Appendix 4. 2MASS Calibration Scan Working Databases and Atlas Images
1. Overview
c. Cautionary Notes
- General Calibration Scan Notes
- Calibration Scan Point Source Working Databases
- Calibration Scan Extended Source Working Databases
- Calibration Scan Image Atlas
Before using the 2MASS extended mission calibration scan 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 calibration
observations were interspersed with the main survey observations
during 2MASS operations, and were made with the same facilities using the
same observing mode. The calibration scan data were reduced with a the
same software pipeline used for the main survey data. Therefore, most
of the characteristics and limitations associated with the All-Sky Release
data products will apply to the calibration scan data products.
The sections below describe features and caveats that are unique to the
calibration scan data products and highlight a few of the important features
that are common to both the calibration and main survey products.
These sections are intended to supplement the
Cautionary Notes to the All-Sky Data Release
but not to replace them.
i. General Calibration Scan Notes
- Please see General Cautionary Notes for
the 2MASS All-Sky Release (I.6.a)
- Using the Calibration Scan Data Products
The 2MASS calibration data were obtained with the same well-characterized
facilities as the main survey and reduced with the same processing software
that incorporates multiple levels of quality assurance.
As a result, the overall quality of the 2MASS calibration Atlas and
WDBs are expected to be comparable to the those from
the survey.
However, the purpose of the 2MASS calibration observations
was to derive the photometric calibration for the survey and not to
generate reliable, uniform "Catalogs" of sources like those that were
produced from the survey observations. As a result,
the calibration data have not been analyzed in as much detail
as the All-Sky Release Catalogs prior to their release, so
all areas of performance have not been characterized and validated.
Users should therefore use the calibration scan
databases and images with care and follow these recommendations:
-
The calibrations WDBs are not "Catalogs"
in the sense of the highly reliable and uniform All-Sky Release
PSC and XSC. They should not be used for statistical
studies of source populations because they contain a large number of repeated,
independent measurements of astrophysical sources, as well as a
significant number of
spurious detections of faint noise excursions, image artifacts and
transients.
- Use the high degree of redundancy in the calibration measurements
to learn more about sources found in the 2MASS All-Sky Catalogs,
or to search for objects with special characteristics in a limited area.
The hundreds to thousands of independent measurements of objects
can be combined to provide precision photometry, analyzed to search
for flux variability or source motion, and used to discriminate
real source detections from random noise excursions and other
spurious detections (with the exception of spatially-repeating
image artifacts).
- Pay attention to and take advantage of the numerous extracted source
quality flags that are provided with each point and extended source
database entry (e.g. rel,
cc_flg,
ph_qual,
vc)
when assessing calibration database source reliability and measurement
quality.
- When in doubt about the reliability or measurement accuracy of
a source in the calibration scan WDBs, use confirmation statistics
when available, and most importantly, examine its images using
the calibration scan Image Atlas.
- Limited Sky Coverage
2MASS calibration observations cover only approximately
6 deg2 of sky distributed among 40
fields or tiles. The locations of these fields are shown
in Figure 1 in A4.1.
The calibration fields sample a variety of sight lines and
astrophysical environments ranging from sparse high galactic
latitude regions to dense fields in the galactic plane.
Each calibration observation
consisted of six consecutive 1° long (in declination) scans
made in alternating north-south directions that were made using the
same freeze-frame scanning technique
as the main survey observations.
The 2MASS telescopes were stepped 5" east in RA between each
scan within a calibration observation, so the total area covered is
usually ~0.15 deg2. In most fields, the footprints
of the hundreds to thousands of scans were well-registered
(Figure 1). However, a small fraction (<5%) of the
scans in a few of the fields were displaced from the nominal
position due to occasional telescope pointing variations. This
results in irregular coverage patterns such as shown in Figure 2.
The depth-of-coverage in the calibration fields ranges from one scan at the
extreme edges, up to several hundred or thousand in the central
parts of the fields.
 |
 |
| Figure 1 - 90182 Field Coverage | Figures 2 - 92202 Field Coverage |
| Coverage maps showing footprints of 1703 scans of the
90182 calibration field (left) and 2802 scans of the 92202
field (right). The maps show a 1°x2° area in cartesian projection
with the RA scale exaggerated to emphasize the scan cross-stepping.
The scans of 90182 were all well-registered, while ~1% of the
scans of 92202 were off the nominal field position. |
- Photometric Considerations
- Photometric Calibration
Photometric calibration for the 2MASS calibration
scan data was handled differently than for the main survey scans.
Rather than
applying a zero point offset derived from fits to the nightly
photometric solutions, data in each calibration observations were calibrated
by applying the instantaneous zero
point offset derived directly from the measurements of the
standard stars from that observation. The zero point offset
applied to a scan is the mean offset measured using all standard stars
in the six scans making up that particular calibration observation.
The zero point offsets and their RMS measurement errors applied
to each scan are given by the
[jhk]_zp_ap and
[jhk]_zperr_ap columns
in the calibration scan metadata table.
The in-situ calibration process is described
in A4.4.a
Using the in situ measurements of standard stars results in
slightly better calibration accuracy in the calibration scans
than in the main survey data. The mode of the zero point measurement
errors in the calibration scans is approximately 0.003-0.004 mag
(A4.4.a), compared
to 0.007-0.011 mag for the main survey
(IV.8.c).
- Combined Photometric Uncertainties
The combined photometric uncertainties given in the
[jhk]_msigcom
columns of the calibration point source WDB records
are usually overestimated.
The [jhk]_msigcom values
are computed by combining in quadrature the photometric measurement
errors with flat-fielding and photometric normalization and calibration
error factors:
[jhk]_msigcom = sqrt([jhk]_cmsig2 + [jhk]_zperr2 + fferr2 + [r1normrms2])
The flat-fielding, normalization and calibration error factors
from the main survey were used for the calibration scans for
consistency between data sets. However,
as discussed above the photometric
calibration uncertainties for calibration scans are typically a factor
of two smaller for calibration scans than survey scans.
To compute a more precise combined photometric
uncertainty for sources in the calibration point source WDB,
back out the constant calibration error factors (0.011, 0.007 and 0.007 mag
for J, H and Ks, respectively) from the
quoted [jhk]_msigcom values, and add back in quadrature
the measured calibration uncertainties for the scan scan given
in the [jhk]_zperr_ap
columns in the scan metadata table.
- Astrometric Considerations
- Astrometric Calibration
Position reconstruction for the calibration
scan observations (A4.4.b) used
USNO-A2.0
as the primary astrometric reference catalog, rather than the
Tycho-2 Catalog
that was used as the reference for the main survey.
Because of small systematic astrometric differences
between USNO-A2.0 and Tycho-2, the 2MASS calibration scan source positions
have systematic biases of up to ~0.6" with respect to the 2MASS
All-Sky PSC and XSC.
The astrometric biases are position-dependent within
calibration scans, and they differ in amplitude and structure
between calibration fields. However, they are consistent
among all scans of a given calibration field.
The average offsets with respect to the main survey have been computed
for each calibration field using the Merged
Calibration Point Source Database and
are shown in A4.2. The largest biases
are found in the 90004 calibration field, which has
an offset relative to the All-Sky PSC of approximately 0.6" in declination.
Source positions in the All-Sky PSC and XSC and Survey WDBs are
more accurate relative to the J2000 reference frame than those
in the calibration WDBs, and should be be used wherever possible.
However, the internal consistency of the position reconstruction
within scans of individual calibration fields is excellent.
This consistency, combined with the large number of independent
scans of the calibration fields, make the Cal-PSWDBs resources for
measuring very small-scale source motions.
- Position Reconstruction Uncertainties
A constant factor of of 0.1" on each axis has
been added in quadrature to the
computed position uncertainty error ellipse values for
calibration scan point sources to take into account the absolute
astrometric uncertainty arising from the use of the USNO-A2.0
catalog as the astrometric reference.
Thus, there is an artificial floor to the values of
err_maj and
err_min in the Cal-PSWDB
records. Because of this artificial floor, certain of the
positional statistics that are scaled to position uncertainty
are not computed for the Merged
Calibration Source Information Tables (e.g.
sep_maxrad,
chisq_mx2d,
chisq_grp2d).
- Position Reconstruction Errors
The Atlas Images and extracted source lists from
two scans of the 90009 calibration field have systemmatic position biases of
up to 12-13" with respect to all other scans of that field.
Incorrect astrometric solutions were applied to data from two scans
of the 90009 calibration field during pipeline data processing.
Because the standard stars were unambiguously identified
in these scans, and the source photometry was not adversely affected,
the astrometric error was
not diagnosed during the quality assurance
process and the images and extracted source lists were loaded into the
Calibration Atlas and WDBs.
The affected observations of the 90009 field are scan 106 taken
on 05/15/1999 UT (scan_key=26284) and
scan 102 taken on 05/18/1999 UT
(scan_key=25081),
both from the southern observatory.
Figure 3 shows the radial position offsets of sources in three different
scans relative to the positions in scan 100 on
05/18/1999 UT (scan_key=25079, which had no position
reconstruction errors. The first two scans (shown in red and green)
are those affected by the reconstruction errors. The third
scan, scan 104 on 05/18/1999 UT shown for reference, is correctly
reconstructed and shows no significant offsets.
The incorrect positions of source extractions in the
two scans of the 90009 calibration field occasionally
coincide with the correctly reconstructed positions of nearby objects.
Consequently, if the Cal-PSWDB and Cal-XSWDB are searched for
all available detections of an object at a specific location in the 90009
field, the extractions returned may include one or two
that are not actually associated with the object in question.
Because the Calibration Merged Point and
Extended Source Information Tables were
produced using positional association, some merged source entries
in the region of the 90009 calibration field may be contaminated
by incorrect associations with extractions from the two scans with
position reconstruction errors.
These erroneous associations may manifest
themselves as outliers in light curves from the individual
measurements in the Cal-PSWDB or Cal-XSWDB, as illustrated in Figure 4,
and they will bias calculations of mean source positions and/or
magnitudes, such as those in the merged source tables. The impact of
the incorrect positions on the merged source tables
is discussed further in A6.1.b.
 |
 |
| Figure 3 - Radial position offsets of sources in two scans of
the 90009 calibration field (scan_key=25081 and 26284)
that had position reconstruction errors, relative to positions in
a scan with no errors (scan_key=25079). The relative position
offsets for another scan that correctly reconstructed are also
shown (scan_key=25083). |
Figure 4 - J, H and Ks light curves from the Cal-PSWDB
of a faint source in the 90009 calibration field (gcntr=6370424).
The single epoch outliers in each band are caused by the
detection of nearby bright source in one of the scans of this field
with position reconstruction errors (scan_key=25081) that
coincides with the position of the faint source in the correctly
reconstructed scans. The dashed horizontal lines
show the average brightness levels reported for the faint source in the
Calibration Merged Point Source Information Table.
|
- Low Quality Scans in the Calibration WDBs and Atlas
The calibration Image Atlas and WDBs contain data from scans
that do not necessarily meet all of the quality criteria imposed
on scans in the main survey data..
Because the principal objective of the 2MASS calibration observations was to
derive photometric zero point and atmospheric stability information
during each night of survey operations, they were not subject to
the same rigorous quality assessment as the
main survey scans.
In particular, calibration scan data were not required to meet the
sensitivity thresholds required for the main
survey, or to be free from astrometric errors
as long as standard stars could be identified and their photometry
extracted.
Calibration scans were assigned a
binary quality score (10 or 0 = pass
or fail) that was based only on their photometric quality.
A passing score of 10 was assigned to scans in which standard stars could
be unambiguously identified, photometric zero points could be derived, and
that were part of a six-scan calibration observation that was made under
photometric conditions.
Only data from scans receiving a passing score
were loaded into the calibration scan Atlas Image archive and
point and extended source WDBs.
- Sensitivity Variations
With the same exposure time as the survey observations,
the calibration scans achieve sensitivities
and characteristic depths similar to the main 2MASS survey
in corresponding regions of the sky.
Like the survey observations, though, the calibration observations were made
in a broad range of atmospheric transparency, seeing and OH airglow
conditions, so the achieved sensitivity of even the same field varies
from observation-to-observation.
The distributions of the estimated
SNR=10 point source sensitivity levels for all
calibration scans are shown in Figure 5.
These are very similar in their peak values and
faintest extent to those of the main survey scans shown in
Figure 18 in section
VI.2. However, the Cal-WDBs contain
scans up to several tenths of a magnitude less sensitive than were
accepted for the main survey because they were not required to meet
the same sensitivity thresholds
specified in the survey's Level 1
Requirements.
 |
| Figure 5 - Distribution of estimated SNR=10 point source sensitivity levels
achieved in all 2MASS calibration scans, in the absence of confusion.
Note that some scans fall below the sensitivity requirements
for the main survey (J=15.8, H=15.1, Ks=14.3 mag) |
ii. Calibration Point Source Working Databases (Cal-PSWDB and LMC/SMC Cal-PSWDB)
- Please see Cautionary Notes for the 2MASS
All-Sky Release Point Source Catalog (I.6.b)
- Nature of the Calibration Point Source Working Database
The Cal-PSWDB is not a "Catalog" in the
same sense as the highly reliable, uniform All-Sky PSC.
The Cal-PSWDB contains multiple, independent measurements of
real astrophysical sources, as well as spurious detections of faint,
low SNR noise excursions, image artifacts and transient events such
as cosmic ray strikes and meteor trails. Because of these features,
and because the Cal-PSWDB covers a relatively small amount of sky,
it is not well-suited for statistical investigations of large source
populations.
The distinguishing feature of the Cal-PSWDB is that it contains
hundreds to thousands of independent point source measurements in the
calibration fields. Thus, it is a resource for improved photometric
information about sources found first in the All-Sky PSC,
and to search for and study objects on the basis of flux variability
or spatial motion.
- The Cal-PSWDB "Reliability" Flag (rel)
To help discriminate between real source detections and the many spurious
extractions of noise and artifacts in the Cal-PSWDB,
each databases entry has been assigned a reliability
flag value, rel,
that is related to the probability that it is a detection
of an 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 select Cal-PSWDB sources
having a reliability flag value of rel="A" to minimize the
number of spurious extractions. Caution should be exercised
when using any Cal-PSWDB source with a lower probability
of reliability. When in doubt about the reliability of a
database entry, we strongly recommend examining the image of the source
using the Calibration Image Atlas.
Table 1 contains a breakdown of the Cal-PSWDB according
to reliability flag value, and the probability of reliability
associated with each value of rel.
Table 1 - Breakdown of Cal-PSWDB Entries by Estimated Reliability (rel)
| rel Value | Number (Cal-PSWDB) | Number
(LMC/SMC Cal-PSWDB) | Probability of Reliability |
| A | 90515464 | 7221609 | >90% |
|---|
| B | 6462533 | 537858 | 80-90% |
|---|
| C | 3416630 | 411962 | 70-80% |
|---|
| D | 4453285 | 551256 | 50-70% |
|---|
| E | 10963858 | 954695 | 30-50% |
|---|
| F | 75652250 | 3259310 | <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 Cal-PSWDB entry whose reliability may be in question.
Table 2 - Common Characteristics of Unreliable Cal-PSWDB 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, calibration point source photometry
is measured differently for three brightness regimes.
The J, H and Ks 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 3 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 Cal-PSWDB.
Figures 6, 7 and 8 show the J, H and Ks Cal-PSWDB differential
source counts in the 90273 calibration field, decomposed by
rd_flg.
The majority of detected sources are non-saturated in the
1.3 s READ2-READ1 exposures, and have default photometry
from profile-fitting (rd_flg=2).
Profile-fitting fails for a small fraction of the non-saturated
READ2-READ1 detections, and in these cases the default magnitude
is taken from the curve-of-growth-corrected
aperture photometry (rd_flg=4).
For point sources that are saturated in the READ2-READ1 exposures
but not the 51 ms READ1 exposures, corresponding to brightness ranges
4.5<J<9, 4<H<8.5, 3.5<Ks<8 mag, the
default magnitudes
are taken from aperture photometry
measurements made on the READ1 exposures (rd_flg=1).
Images of stars brighter than approximately J~4.5, H~4 and
Ks~3.5 mag are saturated in even the 51 ms exposures.
The default magnitude for these objects is taken from photometric
estimates made using a 1-d radial profile
fit to the azimuthally-averaged image profile on the READ1 exposures.
The rd_flg value for such objects in the appropriate band is "3".
 |
 |
 |
| Figure 6 - J-band |
Figure 7 - H-band |
Figure 8 - Ks-band |
| Differential J, H and Ks-band
(right) source counts from the Cal-PSWDB in the 90273 calibration field.
Counts are broken down into different rd_flg populations denoted
by color. |
- Origin and Quality of Photometry - Calibration Point Source Quality Flags
The origin and general quality of the default
magnitude photometry
listed in the Cal-PSWDB 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 Calibration 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 Ks value.
The rd_flg denotes the origin of
and algorithm used to derive the quoted default
magnitudes for each
2MASS calibration scan 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 Cal-PSWDBs.
Table 3 - Definition of rd_flg and Occurrences in the Cal-PSWDBs
| rd_flg Value | Source of Photometry | Cal-PSWDB | LMC/SMC Cal-PSWDB |
| 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 Calibration 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. | 59412439 | 72412051 | 89717194 | 2920435 | 4074242 | 6261166 |
|---|
| 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 1.3s "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. | 336475 | 466936 | 402940 | 11434 | 17582 | 13774 |
|---|
| 2 | READ2-READ1 profile-fit photometry. The default magnitude
is derived from a profile-fitting measurement (IV.4.b)
made on the 1.3 sec "READ2" exposures. The profile-fit magnitudes
are normalized to curve-of-growth-corrected aperture magnitudes. This is the
most common type in the Cal-PSWDB, and is used for sources that have no
saturated pixels in any of the 1.3 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. | 130364379 | 116588312 | 98614210 | 9931540 | 8727704 | 6498731 |
|---|
| 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. | 7387 | 10773 | 9043 | 3 | 9 | 8 |
|---|
| 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
1.3 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. | 14129 | 20295 | 14651 | 32 | 527 | 159 |
|---|
| 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 Calibration 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. | 1238528 | 1874675 | 2626989 | 72494 | 115780 | 162014 |
|---|
| 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 Calibration 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. | 90683 | 90978 | 78993 | 752 | 846 | 838 |
|---|
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.
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 Cal-PSWDB.
Table 4 - Definition of ph_qual and Occurrences in the Cal-PSWDBs
ph_qual Value
(1 per band) | Meaning | Cal-PSWDB | LMC/SMC Cal-PSWDB |
| 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".
| 4805990 | 3245298 | 2640016 | 191278 | 135297 | 83124 |
|---|
| 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. |
1036 | 4922 | 1481 | 2 | 7 | 3 |
|---|
| 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. |
60612575 | 74201766 | 92260829 | 2991868 | 4187216 | 6421119 |
|---|
| 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. |
3521 | 4018 | 5032 | 31 | 16 | 30 |
|---|
| 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_chi 10.0);
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. |
362555 | 389526 | 281505 | 14697 | 16478 | 5754 |
|---|
| A | Detections in any brightness regime where valid measurements
were made (rd_flg=1,2 or 3) with [jhk]_snr10
AND [jhk]_msig0.10857. |
61104377 | 50934221 | 38666681 | 4309838 | 3728254 | 2457397 |
|---|
| B | Detections in any brightness regime where valid measurements
were made (rd_flg=1,2 or 3) with [jhk]_snr7
AND [jhk]_msig0.15510. |
17280897 | 15763955 | 11909128 | 1542129 | 1431289 | 956627 |
|---|
| C | Detections in any brightness regime where valid measurements
were made (rd_flg=1,2 or 3) with [jhk]_snr5
AND [jhk]_msig0.21714. |
15970277 | 15653287 | 13048546 | 1836730 | 1676724 | 1205758 |
|---|
| 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. |
31322792 | 31267027 | 32650802 | 2050117 | 1761409 | 1806878 |
|---|
In the 2MASS Cal-PSWDB and LMC/SMC Cal-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 survey and calibration
scan data processing are
discussed in IV.7.
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").
Cal-PSWDB entries with cc_flg!="000" should be viewed with caution.
A cc_flg value of [C,D,G,P] in all detected bands
indicates that the 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 their
photometry may be corrupted and/or biased by artifacts or
confusion with nearby sources in a manner that is not reflected
in the quoted photometric uncertainties.
Table 5 - Definition of cc_flg and Occurrences in the
Cal-PSWDBs
cc_flg Value
(1 per band) | Meaning | Cal-PSWDB | LMC/SMC Cal-PSWDB |
| N(J) | N(H) | N(Ks) | N(J) | N(H) | N(Ks) |
| 0 | Real source unaffected by artifacts, or not detected in that band | 163603798 | 171147262 | 177545661 | 11196666 | 11703240 | 12326475 |
|---|
| p | Real source whose photometry may be contaminated by a persistence (latent) image of a bright star | 1032882 | 887758 | 1253259 | 63805 | 55740 | 18095 |
|---|
| c | Real source whose photometry may be biased because of confusion with nearby brighter source | 21432842 | 15563316 | 10213131 | 1478713 | 1035095 | 495078 |
|---|
| d | Real source whose photometry may be contaminated by a diffraction spike | 40920 | 74609 | 73062 | 2451 | 4329 | 1669 |
|---|
| s | Real source whose photometry may be contaminated by a horizontal "stripe" (electronic cross-talk) due to a bright star | 532529 | 530397 | 524406 | 3471 | 2584 | 11745 |
|---|
| b | Real source that is confused in the bandmerging process because of a nearby source | 15059 | 15380 | 18521 | 306 | 405 | 504 |
|---|
| P | Spurious detection of a Persistence (Latent) image of a bright star | 3112465 | 2108844 | 1253259 | 127307 | 99752 | 43877 |
|---|
| C | Spurious detection in the Confusion "halo" around a bright star | 179225 | 171245 | 133283 | 1561 | 1503 | 1614 |
|---|
| D | Spurious detection on a Diffraction spike from a nearby bright star | 678230 | 776185 | 652546 | 17360 | 26417 | 15163 |
|---|
| G | Spurious detection of a dichroic Glint | 836070 | 189024 | 600928 | 45050 | 7625 | 22470 |
|---|
- Flux Overestimation for Faint Sources
Flux measurements of faint source
extractions with SNR<8-9 in the Calibration PSWDBs are overestimated
because of statistical measurement bias. This bias is ~5%
at SNR~7 and increases rapidly with decreasing SNR.
Photometry of faint source extractions in the Calibration PSWDBs (as well
as the Survey PSRT and 6x PSWDB) is
affected by statistical flux overestimation bias, as described
in V.3.a.ii. This bias, which is also
present in low SNR sources in the All-Sky Release PSC, results naturally
when measuring sources with non-zero uncertainty at
true flux densities near system sensitivity limits. Sources
will preferentially be detected when noise drives up the
apparent flux rather than driving it down. Therefore, sources
detected near the sensitivity limit will have, on average, a measured
brightness higher than their true brightness, or equivalently a higher
SNR than their true value. Such sources will also have measurement
errors that do not accurately represent their true SNR. The closer a
measured brightness is to the detection limit, the larger the amplitude
of the statistical overestimation.
An illustration of the characteristic flux overestimation bias
in the 2MASS 7.8 s READ2-READ1 exposures in scans of the 90067
calibration field is shown in Figure 9.
In the top panels of Figure 9, differences between
photometry extracted from the very deep,
Combined Calibration Field Images of
the 90067 field and the average calibration scan photometry for
the field from the Calibration
Merged Point Source Information Table are shown
plotted as a function of average magnitude from the Merged Information
Table. The combined calibration images have an effective sensitivity
approximately four magnitudes fainter than the single scans and
are therefore are free from bias at the flux limits of the individual scans.
The bottom panels of Figure 9 show the RMS (root variance) of the
merged calibration scan photometry plotted as a function of average
magnitude. The RMS of the merged calibration field photometry
is representative of the sensitivity from individual survey
scans, averaged over all observing conditions.
The top panels in Figure 9 show that statistical flux overestimation
begins to be seen at J>16.3, H>15.2 and Ks>14.7 mag. Reading down
to the bottom panel, these magnitudes corresponds to SNR~8-9.
By SNR=7, the flux overestimation is nearly ~0.05 mag, and rises
steeply exceeding 0.2 mags at SNR=5. This behavior agrees
well with the bias predicted from simulations in
V.3.a.ii.
Also visible in the bottom panel is the flattening of the
RMS vs. magnitude curve below SNR~7. This
is a manifestation of incompleteness and flux overestimation.
At the lower SNR levels, sources are not detected when noise
fluctuations drive the flux down. This effectively truncates
the faint end of the measured flux distribution leading
to an artificially narrow distribution and hence an artificially low
flux uncertainty and biased flux estimate.
 |
| Figure 9 - Illustration of statistical flux overestimation as a
function of magnitude and SNR in the 90067 calibration field.
(top panels) Difference in J, H and Ks magnitudes
measured on deep, combined calibration scan images and the average magnitudes
from single calibration scans plotted as a function
of average magnitude. (bottom panels)
RMS (root variance) of merged (average) calibration scan magnitudes plotted
as a function of average magnitude. Red horizontal lines
indicate SNR=5,7 and 10 levels. |
- Multiple Detections in the Cal-PSWDBs
The Cal-PSWDB and LMC/SMC Cal-PSWDB contain a large
number of repeated, independent measurements of a relatively small
number of unique objects in the small areas covered by
the 2MASS calibration fields. Consequently, the Cal-PSWDBs are not
useful databases for studies based on the statistics of source
counts. The 2MASS All-Sky PSC should be used for
this type of inquiry.
The redundant nature of the Cal-PSWDBs is their distinguishing feature.
While the database's degeneracy makes it inappropriate for use in statistical
studies, it can be used as a resources for obtaining
information about sources well beyond what is possible from the
single survey observations.
Photometric and positional measurements of sources detected
many times can be combined to greatly reduce statistical measurement
uncertainties. The large number of indendent epoch measurements
also enable probing source variability and proper motion.
Finally, source detection statistics provide an independent assessment of
reliability; spurious detections are less likely to be repeatedly
detection.
The Calibration Merged Source
Information Table provides combined brightness
and position measurements for multiply-detected point sources
in the calibration observations, and statistics on source brightness
and position distributions that are useful for identifying variables
and moving objects. Source confirmation statistics
from the Merged Source Information table is also contained
in the Cal-PSWDBs: the spos
and sdet columns
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.
- Solar System Objects
No attempt was made to remove detections of non-inertially fixed
sources from the Cal-PSWDBs.
Sources that are near the positions of known asteroids,
comets, planets or planetary satellites at the time of the calibration
observations
are flagged by having mp_flg="1"
(IV.9).
These are positional associations and are not necessarily identifications.
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.
Contamination by background stars is discussed in
IV.9.b.
The positions of many main belt asteroids were scanned multiple times
during calibration observations because of the repetitive observing
cadence. Figure 10
shows the frequency distribution of known asteroid observations
in the calibration scans.
The most frequently observed asteroid was (1528) Conrada 1940 CA
which was scanned (and detected) 112 times over a two week
period in the 90067 calibration field. The large number of
detections was extraordinary because the asteroid fortuitously
executed its apparent prograde-to-retrograde swing within the 90067 field
 |
| Figure 10 - Frequency distribution of asteroid observations in
the 2MASS calibration scans. The peaks at 6 scan intervals occur
because a calibration field was scanned 6 times in rapid succession
each time it was observed. |
- Optical Associations
All point source extractions in the 2MASS calibration scans were
positionally correlated with the
USNO-A2.0
optical catalog. The calibration scan
point sources were not associated with the
Tycho 2 catalog,
as was done for the main survey and 6x point sources.
2MASS calibration sources that have optical catalog
counterparts within ~5´´ have listed in the Cal-PSWDB
source records the optical catalog identifier
(a="U"),
blue and red magnitudes,
the 2MASS/optical position separation
(dist_opt) and
position angle (phi_opt).
These are positional associations, and
not necessarily identifications. The relative positional accuracy of 2MASS and
USNO-A2.0 are 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.
Incorrect "a" Values for Optical Associations with dist_opt5´´
Optical source association for the calibration point sources was
affected by the same software error in setting the optical catalog
identifier flag,
a, as the main survey data.
This error resulted in an incorrect
value of a="0" being assigned to optical associations that have
dist_opt5´´.
There are 16,943 sources in the Cal-PSWDB with this error. These
objects can be identified as having a="0" and non-null values
in the other optical association fields:
dist_opt,
phi_opt,
b_m_opt, and
vr_m_opt.
The optical catalog identifier, a, should not
be used as the only test for optical counterparts in Cal-PSWDB sources.
iii. Calibration Extended Source Working Databases (Cal-XSWDB and LMC/SMC Cal-XSWDB)
- Please see Cautionary Notes for the
2MASS All-Sky Release Extended Source Catalog (I.6.c)
- Nature of the Calibration Extended Source Working Database
The Calibration XSWDBs are not "Catalogs" in the same
sense as the highly reliable, uniform All-Sky XSC.
The Cal-XSWDBs contain all extractions of
sources believed to be extended relative to the instantaneous
point-spread-function (PSF) in all 2MASS calibration observations.
These include one or more independent detections of real extended
astrophysical sources such as galaxies and galactic nebulae, and detections
of single and multiple single stars and spurious
image artifacts that have extended source signatures.
The distinguishing feature of the Cal-XSWDB is that it contains
hundreds to thousands of independent extended source measurements in the
calibration fields. Thus, it is a resource for improved photometric
information about sources found first in the All-Sky XSC,
and to search for and study objects on the basis of potential
flux variability.
- The Cal-XSWDB "Reliability" Flag (rel)
2MASS calibration scan extended source processing
(see IV.5.d)
attempts to identify all sources that are not well-fit by a
single PSF. Because the
algorithms are not perfect, the Cal-XSWDB
contains detections of both true extended sources as such
as galaxies and nebulae, well as
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 Cal-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 Cal-XSWDB according
to reliability flag value, and the probability of reliability
associated with each value of rel.
Users should
select Cal-XSWDB sources having a reliability flag value of
rel="A" to minimize the number of spurious extractions and
non-extended sources.
Caution should be exercised when using any Cal-XSWDB source with
a lower probability of reliability. When in doubt about the
reliability of a calibration scan database entry, we strongly recommend
examining the image of the source using the calibration Image Atlas.
Table 6 - Breakdown of Cal-XSWDB Entries by Estimated Reliability (rel)
| rel Value | Number (Cal-XSWDB) | Number
(LMC/SMC Cal-XSWDB) | Probability of Reliability |
| A | 352511 | 12797 | >90% |
|---|
| B | 6665 | 79 | 80-90% |
|---|
| C | 897 | 13 | 70-80% |
|---|
| D | 2319 | 39 | 50-80% |
|---|
| E | 2994 | 56 | 30-50% |
|---|
| F | 38425 | 2897 | <30% |
|---|
- Visual Examination: the "vc" flag
Images of a large number of candidate extended sources in the Cal-XSWDBs
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
Cal-XSWDB are listed in Table 7.
Table 7 - Definition of vc and Occurrences in the Cal-XSWDBs
| vc Value | Meaning | Number
Cal-XSWDB | Number
LMC/SMC Cal-XSWDB |
| -2 | Unknown. Sources that were examined but could not be unambiguously classified. These objects are usually faint and compact. | 40780 | 1555 |
| -1 | Not examined visually. | 36344 | 63 |
| 1 | Galaxy or galactic extended source. | 288949 | 11370 |
| 2 | Not true extended source (i.e. single or multiple star or artifact. | 37738 | 2893 |
- Artifacts and Source Contamination: The cc_flg Flag
The cc_flg is used to highlight Cal-XSWDB and LMC/SMC Cal-XSWDB
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.
The possible values of cc_flg and
the number of occurrences of each value in the Cal-XSWDBs
are summarized in Table 8.
Note that the calibration scan cc_flg encoding is slightly different
than used for the All-Sky XSC.
Artifacts are denoted in the Cal-XSWDBs by capital "A" rather than
lower case "a", and there are no instances of large galaxies
or segments of large galaxies (cc_flg="Z" or "z").
Table 8 - Definition of cc_flg and Occurrences in the Cal-XSWDBs
| cc_flg Value | Nature of Source | Number
(Cal-XSWDB) | Number
LMC/SMC Cal-XSWDB |
| 0 | Nominal. Unaffected by artifacts. | 366073 | 12988 |
| A | Artifact or unreliable | 37738 | 2893 |
- Multiple Detections in the Cal-XSWDBs
The 2MASS Cal-XSWDB and LMC/SMC Cal-XSWDB contain
multiple measurements
of the same objects. Some of these multiple detections are independent
measurements of ojects in regions observed more than once during 2MASS
calibration observations, and some are duplicate source entries from the
overlapping
Calibration Atlas Image declination boundaries within the same scan and are
not independent. Because of this "over-completeness," the Cal-XSWDB
is not suitable for studies based on source count statistics. Use the
2MASS All-Sky XSC for this type of inquiry.
Because the 2MASS calibration observations scanned the
same fields repeatedly during the survey,
and because the Cal-XSWDBs contain extractions of all extended source
candidates from all calibration observations taken under photometric
conditions, the Cal-XSWDBs contain many independent measurement of
a relatively small number of objects. The number of duplicate source
detections for a given source depends on the number of scans of each
field, and can vary across the calibration fields, particularily
at tile edges (See A4.2.b.i
The Calibration Merged Extended
Source Information Table provides combined position, brightness and
shape information for multiply-detected extended sources in the calibration
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 Cal-XSWDB: the
spos and
sdet columns in the Cal-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 Calibration Atlas Images during 2MASS survey data processing.
Because the Atlas Images within each calibration 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 Cal-XSWDBs. 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
Cal-XSWDBs 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 Cal-XSWDBs
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 Cal-XSWDBs, they typically have underestimate fluxes, positions
that are biased away from the image edge, and incorrect size and
shape attributes.
Because the 2MASS calibration tiles did not have adjacent, overlapping
tiles, it is usually not possible to find alternative measurements
of extended sources that are truncated by scan edges in the
Cal-XSWDB itself. However, the 2MASS survey measurements covered
the area of the calibration fields, and all their surrounding areas
with at least 1' of overlap between adjacent scans.
Therefore, there are usually good measurements available for
truncated objects up to 1' in size available in the 2MASS All-Sky XSC.
In addition, large extended objects where specially characterized
for the survey in the
Large
Galaxy Atlas (LGA; Jarrett et al. AJ, 125, 525).
- Large Galaxies
The 2MASS Cal-XSWDBs do
not contain accurate flux, position or size measurements
for objects that are large relative to the size of a Calibration 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 Cal-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.
The two largest galaxies in the Cal-XSWDBs are NGC 5719 (SAB pec) and
NGC 2950 (SB0), in the 90867 and 90091 calibration fields
respectively. Both galaxies have near infrared isophotal semi-major
axis radii of approximately one arcminute. NGC 5719 is close to the
east-west centerline of the 90867 field, so is usually
completely covered by the calibration scans and therefore
well measured in the Cal-XSWDBs. NGC 2950 lies within one arcminute
of the western edge of most scans of 90091, so the measurements
in the Cal-XSWDBs may not provide accurate positions
and photometry for it.
iv. Calibration Scan Image Atlas
- Please see Cautionary Notes for the 2MASS
All-Sky Release Image Atlas (I.6.d)
- Backgrounds
The calibration scan 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 Ks band,
although it can be larger in the H-band due to atmospheric OH airglow
emission.
The only background compensation that is made during calibration 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. An example of moderate airglow structure
can be seen in the 3-color
image constructed from one scan of the 92397 calibration
field.
- Saturation
No correction for saturation is made during Atlas Image construction.
Saturation occurs in the READ2-READ1 frames from which the calibration
Atlas Images are constructed approximately
at J<9, H<8.5 and Ks<8.0.
Saturation is handled correctly during source photometry in calibration scan
data pipeline processing.
Therefore, users are advised to defer to the Cal-PSWDBs
and Cal-XSWDBs for photometry of bright point and extended sources.
- Multiple Images at a Specific Location
Because of the repetitive nature of the calibration observations,
the calibration Image Atlases may contain hundreds to thousands of independent
images covering a particular location in the calibration fields.
When examining the image of a particular source selected
from the Cal-PSWDBs or Cal-XSWDBs, it is important to select
the 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 calibration scan Atlas. If these
are not specified, you will most likely not receive the
image from the correct scan.
[Last Updated: 2010 February 10; by R. Cutri]
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