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
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:
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 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).
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:
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.
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.
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).
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.|
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
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
Only data from scans receiving a passing score
were loaded into the calibration scan Atlas Image archive and
point and extended source WDBs.
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)
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.
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.
|rel Value||Number (Cal-PSWDB)||Number|
|Probability of Reliability|
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.
|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" and j_m>11) or (rd_flg="1" and h_m>10.5) or (rd_flg="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"|
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.|
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.
|rd_flg Value||Source of Photometry||Cal-PSWDB||LMC/SMC Cal-PSWDB|
|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|
|ph_qual Value |
(1 per band)
|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_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.||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.
|cc_flg Value |
(1 per band)
|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 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.|
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,
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.|
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)
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.
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.
|rel Value||Number (Cal-XSWDB)||Number|
|Probability of Reliability|
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.
|-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|
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").
|cc_flg Value||Nature of Source||Number|
|0||Nominal. Unaffected by artifacts.||366073||12988|
|A||Artifact or unreliable||37738||2893|
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.
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.
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).
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
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
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.
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]