraction may be associated with more than one merged group in confused situations. The cntr/pts_key values for such WDB extractions will appear more than once in the Cross-Reference Table, accompanied by the gcntr of each of the groups to which it has been associated.
As seen in Table 2, the Survey, 6x and Calibration Scan Merged Point Source Cross-Reference Tables contain 396,697,288, 12,267,173, 204,143,440 and 42,50,232 entries, respectively. Contrasting these numbers with the number of entries in the respective Point Source Merged Information Tables gives the approximate mean number of independent detections of each merged source within each WDB. For the Survey and 6x data sets this ranges from 2-3 detections per group. For the highly redundant Calibration Scan data, the mean number of detections is ~470, emphasizing the value of these data as a source for precision photometry and time domain studies.
The Merged Point Source Cross-Reference Tables are discussed
in the context of the general WDB merging process in
Appendix 6. The formats of the
Survey, 6x, Calibration and LMC/SMC Calibration Cross-Reference Tables
are the same,
and are given in A6.3c.
iii. Merged Extended Source Information Tables
The 2MASS Survey, 6x, Calibration and LMC/SMC Calibration Merged Extended Source Information Tables contain combined position and brightness data for resolved sources detected more than once in each of the respective data sets. All sightings of a source are identified by performing a positional autocorrelation of each Extended Source WDB, and finding groups of extractions from different scans that fall within a 5" correlation radius. For each group of positionally associated extractions, the Merged Source Information Tables contain the variance weighted mean positions, mean and variance weighted mean selected fluxes in the three survey bandpasses, average basic shape information, various statistics on the distribution of positions, fluxes, and shape parameters of all the sightings, and confirmation statistics summarizing the number of times a source was detected in a given WDB and the estimated number of times the mean position of the source was scanned. The source merging process is limited by possible confusion with intermittently resolved multiple sources and noise, and the internal consistency of 2MASS source position reconstruction.
As shown in Table 2, the Survey, 6x and Calibration Scan Merged Extended Source Information Tables contain 406,636, 30,252, 2,146 and 101 entries, respectively. These are much smaller than the sizes of each of the Extended Source WDBs given in Table 1 for several reasons:
- The multi-epoch sightings of each source have theoretically been collapsed down into one entry. This is most dramatic for the heavily observed Calibration WDBs.
- Only sources observed two or more times are included in the Merged Source Information Tables.
- The merging process for each WDB was restricted to extractions that have >50% probability of being real extended source detections (rel = [A-D]; see Section A5_1). This reduces the total number of candidate extractions in each WDB by approximately 10%.
The Merged Extended Source Information Tables are designed to be used in conjunction with the respective WDBs as a source of additional data for some sources, rather than as a replacement for them. For example, you cannot necessarily query the Survey Merged Extended Source Information Table to find a complete list of detections down to some flux level in a particular region of the sky, even though the Survey observations covered the entire sky. The more appropriate use is to identify objects of interest in the WDBs (or Catalogs), and if they were detected more than once, use the Merged Source Information tables to obtain improved photometry and positions, or to investigate if the sources have varied in brightness.
Within the Survey, 6x and Calibration Merged Extended Source Information Tables, each entry corresponds to an associated group of extractions and is assigned a unique identifier, gcntr. This identifier is cross-referenced in the individual Survey, 6x and Calibration Scan Extended Source WDB entries. Each WDB entry belonging to a merged group of extractions also carries the number of times the source was detected (sdet) and an estimate of the number of times the merged source position was scanned within a given data set (spos).
The general WDB merging process is described in
Appendix 6. The contents and formats of the
Survey, 6x and Calibration Merged Extended Source Information Tables are
given in A6.3b.
iv. Merged Extended Source Cross-Reference (Link) Tables
The Survey, 6x and Calibration Scan Merged Extended Source Cross-Reference Tables contain the information linking each associated group of extractions with their individual WDB entries. Each table contains one row for each WDB extraction that has been associated with a merged extended source group. Each row in the tables provides the unique merged group identifier (gcntr), the unique WDB identifier (cntr/pts_key) and the number of groups with which that particular WDB extraction has been associated (ngrp). A WDB extraction may be associated with more than one merged group in confused situations. The cntr/pts_key values for such WDB extractions will appear more than once in the Cross-Reference Table, accompanied by the gcntr of each of the groups to which is has been associated.
As seen in Table 2, the Survey, 6x and Calibration Scan Merged Extended Source Cross-Reference Tables contain 960,841, 76,636, 473,971 and 8,660 entries, respectively. Contrasting these numbers with the number of entries in the respective Extended Source Merged Information Tables gives the approximate mean number of independent detections of each merged source within each WDB. For the Survey and 6x data sets this ranges from 2-3 detections per group. For the highly redundant Calibration Scan data, the mean number of detections is ~220, emphasizing the value of these data as a source for precision photometry and time domain studies.
The Merged Extended Source Cross-Reference Tables are discussed
in the context of the general WDB merging process in
Appendix 6. The formats of the
Survey, 6x, Calibration and LMC/SMC Calibration Cross-Reference Tables
are the same,
and are given in A6.3c.
e. Combined Calibration Scan Images and Source Lists
| Product | Number |
|---|---|
| Images | 735 |
| Extracted Sourcesa | 246,760 |
Notes to Table 3:
a - Includes extractions identified
as spurious detections of image artifacts.
i. Combined Calibration Scan Images
The Combined Calibration Scan images are a set of J, H and Ks FITS images that were produced by combining the Atlas Images from the 600-3700 individual scans of each of the 35 Calibration Fields acquired during the Survey. The combined images are the deepest observations enabled by 2MASS, reaching nominal sensitivities 3.5 to 4.5 magnitudes deeper than the Survey or single Calibration scan measurements (not taking into account confusion which can be significant in the Galactic Plane fields).
There are 21 FITS images available for each Calibration field. In each of the 3 survey bands, J, H and Ks, the images are:
- Full-area combined images for all north-going scans
- Full-area combined images for all south-going scans
- Full-area combined images of all scans (north+south-going)
- Depth-of-coverage maps for the full-area north- and south-going combined images
- Combined north-going scan images masked to an area covered by >30% of the available scans
- Combined south-going scan images masked to an area covered by >30% of the available scans
- Combined north+south scan images masked to an area covered by >30% of the available scans
The north- and south-going scan data were combined separately for each field to facilitate the identification of bright star latent images (IV.7). Latents trail in opposite directions from the parent star in the different scan directions. Within each calibration observation (set of six calibration scans), each scan was displaced by 5" in RA from the previous scan to minimize systematic pixel effects. These offsets, combined with small telescope pointing variations resulted in a spread in the footprint of the Calibration Scans on the sky. The full-area combined images include most of the area covered by all scans, regardless of the depth of coverage. The masked combined images are truncated to include only the area covered by at least 30% of the available scans.
Production of the Combined Calibration Scan Images is described in Appendix 7. This was accomplished by registering the images from each individual scan onto a common position grid (A7.2a), identifying and filtering out the noisiest or otherwise contaminated scans (A7.2d), scaling the images from each scan according to the measurement sensitivity (A7.2c) and forming the noise-variance-weighted averages of the selected images. The FITS headers of the combined images contain full WCS information and photometric zero points.
Users are strongly encouraged to familiarize themselves with the Cautionary Notes concerning the Combined Calibration Scan Images given in A7.1.b.
The Combined Calibration Scan FITS images can be accessed
via the web-based Combined Calibration
Field Data Retrieval Page (A7.4).
ii. Combined Calibration Scan Extracted Source Lists
The Combined Calibration Scan Extracted Source Lists contain positional and three-band photometric data for 246,760 sources detected on the deep, Combined Calibration Scan FITS Images.
Preparation of the extracted source lists is described in A7.3. Conservative source detection was performed separately on the north- and south-going combined images. The final source lists include only sources that were detected on both directions - this eliminates a large fraction of the spurious detections of random noise and latent images. Simple circular aperture photometry of extracted sources, bootstrapped to the measured brightness of the 2MASS Survey calibration stars in each field, is provided. The format of the extracted source tables is presented in A7.3d. The general properties of combined calibration scan extracted source lists are presented in A7.5, including source counts, comparisons with the 2MASS All-Sky Point Source photometry and astrometry.
It is expected that most users will perform custom PSF-fitting source extraction on the Combined Calibration Images because of the high degree of confusion and complexity in the fields. Therefore, the extracted source lists are intended to be "introductory" only, and to provide guidance on issues such as photometric and astrometric calibration. Source detection used a relatively conservative SNR~5 threshold. The circular aperture photometry will be adequate for isolated, unresolved sources, but will be biased for extended and/or multiple objects that will be better characterized using specialized apertures and source deblending. The extracted source lists have not been rigorously cleaned of spurious artifact detections. The lists do contain a "confusion and contamination" flag, cc_flg, that is analogous to that in the Catalogs, Reject Tables and Working Databases. However, there may be unflagged artifact detections in the lists.
Users are strongly encouraged to review the Cautionary Notes concerning the Combined Calibration Scan Extracted Source Lists given in A7.1.b.
The Combined Calibration Scan Source Lists may be accessed via the web-based Combined Calibration Field Data Retrieval Page (A7.4), or using the GATOR Catalog Access Service at the Infrared Science Archive.
[Last Updated: 2010 February 10; by R. Cutri]
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