Appendix 3. Long Exposure (6x) Scan Databases, Catalogs and Images

1. Overview of the Long Exposure (6x) Observations and Data Products

d. Cautionary Notes

1. General 6x Notes
2. 6x Point Source Working Database and Catalog
3. 6x Extended Source Working Database and Catalog
4. 6x Image Atlas

Before using the 2MASS extended mission 6x data products, users are strongly encouraged to familiarize themselves with the Cautionary Notes to the All-Sky Data Release data products in 1.6. The 2MASS 6x observations were made using the same facilities and the same basic strategy as the main survey, and the 6x data were reduced with a software pipeline very similar to that used for the main survey. Therefore, most of the characteristics and limitations associated with the All-Sky Release data products will apply to the 6x data products.

The sections below describe features and caveats that are unique to the 6x data products and highlight a few of the important features that are common to the 6x and main survey data products. These sections are intended to supplement the Cautionary Notes to the All-Sky Data Release but not to replace them.

Section A3.1.d.i presents general Cautionary Notes, and A3.1.d.ii, A3.1.d.iii and A3.1.d.iv provide essential information for the 6x point and extended source Working Databases, Catalogs and Atlas Images, respectively.

i. General 6x Notes

• Please see General Cautionary Notes for the 2MASS All-Sky Release (I.6.a)
• Quality of 6x Data Products

Although the objective of the 2MASS Extended Mission is to produce Databases, Catalogs and an Image Atlases that meet the same standards of reliability and quality as those of the main survey, the 2MASS 6x data products have not received the same level of scrutiny as the All-Sky Release products. The 6x data were obtained with same well-characterized facilities as the main survey and reduced with essentially the same processing software that incorporates multiple levels of quality assurance. The 6x release data products were generated using similar criteria and best practices developed to produce the 2MASS All-Sky Catalogs and Image Atlas. However, the reliability of the 6x products has not been thoroughly validated. Consequently, users are encouraged to use the 6x databases, catalogs and images with care, and to follow these recommendations:

• Understand the value and limitations of the 6x measurements relative to the main survey data products documented in this section.
• Understand and heed the differences between the 6x Working Databases, which contain one or more detections of real astrophysical source along with spurious extractions of noise excursions, image artifacts and transient events, and the 6x Catalogs, which contain only one measurement of multiply-detected sources and have been cleaned of most spurious detections.
• Pay attention to and take advantage of the numerous extracted source quality flags that are provided with each database and catalog entry (e.g. rel, cc_flg, ph_qual, vc) when assessing source reliability and measurement quality.
• Defer to the All-Sky Release PSC and XSC for measurements of sources near to or brighter than the 6x READ2 7.8s exposure saturation level (see below), and wherever practical to the general All-Sky Catalog source extractions because they have received a greater level of review and validation.
• When in doubt about the reliability or measurement accuracy of a source in the 6x WDBs or Catalogs, examine its images using the 6x Image Atlas.



• Sky Coverage Considerations

  Limited Area

  Unlike the main survey, 2MASS 6x observations covered only approximately 589 deg² distributed among 30 different fields containing targets of astronomical interest (see Figure 1 in A3.2). Most of the 6x area is contained in two large regions covering the Large and Small Magellanic Clouds, in which 383 deg² and 127 deg² are surveyed, respectively. The remaining ~80 deg² is in 28 fields, the largest of which is the Lockman Hole in which 28.3 deg² were observed.

  Duplicate 6x Tile Numbering

  The 6x fields are made up of a set of predefined 6° and 1° long (in declination) tiles, analogous to those used for the main survey and survey calibration observations, respectively. Duplicate tile numbers were incorrectly assigned to several scans in the Chameleon 2 and Hydra 6x fields that did not cover the same region of sky. Therefore, caution should be used when selecting or searching for observations according to tile number in those fields. See the Chameleon 2 and Hydra field description pages linked to Figure 1 in A.3.2.b for details.

  Coverage Gaps

  Within individual 6x fields, there may be gaps in coverage for two reasons. First, some tiles were not scanned because of scheduling considerations, or were not observed in photometric conditions. Second, a telescope commanding script error caused the starting position of some 1° 6x scans to be displaced in declination from their nominal locations. This resulted in coverage gaps between adjacent declination bands of tiles ranging from a few arcseconds to as much as 4°. The Lupus 6x field scan coverage map shown in Figure 1 illustrates gaps from both missing scans scan offsets at the declination boundary

  Observations of the Chameleon II, Hydra Abell 754 and Abell 3420 6x fields were most severely affected by the 1°-scan starting point problem. Coverage in those fields is extremely fragmented, and is split into two distinct regions separated by large distances in declination. Coverage in the northern and southern sections of these fields can also be irregular. Users should consult the sky coverage maps before using data from the Chameleon 2, Hydra, Abell 754 and Abell 3420 fields.

  Effective Coverage of the Catalogs

  The 6x-PSC and 6x-XSC cover slightly smaller areas than shown in the sky coverage maps for each field because Catalog sources were required to lie >10´´ and >15´´ away from Tile edges, respectively (A3.6.c). This tile-edge safety boundary minimizes edge-effects that include non-three-band coverage due to slight misalignments of the focal plane arrays, distortion at the edges of focal planes, and partial coverage of extended sources close to tile edges.

  As in the main survey, coverage area is also lost in the 6x Catalogs due to the presence of bright stars and the influence of their bright image wings, ghosts images, diffractions spikes, latent images and other image artifacts (A.3.5.e). Extractions made near bright stars are carried in the 6x Working Databases, but are generally flagged as spurious detections of artifacts and therefore not passed to the reliable Catalogs. This has the effect of masking out an area around bright stars that scales with the star brightness. Figure 2 shows the spatial distribution of 6x-PSC sources in the Lupus field, illustrating the impact of bright star masking.

  Between two and three square degrees of effective coverage was omitted from the 6x Catalogs because scans in several 6x fields were incorrectly labeled with the same tile numbers even though they did not cover the same region of sky (see A3.6.b). Because only one scan of tiles apparently observed more than once were selected for the catalogs, this duplicate labeling caused several scans and parts of scans with unique sky coverage to be excluded from the Catalogs. The extracted point and extended source information for the omitted area is still accessible in the 6x WDBs.

  Figure 1 - Sky map showing the outlines of 6x scans covering the Lupus field (blue lines), and the distribution of 6x-PSWDB extractions (red dots). Note the coverage gaps at the declination boundaries between some tiles, and the missing scans in the eastern half of the southern band of tiles.	Figure 2 - Sky map showing the outlines of 6x Lupus field scans (blue lines) and distribution of 6x-PSC sources (red dots). Note the effective masking of 6x-PSWDB extractions around several bright stars in the northern portion of the field.




• The READ1/READ2-READ1 Sensitivity Gap

For the 6x observations, the READ2 integration time was increased from 1.3 s to 7.8 s, but the READ1 integration time remained at 51 ms, the same as in the main survey (A3.4.a). The brightest unsaturated sources in the main survey's 1.3 s READ2-READ1 images were also detectable as the faintest sources in the 51 ms READ1 frames. This photometric overlap was essential for providing contiguous and internally consistent magnitude coverage across the dynamic range of the survey. The 6x observations do not necessarily have overlapping READ1 and READ2-READ1 photometry because of the increased READ2 integration time. Thus, many saturated sources in the 6x READ2-READ1 exposures are too faint to be detected in the 51 ms READ1 frames, or are detected only at very low SNR levels. The resulting incompleteness and/or faint detections in the 8-11 mag READ1/READ2-READ1 boundary introduces artificial features in source counts and color-magnitude diagrams that are discussed below.

The 2MASS 6x data primarily exist to report the faint extension of the original survey results provided by the 6x increase in READ2 exposure time. Photometry of the brighter 7.8 s READ2 and 51 ms READ1 extractions are included in the 6x catalogs given their utility for variability and proper motion studies.

Photometry for bright sources that are near to or brighter than the saturation level of the 6x 7.8 s READ2 exposures should be obtained from the 2MASS All-Sky Catalogs. In general, photometry for any source with high SNR measurements in the All-Sky Catalogs should take precedence over the 6x measurements because they have received a greater level of review and validation



• Data Reduction

The raw 6x imaging data were reduced with a modified version of the 2MAPPS v3.0 software pipeline that was used for the main survey. The changes made to 2MAPPS are summarized in A3.5. Basic processing steps and pipeline output products remained essentially unchanged from main survey data processing.

Most modifications in the reduction software were designed to accommodate the longer READ2 exposure times of the 6x data and the resulting higher source and background count rates. Secondary changes included corrections to software bugs that were known to exist from main survey data processing, and a small number that further improved data quality and brought final output data formats closer to those of the release products.



• Photometric Considerations
• Photometric System

Magnitudes reported in the 6x Point and Extended Source Working Databases and Catalogs are in the same natural 2MASS photometric system as the All-Sky PSC and XSC. The system bandpasses are described in III.1.b. Transformations between the 2MASS and selected other photometric systems, updated from the initial forms derived by Carpenter et al. (2001, AJ, 121, 2851), are given in VI.4.b.

• Photometric Calibration
• 6x measurements were calibrated using photometric zero points derived from the same hourly observations of calibration fields each night that were used to calibrate the main survey. The calibration fields were observed using the normal survey 1.3 s READ2-READ1 exposure time, and instrumental zero points were adjusted to compensate for the longer 6x READ2-READ1 exposure time, as described in A3.5.c.
• Calibration of the LMC/SMC 6x observations were supported by the addition of five new calibration fields located in and around the fields of those galaxies. The location of the new calibrations fields and the secondary standard stars are described in A3.5.c.



• Astrometric Considerations

Position reconstruction for the 6x observations (A3.5.b) used the 2MASS All-Sky PSC as the primary astrometric reference catalog, rather than the Tycho-2 Catalog that was used as the reference for the main survey.

The relatively high density of astrometric reference stars provided by the All-Sky PSC results in excellent global 6x astrometric performance relative to the All-Sky PSC and external catalogs, as documented in A3.2.c. For example, the mean radial offset between the reported positions of stars in the 6x-PSC source and the UCAC2 astrometric catalog is 95 mas which is essentially the same as measured between All-Sky PSC and UCAC astrometry. In addition, the software error that resulted in uncorrected distortion for Read_1 sources in the main survey data processing has been corrected for the 6x processing. Thus, there is no systematic bias with cross-scan position between READ1 and READ2-READ1 astrometry for 6x point sources.

ii. 6x Point Source Working Database and Catalog

• Please see Cautionary Notes for the 2MASS All-Sky Release Point Source Catalog (I.6.b)
• 6x Point Source Catalog vs. Working Database

  The 6x-PSC is released as part of the 6x-PSWDB and not as a separate table as was done for the All-Sky Survey. The subset of rows in the 6x-PSWDB that comprise the 6x-PSC are identified as having a value of cat=1 in their source entry.

  The 6x-PSWDB contains all extractions from all observations made in the long exposure mode These include one or more independent detections of real astrophysical sources, detections of low SNR noise excursions, and spurious detections of image artifacts and transient events such as cosmic ray strikes and meteor trails.

  The 6x-PSC is comprised of the subset of 6x-PSWDB entries that define a uniform, complete and reliable list of near infrared sources. One unique detection of inertial near infrared sources is selected for objects observed multiple times, so the 6x-PSC can be thought of as a "snapshot" of the near infrared sky. The criteria used to select the 6x-PSC from the 6x-PSWDB are described in A3.6, and are analogous to those used to draw the 2MASS All-Sky PSC from the Survey Point Source Working Database.

• The 6x-PSWDB "Reliability" Flag (rel)

The 6x-PSWDB contains a large number of spurious detections of faint noise excursions, image artifacts, and transient events such as cosmic ray strikes and meteor trails in addition to detections of real astrophysical sources. To help discriminate between real and spurious extractions, each 6x-PSWDB entry has been assigned a reliability flag value, rel, that is related to the probability that it is a detection of a real astrophysical object at the time of the 2MASS observation. The reliability flag is a single character in the range "A" to "F", with "A" representing sources with the highest reliability and "F" the lowest. Appendix 5 contains a description of the algorithm used to assign the reliability flag values and limitations of the estimator.

Users should use the 6x-PSC, or should select 6x-PSWDB sources having a reliability flag value of rel="A" to minimize the number of spurious extractions. Caution should be exercised when using any 6x-PSWDB source with a lower probability of reliability. When in doubt about the reliability of a 6x catalog or database entry, we strongly recommend examining the image of the source using the 6x Image Atlas.

Table 1 contains a breakdown of the 6x-PSWDB and 6x-PSC according to reliability flag value, and the probability of reliability associated with each value of rel. The 6x-PSC source selection criteria are designed to identify reliable sources in the 6x-PSWDB, so the number 6x-PSC entries declines rapidly with decreasing probability of reliability. There are still a small number of low reliability sources in the 6x-PSC. Visual examination of the 42 6x-PSC sources with rel="F" reveals that most (29) are in fact detections of real objects that are usually confused, in close multiple systems, or are measurements of brighter stars in the difficult READ1 and READ_2 sensitivity "gap." The remaining 13 rel="F" extractions in the 6x-PSC are spurious detections in the disks of large galaxies, in the halos of bright stars, and glints at the edge of images.

Table 1 - Breakdown of 6x-PSWDB and 6x-PSC Entries by Estimated Reliability (rel)

rel Value	Number (6x-PSWDB)	Number (6x-PSC)	Probability of Reliability
A	15604449	8277215	>90%
B	1390727	331734	80-90%
C	570639	24476	70-80%
D	583122	3672	50-70%
E	1007938	1	30-50%
F	4866827	496	<30%





In general, unreliable database and catalog entries exhibit one or more of the characteristics described in Table 2. Users should scrutinize such sources carefully. However, possession of any one or more of these attributes does not necessarily guarantee that a detection is unreliable, only that it merits examination. We strongly advise examining images of any 6x-PSWDB or 6x-PSC entries whose reliability may be in question.

Table 2 - Common Characteristics of Unreliable 6x-PSWDB and 6x-PSC Sources

Condition	Source Attribute
Single band detection	rd_flg="0" in two bands
Contamination or confusion flagging	cc_flg != "000"
Large value of profile-fit photometry 2 value	rd_flg="2" and [jhk]_psfchi >> 1
Low frame-detection count for SNR>7 sources	ndet[1,3,5] 1 and [jhk]_snr >7
READ1 detection that is fainter than the READ2 saturation limits	(rd_flg[1]="1" and j_m>11) or (rd_flg[2]="1" and h_m>10.5) or (rd_flg[3]="1" and k_m>10)
Source is fragment of an extended source	gal_contam="2"
Source is associated with a solar system object	mp_flg="1"




• The Default Magnitude

  As was the case for the main survey, 6x point source photometry is measured differently for three brightness regimes. The J, H and K_s default magnitudes (j_m, h_m, k_m) listed for each object represent what are the best available measurements in each band, as determined by the automated processing. The rd_flg specifies the origin of the default magnitude in each band. Table 1 below contains the possible values of rd_flg, a description of the photometry algorithm corresponding to each value, and the number of instances of each value, by band, in the 6x-PSWDB and 6x-PSC.

  Figures 3-5 show the J, H and K_s 6x-PSWDB differential source counts in the 6x Lockman Hole field, decomposed by rd_flg. The majority of detected sources are non-saturated in the 7.8 s READ2-READ1 6x exposures, and have default photometry from profile-fitting (rd_flg=2). The 6x counts extend approximately one magnitude fainter than in the main survey, and 6x READ2-READ1 saturation occurs at J>11, H>10.5 and K_s>10 mag, approximately two magnitudes fainter than in the main survey observations. For a small fraction of the non-saturated READ2-READ1 detections profile-fitting photometry fails, and curve-of-growth-corrected aperture photometry provides the default magnitude (rd_flg=4).

  The default magnitudes for point sources in the brightness range 4.5<J<11, 4<H<10.5, 3.5<K_s<10 mag are taken from aperture photometry measurements made on the 51 ms READ1 exposures (rd_flg=1). Because the READ1 exposure time was not increased for the 6x observations, measurements of sources at the faint end of the READ1 regime will be of relatively low SNR. Higher precision measurements of such stars are provided by the All-Sky PSC wherein these objects were in the high SNR, non-saturated READ2-READ1 range.

  Stars brighter than approximately J~4.5, H~4 and K_s~3.5 mag saturate even the 51 ms exposures. Photometry of such objects is performed using a 1-d radial profile fit to the azimuthally-averaged image profile on the READ1 exposures, and the rd_flg value for the appropriate band is "3". The boundary between rd_flg=1 and rd_flg=3 for the 6x data is the same as for the main survey and users should defer to the All-Sky PSC for fluxes and positions for these sources. Inclusion of these saturated observations in the 6x databases is largely to support variability and proper motion studies.

  Figure 3 - Differential 6x PSWDB J-band source counts for the Lockman Hole field. Counts are broken down into different rd_flg populations denoted by color..	Figure 4 - Differential 6x PSWDB H-band source counts for the Lockman Hole field. Counts are broken down into different rd_flg populations denoted by color..	Figure 5 - Differential 6x PSWDB Ks-band source counts for the Lockman Hole field. Counts are broken down into different rd_flg populations denoted by color..

  
  

• Origin and Quality of Photometry - 6x Point Source Quality Flags

  The origin and general quality of the default magnitude photometry listed in the 6x-PSWDB and 6x-PSC are summarized by a number of informative flags that accompany each source. It is essential that users refer to these flags when interpreting photometry for any source in the 6x Point Source products. The primary quality indicator flags include rd_flg, ph_qual, cc_flg and bl_flg. Each of these is comprised of three characters, each corresponding to one band; the first character is the J-band value, the second is the H value, and the third is the K_s value.

  rd_flg - Origin of Point Source Photometry

  The rd_flg denotes the origin of and algorithm used to derive the quoted default magnitudes for each 2MASS 6x Point Source. Table 3 contains the possible values of rd_flg, their interpretation and the number of instances of each value in each band in the 6x point source tables.

  Table 3 - Definition of rd_flg and Occurrences in the 6x-PSWDB and 6x-PSC

  rd_flg Value	Source of Photometry	6x-PSWDB			6x-PSC
  		N(J)	N(H)	N(Ks)	N(J)	N(H)	N(Ks)
  0	Not detected in this band. The default magnitude is the 95% confidence upper limit derived from a 4" radius aperture measurement taken at the position of the source on the 6x Atlas Image. The sky background is estimated in an annular region with inner radius of 14" and outer radius of 20". The uncertainty in the default magnitude, [jhk]_msig is null in these cases.	4002889	7551654	11487578	90851	1001170	2881445
  1	"READ1" aperture photometry. The default magnitude is derived from aperture photometry (IV.4.a.i) measurements on the 51 ms "READ1" exposures. The aperture radius is 4", with the sky background measured in an annulus with an inner radius of 14" and an outer radius of 20". Used for sources that saturate one or more of the 7.8s "READ2" exposures, but are not saturated on at least one of the 51 ms "READ1" frames. The photometric uncertainty in these cases is the RMS variation of the measured flux from all frames on which a source falls. If the source was measured on only one frame, the [jhk]_msig value becomes a flag, and has a value of >8.0 mag in the appropriate band.	61596	78655	85856	35317	41590	46922
  2	READ2-READ1 profile-fit photometry. The default magnitude is derived from a profile-fitting measurement (IV.4.b) made on the 7.8 sec "READ2" exposures. The profile-fit magnitudes are normalized to curve-of-growth-corrected aperture magnitudes. This is the most common type in the 6x-PSWDB, and is used for sources that have no saturated pixels in any of the 7.8 sec exposures. The uncertainties for the profile-fit measurements, [jhk]_msig, are derived from a goodness-of-fit metric that includes terms for uncertainties in the photon noise in the sky background reference, the noise in the source brightness, and how well the fitted profile matches the actual point-spread-function of the source.	19771707	15960573	11935820	8453862	7428661	5503591
  3	Saturated in READ1. The default magnitude is derived from a 1-d radial profile fitting measurement made on the 51 ms "READ1" exposures. Used for very bright sources that saturate all of the 51 ms "READ1 1" exposures. The photometric uncertainty in these cases is the RMS variation of the measured flux from all frames on which a source falls. If the source was measured on only one frame, the [jhk]_msig value becomes a flag, and has a value of >8.0 mag in the appropriate band.	260	345	304	158	201	185
  4	READ2-READ1 aperture photometry. The default magnitude is derived from curve-of-growth-corrected 4" radius aperture photometry measurements (IV.4.c) on the 7.8 s "READ2" exposures. This is used for sources that are not saturated in any of the READ2 frames, but where the profile-fitting measurements fail to converge to a solution. These magnitudes are the same as the standard aperture magnitudes (j_m_stdap, h_m_stdap, k_m_stdap), but when they are the default magnitudes, it generally implies that they are low quality measurements. The photometric uncertainty in these cases is the RMS variation of the measured flux from all frames on which a source falls.	12542	9163	6940	56	36	29
  6	Inconsistent deblend. The default magnitude is the 95% confidence upper limit derived from a 4" radius aperture measurement taken at the position of the source on the 6x Atlas Image. The sky background is estimated in an annular region with inner radius of 14" and outer radius of 20". This is used for pairs of sources which are detected and resolved in another band, but are detected and not resolved in this band. This differs from a rd_flg="0" because in this case there is a detection of the source in this band, but it is not consistently resolved across all bands. The [jhk]_msig value is null for these cases.	150997	400713	456392	57347	165206	205308
  9	Detection at this location, but no useful brightness measurement possible. The default magnitude is the 95% confidence upper limit derived from a 4" radius aperture measurement taken at the position of the source on the 6x Atlas Image. The sky background is estimated in an annular region with inner radius of 14" and outer radius of 20". This is used for sources that were nominally detected in this band, but which could not have a useful brightness measurement from either profile fitting or aperture photometry. This often occurs in highly confused regions, or very near Tile edges where a significant fraction of the measurement aperture or sky annulus falls off the focal plane.	23711	22599	50812	3	730	114

  
  

  ph_qual - Photometric Quality

  The photometric quality flag, ph_qual, provides a concise guide to the quality of the default point source photometry that is based on signal to noise ratio, measurement quality, detection statistics, etc. Sources with ph_qual="A" in a band, have <10% measurement uncertainties, and generally have the highest quality characterization. Measurement quality decreases with alphabetically increasing values of ph_qual. The values of "U" and "X" are reserved for non-detections/inconsistent deblends or otherwise unmeasurable detections in a band. The value "Z" denotes extractions that are believed to be spurious detections of image artifacts. Sources with ph_qual="Z" are not selected for the 6x Point Source Catalog. The ph_qual value is set in each band according to the precedence listed in Table 4 below. For example, a measurement in a band that satisfies the conditions for "Z" is not tested for other conditions. Table 4 also gives the number of sources in each band that have the corresponding quality value in the 6x-PSWDB and 6x-PSC.

  Table 4 - Definition of ph_qual and Occurrences in the 6x-PSWDB and 6x-PSC

  ph_qual Value (1 per band)	Meaning	6x-PSWDB			6x-PSC
  		N(J)	N(H)	N(Ks)	N(J)	N(H)	N(Ks)
  Z	Detection in this band is identified with an image artifact. The corresponding cc_flg value in this band is "C", "D", "G" or "P". Sources with ph_qual="Z" in any band are excluded from the 6x-PSC.	604809	471768	273603	0	0	0
  X	There is a detection at this location, but no useful brightness estimate can be extracted using any algorithm. rd_flg=9 and default magnitude is null.	66	4165	32359	3	730	114
  U	Upper limit on magnitude. Either source is not detected in this band (rd_flg=0), or it is detected, but not resolved in a consistent fashion with other bands (rd_flg=6). A value of ph_qual="U" does not necessarily mean that there is no flux detected in this band at the location. Whether or not flux has been detected can be determined from the value of rd_flg. When rd_flg="0", no flux has been detected. When rd_flg="6", flux has been detected at the location where the images in all three bands (JHKs) were not deblended consistently.	4150606	7944790	11937902	148198	1166376	3086753
  F	This category includes rd_flg=1 or rd_flg=3 sources where a reliable estimate of the photometric error, [jhk]_msig, could not be determined. The uncertainties reported for these sources in [jhk]_msig and [jhk]_msigcom are flags and have numeric values >8.0.	3913	6773	9593	1658	3211	4325
  E	This category includes detections of any brightness or SNR where: 1) the goodness-of-fit quality of the profile-fit photometry was very poor (rd_flg=2 and [jhk]_psf_chi10.0); or 2) where profile-fit photometry did not converge, and an aperture magnitude is reported (rd_flg=4); or 3) where the number of frames on which a source was detected was too small in relation the number of frames in which a detection was geometrically possible (rd_flg=1 or rd_flg=2). The 2 limit results in many relatively bright (SNR>>10) unresolved double stars being found in this category.	63602	53217	33655	29331	22287	12007
  A	Detections in any brightness regime where valid measurements were made (rd_flg=1,2 or 3) with [jhk]_snr10 AND [jhk]_msig0.10857.	10429521	7098971	5016932	5919095	4055241	2843514
  B	Detections in any brightness regime where valid measurements were made (rd_flg=1,2 or 3) with [jhk]_snr7 AND [jhk]_msig0.15510.	3987039	2759642	1736451	2148894	1600838	978771
  C	Detections in any brightness regime where valid measurements were made (rd_flg=1,2 or 3) with [jhk]_snr5 AND [jhk]_msig0.21714.	2731914	3129174	2106271	362428	1455871	1083313
  D	Detections in any brightness regime where valid measurements were made (rd_flg=1,2 or 3) with no [jhk]_snr or [jhk]_msig requirement.	2052232	2555202	2876936	27987	333040	628797

  Contamination and Confusion - cc_flg

  In the 2MASS 6x-PSWDB, the cc_flg values identify extractions that are either real astrophysical sources whose measurements are confused and/or contaminated by other nearby sources or image artifacts, or that are spurious detections of image artifacts. The algorithms used to identify confused and/or contaminated sources and to set the values of the cc_flg during 6x data processing are discussed in A3.5.e. The encoding of the cc_flg is summarized in Table 5 below.

  Users should select sources with cc_flg="000" if they need samples of sources that have the lowest probability of contamination. However, this constraint may compromise the completeness of source selection, particularly in crowded fields where a significant fraction of sources are affected by photometric confusion (cc_flg="c").

  6x-PSWDB or 6x-PSC entries containing a cc_flg value other than "0" in any detected band should be viewed with caution. A cc_flg value of [C,D,G,P] indicates that the entry is associated with an image artifact. If the cc_flg has any of these values in all detected bands, then the WDB entry is most likely a spurious detection of the artifact. WDB entries with non-zero but lower case letter cc_flg values [b,c,d,p,s], or that have a capital letter cc_flg value in fewer than all detected bands, are probably real sources but with photometry that may be corrupted and/or biased by artifacts or confusion with nearby sources.

  6x-PSWDB entries that are identified as artifacts (cc_flg = [C,D,G,P]) in any band are not passed to the 6x-PSC. However, the 6x-PSC does contain sources that may be contaminated by artifacts or confusion (cc_flg = [b,c,d,p,s]) in one or more bands.
  
  

  Table 5 - Definition of cc_flg and Occurrences in the 6x-PSWDB and 6x-PSC

  cc_flg Value (1 per band)	Meaning	6x-PSWDB			6x-PSC
  		N(J)	N(H)	N(Ks)	N(J)	N(H)	N(Ks)
  0	Real source unaffected by artifacts, or not detected in that band	19068694	20954202	22383866	6764368	7387179	8002265
  p	Real source whose photometry may be contaminated by a persistence (latent) image of a bright star	108966	122566	42175	30689	56385	12142
  c	Real source whose photometry may be biased because of confusion with nearby brighter source	3692108	2121811	1132896	1607010	1036306	541756
  d	Real source whose photometry may be contaminated by a diffraction spike	6068	1027	10083	2755	4654	4629
  s	Real source whose photometry may be contaminated by a horizontal "stripe" (electronic cross-talk) due to a bright star	492934	303268	150718	232664	152899	76543
  b	Real source that is confused in the bandmerging process because of a nearby source	1319	2598	1629	108	171	259
  P	Spurious detection of a Persistence (Latent) image of a bright star	286063	292724	103307	0	0	0
  C	Spurious detection in the Confusion "halo" around a bright star	154010	103995	77539	0	0	0
  D	Spurious detection on a Diffraction spike from a nearby bright star	91575	98060	70611	0	0	0
  G	Spurious detection of a dichroic Glint	121965	16074	50878	0	0	0

  
  
  

• Photometric Uncertainties

The photometric uncertainties given in the [jhk]_msig columns in the 6x-PSWDB and 6x-PSC are equivalent to the "corrected" uncertainties in the [jhk]_cmsig columns in the All-Sky PSC and Survey Point Source Reject File for profile-fit photometry.

One of the modifications made to the data reduction pipeline for 6x data processing was a change to the profile-fitting photometry error model to compensate for the overestimation of photometric uncertainties that affected the main survey point source photometry. As a result, there was no need to apply the a posteriori corrections to the 6x photometric uncertainties that were applied to the main survey data. There are no [jhk]_cmsig columns in the 6x PSWDB and 6x-PSC.



• Depth of the 6x-PSWDB and 6x-PSC

While the 2MASS 6x observations probe one magnitude deeper than the main survey on average, the actual sensitivity achieved by individual scans varied with the atmospheric transparency, seeing and background emission levels. Histograms of the estimated magnitudes at which point source SNR=10 for all northern and southern observatory 6x scans are shown in Figure 6. As with the main survey, the achieved sensitivities range by nearly +0.5 mag in the extreme depending on the atmospheric conditions.

The point source sensitivity of the 6x observations are also constrained by source confusion noise. Differential source counts from the Lockman Hole and Cygnus 6x fields are compared with the counts from the same areas of the main survey in Figures 7 and 8, respectively. The 6x counts in the high galactic latitude Lockman Hole field turn over approximately one magnitude fainter than those in the All-Sky PSC, near J=17.6, H=16.9 and K_s=16.1 mag. The improvement relative to the All-Sky PSC is not as great in the Cygnus field, which has an order of magnitude higher source density than the Lockman Hole region.

Figure 6 - Distribution of the estimated SNR=10 magnitudes for all photometric 6x scans, based on atmospheric transparency, seeing and background. These estimates do not take into account the effects of confusion.	Figure 7 - Area-normalized 6x-PSC and 6x-XSC differential source counts in the Lockman Hole 6x field, compared with those in the same area in the All-Sky PSC and XSC. 6x-PSC and 6x-XSC counts are shown by the red and blue lines, respectively. All-Sky PSC and XSC counts are shown in the grey and cyan shaded regions.	Figure 8 - Same as Figure 7, but for the Cygnus 6x field region.




• Effect of the READ1/READ2-READ1 Sensitivity Gap

The effective sensitivity "gap" between the 51 ms READ1 and 7.8s READ2 6x exposures results in photometric biases between 6x and All-Sky PSC data, and can produce artifacts in source count curves and color-magnitude diagrams generated using 6x point source data. Differential point source counts from the Lockman Hole 6x field are shown in Figure 9. The small discontinuities near 10.5-11 mag range are caused by the incompleteness and systematic flux overestimation of low SNR measurements in the READ1 exposures. The same effects cause the horizontal discontinuity of points near K_s=10.5 mag in the 6x color-magnitude diagram from the same field shown in Figure 10.

Photometry for bright sources that are near to or brighter than the saturation level of the 6x 7.8 s READ2 exposures should be obtained from the 2MASS All-Sky PSC. In addition, photometry for any point source with high SNR measurements in the All-Sky Catalogs should take precedence over the 6x measurements because they have received a greater level of review and validation


Figure 9 - Differential point source counts from the 6x Lockman Hole observations showing the discontinuities at the READ1/READ2-READ1 boundaries.	Figure 10 - Color-magnitude diagram for point (red) and extended source (green) extractions 6x WDBs in the Lockman Hole field. The black contours trace the density of point sources. The horizontal discontinuity near Ks=10.5 mag is at the READ1/READ2-READ1 sensitivity boundary.




• Photometric Biases Relative to the All-Sky PSC

Point source photometry from the 6x-PSWDB/PSC and All-Sky PSC typically agree on average to within 1-2%. However, there are systematic deviations from this agreement in certain brightness regimes that are caused by flux overestimation in both the 6x and survey data and possible non-linearity in the 6x measurements.

Diagrams showing the photometric differences between the 6x and All-Sky PSC point source photometry plotted as a function of 6x magnitude are included in the detailed field summary pages linked to Figure 1 in A3.2.b. An example of one such diagram from the Abell 3558 6x field is shown in Figure 11. Several features can be seen in this diagram that highlight biases in the the 6x photometry relative to the main survey measurements:

• The average offset between 6x and survey photometry is small over most of the 51 ms READ1 and 7.8 s READ2-READ1 exposure ranges. The overall agreement in the photometric levels was in fact imposed by minor adjustments to some of the 6x instrumental zero points.
• There is a downward trend in the photometric residuals near 10-10.5 mag where the default 6x photometry comes from low SNR measurements on the 51 ms READ1 exposures. This is caused by statistical flux overestimation of the low SNR 6x READ1 (rd_flg="1") detections, and is seen in all bands in all of the 6x fields.
• There is a slight upturn in the photometric residuals near 11 mag, where the default 6x photometry comes from profile-fit photometry of sources near the saturation limit of the 7.8 s READ2 exposures. Because the amplitude of this bias increases with increasing source brightness in the READ2 (rd_flg="2") regime, it is probably caused by non-linear response of the 2MASS detectors. However, the strength of this bias varies between the detectors and is not the same for all of the 6x measurements. Southern J-band 6x measurements show the strongest bias, reaching 5-8% for the brightest rd_flg="2" sources. The amplitude of the bias in the southern H-band data is about half of the J-band value, and it is essentially zero in the southern K_s-band data. There is little if any bias between 6x and Survey photometry at the bright READ2 exposure limit visible in the northern 6x measurements, as illustrated by the photometric residual plot for the Pleiades 6x field.
• At the faint end of the distributions, the mean residuals become increasingly positive as a result of statistical flux overestimation of low SNR sources in the survey photometry relative to the higher SNR 6x measurements. This upturn is seen in all bands in all of the 6x fields.

In certain high source density 6x fields, such as Cygnus shown in Figure 12, the 6x point source photometry for sources in the range 12-15 mag becomes systematically brighter than the that from the All-Sky PSC photometry, up to ~5%, until the flux overestimation in the faint All-Sky PSC dominates. The origin of this bias is not understood. It is not the same in all bands, nor is it seen in all high density fields.

Because of the potential bias between 6x and survey point source photometry in the transition region between 51 ms READ1 and 7.8s READ2 measurements from the 6x exposures, high SNR measurements from the 2MASS All-Sky PSC should always take precedence over 6x measurements of the same objects. The 2MASS All-Sky PSC has also benefited from greater validation and scrutiny.

Figure 11 - Differences between 6x and main survey default point source photometry as a function of 6x source default magnitude in the Abell 3558 6x field. Black contours trace the density of individual sources that are shown as light grey points. The large blue points and error bars show the trimmed average and RMS of the 6x-survey magnitude differences for all sources in 0.25 magnitude wide bins.

Figure 12 - Differences between 6x and main survey default point source photometry as a function of 6x source default magnitude in the Cygnus 6x field. Black contours trace the density of individual sources that are shown as light grey points. The large blue points and error bars show the trimmed average and RMS of the 6x-survey magnitude differences for all sources in 0.25 magnitude wide bins.



• Position Reconstruction

A detailed analysis of 2MASS-6x point source astrometry based on comparisons with the 2MASS All-Sky PSC, Tycho 2 and UCAC2 catalogs is presented in A3.2.c. The overall astrometric precision of the 2MASS-6x point source measurements is on par with that of the 2MASS All-Sky PSC. For example, the mean radial offset between positions of stars reported in the All-Sky PSC and the UCAC2 catalog is 95 mas.

• Position Uncertainties

The quoted position uncertainties for 6x point sources not saturated on the 7.8 s READ2-READ1 exposures (rd_flg="2") are approximately 20% larger than expected based on comparisons with the external catalogs. The uncertainty overestimation is most likely the result of an overly conservative correction for systematic errors in the 2MASS All-Sky PSC astrometric reference catalog. The quoted uncertainties for sources in the READ1 (rd_flg="1") brightness regime have are lower than expected from the same comparisons.

• READ1 Distortion Problem Correction

An error in how frame distortion corrections were applied in survey data processing was corrected for the reduction of 6x observation data (A3.5.b.ii). As a result, the small (±50 mas) position bias between sources in the READ1 and READ2-READ1 regimes that is present in the All-Sky PSC is not seen in the 6x-PSWDB astrometry.



• Multiple Detections in the 6x-PSWDB

The 6x-PSWDB contains the point source extractions from all 2MASS 6x observations. Sources falling in regions that were scanned more than once may have multiple, independent measurements in the database. Consequently, source counts derived from the 6x-PSWDB may overestimate the true distributions because of the presence of redundant source detections. Use the 6x-PSC, in which duplicate detections have been removed, to avoid this error.

The spos and sdet columns in the 6x-PSWDB give the number of scans that covered the position of a source and the number of unique scans in which the source was detected, respectively. The average positions and photometry are available for sources detected more than once in the "Merged 6x-PSWDB".



• Duplicate Source Detections in the 6x-PSC

The 6x-PSC Catalog Generation process selected one, unique apparition of sources in scan overlap regions that have multiple detections present in the 6x-PSWDB. There are approximately 1000 cases in the 6x-PSC where this multiple detection resolution process failed and two detections of the same source from different scans were promoted to the Catalog. These failures were caused by confusion that ensued when one or more of the multiply-detected sources were themselves artificially split within their respective scans because of blending with nearby objects or noise excursions. The sources involved are often faint and/or complex, such as small extended edge-on galaxies that trigger multiple point source detections.

Figure 13 shows a histogram of radial separations between pairs of 6x-PSC sources in scan overlap regions. The roll-off in number of pairs with separations less than 5-6" is a result of the effective resolution limit of the 2MASS telescope and camera system. The sharp drop below 2" separation is caused by the 2" match window using in the multiple detection resolution process, and the the pairs with separations <2" are those that were not properly dealt with by the multiple detection resolution process. A similar population of small separation source pairs exists in the All-Sky PSC, as seen in Figure 14 in I.6.b.ix.

Figure 13 - Histogram of separations between pairs of 6x-PSC sources in the scan overlap regions. There are ~1000 pairs with separations <2" that are a result of errors in the multiple detection resolution process.

• Solar System Objects

No attempt was made to remove non-inertially fixed sources from the 6x-PSWDB or 6x-PSC. Sources that are at the positions of known asteroids, comets, planets or planetary satellites at the time of the 6x observations are flagged by having mp_flg="1" (IV.9). These are positional associations and are not necessarily identifications, so they remain in the 6x source lists. Some fraction of the putative detections are chance superpositions of the predicted solar system object positions with background sources. This is particularly likely at low Galactic latitudes, where the density of background stars is large.



• Optical Associations

All 6x point source extractions were positionally correlated with the Tycho 2 and USNO-A2.0 optical catalogs. 2MASS 6x sources that have optical catalog counterparts within ~5´´ have listed in the 6x-PSWDB and 6x-PSC source records the optical catalog identifier (a="T" or "U"), blue and visual-or-red magnitudes, the 2MASS/optical position separation (dist_opt) and position angle (phi_opt). Matches with the Tycho 2 catalog take precedence over the USNO-A2.0 matches in the listing.

These are positional associations, and not necessarily identifications. The positional accuracy of 2MASS and the optical catalogs is sufficiently high that inertial sources should have positions that match to within ~1´´. Optical associations with separations dist_opt >1´´ are either proper motion candidates, or possible cases of chance alignments with the IR sources.

The optical magnitudes listed for the optical associations in the 6x-PSWDB are derived from the Tycho 2 and USNO-A2.0 values. In the case of Tycho 2 associations, the optical magnitudes listed are Johnson B and V magnitudes and are derived from the Tycho blue (B_T) and visual (V_T) magnitudes using the transformations given by (Høg et al. 2000):

V = V_T - 0.090*(B_T - V_T)
B-V = 0.850*(B_T - V_T)

For USNO-A2.0 associations, the optical magnitudes are the photographic blue and red magnitudes taken explicitly from the USNO-A2.0 Catalog.

Two 2MAPPS software errors that affected the optical associations in the main survey data reduction were corrected in 6x data processing:

• The first error incorrectly set the optical association flag to a="0" for optical associations that had dist_opt5´´. Because of this correction, the optical catalog identifier, a, can be used as a reliable test for optical counterparts in the 6x-PSWDB and 6x-PSC.
• The second error had systematically missed 2MASS-USNO-A2.0 source associations in the vicinity of the north and south equatorial poles in the main survey survey data. The 6x observations did not scan close enough to the poles to encounter this problem, but the error was corrected nonetheless.

iii. 6x Extended Source Working Database and Catalog

• Please see Cautionary Notes for the 2MASS All-Sky Release Extended Source Catalog (I.6.c)
• 6x Extended Source Catalog vs. Working Database

  The 6x-XSC is released as part of the 6x-XSWDB and not as a separate table as was done for the All-Sky Survey. The subset of rows in the 6x-XSWDB that comprise the 6x-XSC are identified as having a value of cat=1 in their source entry.

  The 6x-XSWDB contains all extractions of sources believed to be extended relative to the instantaneous point-spread-function (PSF) in all observations made in the 2MASS long exposure mode. These include one or more independent detections of real extended astrophysical sources such as galaxies and galactic nebulae, and detections of single and multiple stars and spurious image artifacts that have extended source signatures.

  The 6x-XSC is comprised of the subset of 6x-XSWDB entries that define a uniform, complete and reliable list of extended near infrared sources. One unique detection of each source is selected for objects observed multiple times, so the 6x-XSC can be thought of as a "snapshot" of the near infrared sky. The criteria used to select the 6x-XSC from the 6x-PSWDB are described in A3.6, and are analogous to those used to draw the 2MASS All-Sky XSC from the Survey Extended Source Working Database.
  
  

• The 6x-XSWDB "Reliability" Flag (rel)

2MASS 6x extended source processing (see A3.5.d) attempts to identify all sources that are not well-fit by a single PSF. Therefore, the 6x-XSWDB includes true extended sources, such as galaxies and nebulae. Because the algorithms are not perfect, the 6x-XSWDB also contains detections of close multiple stars and spurious detections of artifacts produced by background gradients around bright stars.

To help discriminate between real extended sources and multiple stars and spurious extractions, each 6x-XSWDB entry has been assigned a reliability flag value, rel, that is related to the probability that it is a detection of a real, extended astrophysical object at the time of the 2MASS observation. The reliability flag is a single character in the range "A" to "F", with "A" representing sources with the highest reliability and "F" the lowest. Appendix 5 contains a description of the algorithm used to assign the reliability flag values and limitations of the estimator.

Table 6 contains a breakdown of the 6x-XSWDB and 6x-XSC according to reliability flag value, and the probability of reliability associated with each value of rel. The 6x-XSC source selection criteria are designed to identify reliable sources in the 6x-XSWDB, so the number 6x-XSC entries declines rapidly with decreasing probability of reliability. There are still a small number of low reliability sources in the 6x-XSC. The single 6x-XSC source with rel="F" is a compact red object superimposed on diffuse nebulosity in the IC1396 6x Field. It was classified as "ambiguous" in visual examination (see below).

Use the 6x-XSC, or select 6x-XSWDB sources having a reliability flag value of rel="A" to minimize the number of spurious extended sources. Caution should be exercised when using any 6x-XSWDB source with a lower probability of reliability. When in doubt about the reliability of a 6x catalog or database entry, we strongly recommend examining the image of the source using the 6x Image Atlas.

Table 6 - Breakdown of 6x-XSWDB and 6x-XSC Entries by Estimated Reliability (rel)

rel Value	Number (6x-XSWDB)	Number (6x-XSC)	Probability of Reliability
A	106860	59459	>90%
B	6672	4334	80-90%
C	28186	18434	70-80%
D	582	235	50-80%
E	69421	1531	30-50%
F	35370	48	<30%




• Visual Examination: the "vc" flag

Images of a large number of candidate extended sources in the 6x-XSWDB were examined visually as part of the reliability scoring. The results of these examinations are encoded in the visual classification score (vc) database parameter. The possible values of vc and frequency of occurrence in the 6x-XSWDB and 6x-XSC are listed in Table 7. Note that objects that were visually classified to be not truly extended (i.e. single or multiple stars) or image artifacts (vc=2) where not selected for the 6x-XSC.

Table 7 - Definition of vc and Occurrences in the 6x-XSWDB and 6x-XSC

vc Value	Meaning	Number 6x-XSWDB	Number 6x-XSC
-2	Unknown. Sources that were examined but could not be unambiguously classified. These objects are usually faint and compact.	16016	5156
-1	Not examined visually.	120714	39152
1	Galaxy or galactic extended source.	78477	39733
2	Not true extended source (i.e. single or multiple star or artifact.	31884	0





• Artifacts and Source Contamination: The cc_flg Flag

The cc_flg is used to highlight 6x-XSWDB and 6x-XSC entries that have some probability of being artifacts or contaminated by nearby large galaxies.

Users should select extended sources with cc_flg="0" if they need samples that have the lowest probability of contamination.

Sources flagged as artifacts (cc_flg="A") include those corrupted by a bright stars and those that are outright spurious detections of image artifacts produced by bright stars. These sources were identified as such by visual inspection of their images. They are not selected for the 6x-XSC.

Table 8 summarizes the possible values of cc_flg and the number of occurrences of each value in the 6x-XSWDB and 6x-XSC. Note that the 6x cc_flg encoding is slightly different than used for the All-Sky XSC.

Table 8 - Definition of cc_flg and Occurrences in the 6x-XSWDB and 6x-XSC

cc_flg Value	Nature of Source	Number (6x-XSWDB)	Number (6x-XSC)
0	Nominal. Unaffected by artifacts.	213698	84040
A	Artifact or unreliable	31884	0
z	Fragment of a large galaxy or nebula. Extracted information is generally not useful.	1508	0
X	Associated with large galaxy.	1	1





• Multiple Source Detections in the 6x-XSWDB

The 2MASS 6x XSWDB may contain multiple measurements of the same object. Some of these multiple detections are independent measurements of objects in regions observed more than once during 2MASS 6x observations, and some are duplicate source entries from the overlapping 6x Atlas Image declination boundaries within the same scan and are not independent. Because of this "over-completeness," the 6x-XSWDB is not suitable for studies based on source count statistics. Use the 2MASS All-Sky XSC and 6x XSC for this type of inquiry.

Because the 6x-XSWDB contains extractions of all extended source candidates from all 2MASS 6x observations taken under photometric conditions, it may contain more than one independent measurement of objects in regions that were scanned multiple times. The number of duplicate source detections varies across the 6x fields, depending on 6x tile overlap geometry and the number of times 6x tiles were scanned. See A3.2 for a description of the coverage of the 6x observations.

The 6x Merged Extended Source Information Table provides combined position, brightness and shape information for multiply-detected extended sources in the 6x observations, and statistics on source brightness that are useful for identifying variables. Source confirmation statistics from the Merged Source Information table is also contained in the 6x-XSWDB: the spos and sdet columns in the 6x-XSWDB give the number of scans that covered the position of a source and the number of unique scans in which the source was detected as an extended source, respectively.

In-Scan Duplicates

Extended sources were detected and characterized on the 6x Atlas Images during 2MASS 6x data processing. Because the Atlas Images within each 6x scan were constructed with 54 pixels (54") of declination overlap between adjacent images, objects that fall in the overlap region between images, or that span more than one image in a scan may have been extracted more than once. All extended source detections in each scan, including the "in-scan duplicates," were loaded into the 6x XSWDB. Because they are based on the same image data, the in-scan duplicates are not independent measurements and thus they are different than the multiple detections of extended sources that were scanned repeated times.

In-scan duplicate detections can be identified in the 6x XSWDB as two extended source entries from the same observation (e.g. with the same scan_key value) that also have nearly the same position and possibly magnitudes, but that have different values of the coadd_key which identifies the Atlas Image from which the source was extracted.



• Extended Sources Near Scan Edges

The 2MASS 6x-XSWDB and 6x-XSC contain some extended sources that are truncated by Atlas Image edges, and consequently have distorted position, flux and shape measurements. Use the measurements of any extended source that lies within approximately 1' of a scan or image edge with caution. Refer to the All-Sky XSC for the best available measurements of any objects on scan or image boundaries.

2MASS extended source processing detected and characterized objects on a single Atlas Image. Sources that are truncated by image edges were not accurately measured. Although detections of truncated objects are contained in the 6x XSWDB, they typically have underestimate fluxes, positions that are biased away from the image edge, and incorrect size and shape attributes.

The overlap size between 2MASS 6x tiles and between the declination boundaries of Atlas Images was designed in part so that objects up to approximately one arcminute in diameter would be fully covered by at least one scan. Small extended objects in the 6x XSWDB that lie on Atlas Image edges may have a duplicate entry from another scan if there is an adjacent overlapping scan. Because of the finite size of the 6x fields, though, many scans do not have overlapping scans.

Extended sources larger than one arcminute in diameter that are truncated by scan or image edges may not always be fully contained on a single Atlas Image, and may therefore not have an accurate measurement available in any 2MASS extended source table (survey, 6x or calibration). The largest objects detected in 2MASS survey observations were characterized using special post-processing for the Large Galaxy Atlas (LGA; Jarrett et al. AJ, 125, 525), and the LGA entries are contained in the All-Sky XSC. However, the LGA is not complete for objects smaller than a few arcminutes, so care should be taken when using any extended source measurement in close proximity to a scan edge.



• Large Galaxies

The 2MASS 6x-XSWDB and 6x-XSC do not contain accurate flux, position or size measurements for objects that are large relative to the size of a 6x Atlas Image, or that span more than one Atlas Image. Consult the 2MASS All-Sky XSC or Large Galaxy Atlas for measurements of galaxies that are larger than a few arminutes in diameter, or that are truncated by the boundaries of a 6x Atlas Image.

2MASS extended source processing detected and characterized objects on a single Atlas Image. Extended sources that are large with respect to the size of an Atlas Image were often artificially "shredded" into multiple components which generally have misleading flux, position and shape measurements. Even when very large objects were correctly detected as a single source, their positions and photometry were usually biased due to incorrect background subtraction and/or because sections of the objects are missing from the image. Because the 2MASS 6x XSWDB contains the default processing output for large extended objects, they should not be used to obtain information about objects with very large size or that are partially truncated by scan edges. Many large galaxy/nebula segments in the 6x extended source tables are flagged by the confusion and contamination flag and have values of cc_flg='z' or 'Z'.

Special post-processing was used to to characterize very large objects detected in 2MASS survey observations. This processing utilized measurements on image mosaics that fully contained the large sources, and from which accurate background fitting could be performed. The resulting 2MASS Large Galaxy Atlas (LGA; Jarrett et al. 2003 AJ, 125, 525) entries were added to the 2MASS All-Sky XSC to insure the catalog's bright source completeness. Comparable LGA post-processing was not carried out for the 6x data. Therefore, the All-Sky XSC should be consulted for the best available measurements for the 6x-XSWDB or 6x-XSC.

iv. 6x Image Atlas

• Please see Cautionary Notes for the 2MASS All-Sky Release Image Atlas (I.6.d)
• Backgrounds

The 6x Atlas Images preserve the observed background sky levels measured relative to camera dark frames with the shutter closed. The mean background level is normally largest in the K_s band, although it can be larger in the H-band due to atmospheric OH airglow emission.

The only background compensation that is made during 6x Atlas Image construction is to adjust the individual frame backgrounds by a constant to produce seamless combined images. Because the OH airglow (especially at H-band) often contains structure on scales at or below the 2MASS frame size, the resulting Atlas Images sometimes show large background spatial variations. Th low frequency airglow structure can be seen in the mosaic constructed from the 6x Atlas Images of the IC1396 field shown in Figure 14.

Figure 14 - 6x J+H+Ks image mosaic showing the field containing IC1396 (the Elephant Trunk nebula). The green-hued background structure is caused by time-variable atmospheric airglow emission that is strongest in the H-band.




• Saturation

No correction for saturation is made during Atlas Image construction. Saturation occurs in the READ2-READ1 frames from which the 6x Atlas Images are constructed approximately two magnitudes brighter than in the main survey observations, at J<11, H<10.5 and K_s<10.0. Saturation is handled correctly during source photometry in 6x pipeline processing. Therefore, users are advised to defer to the 6x Catalogs for photometry of bright point and extended sources.



• Multiple Images at a Specific Location

Because some regions of the sky were observed more than once during 6x observations, the 6x Image Atlas may contain more than one independent image covering a particular location on the sky. When examining the image of a particular source selected from the 6x-PSWDB/PSC or 6x-XSWDB/XSC, it is important to select the 6x Atlas Image from the same scan in which the source was detected.

The IRSA/2MASS Image Services allow specification of a particular date, observatory (hemisphere) and scan number when requesting images from the 6x Atlas. If these are not specified, you may not receive the image from the correct scan.

[Last Updated: 2008 March 13; by R. Cutri]