Appendix 2. Full Survey Image Atlas and Source "Reject" Tables
1. Overview
c. Cautionary Notes
- General Survey Scan Notes
- Survey Point Source Reject Table
- Survey Extended Source Reject Table
- Full Survey Image Atlas
Before using the 2MASS Extended Mission Survey Reject Tables and Full Image Atlas, users are strongly encouraged to familiarize themselves with the Cautionary Notes to the All-Sky Data Release data products in 1.6, and the Guidelines for Using the PSC and XSC in II.2.b and II.3.b, respectively. The Survey Reject Tables and Full Image Atlas are derived from the same observations made with the same facilities as the 2MASS All-Sky Release Catalogs and Atlas. Therefore, most of the characteristics and limitations associated with the All-Sky Release data products will apply.
The sections below describe features and caveats that are unique to the
Extended Mission survey scan data products and highlight a few
important features that are common to both the Extended Mission
and All-Sky Release products.
These sections are intended to supplement the
Cautionary Notes to the All-Sky Data Release
but not to replace them.
- Please see General Cautionary Notes for the 2MASS All-Sky Release (I.6.a)
- Using the Survey Reject Tables and Full Image Atlas
- The Survey Source Reject Tables are not "Catalogs" in the same sense as the highly reliable and uniform All-Sky Release PSC and XSC. Do not use them to conduct statistical studies of source populations because they contain a large number of unreliable extractions of noise, image artifacts and transient events. The Reject Tables may also contain multiple, independent detections of astrophysical sources in regions of the sky observed two or more times during 2MASS survey operations.
- Use the redundancy in the survey measurements to learn more about sources discovered in the 2MASS All-Sky Release Catalogs or to search for objects with special characteristics in a limited area. This includes obtaining precision photometry by averaging together multiple, independent measurements, testing for variability or source motion, and investigating source reliability through detection confirmation statistics.
- Pay attention to and take advantage of the numerous extracted source quality flags that are provided with each point and extended source Reject Table entry (e.g. rel, cc_flg, ph_qual, vc) when assessing source reliability and measurement quality.
- When in doubt about the reliability or measurement accuracy of a source in the Survey Reject Tables, use confirmation statistics when available, and most importantly, examine its images using the Full Survey Image Atlas.
- Non-Uniform Effective Sky Coverage
- In the 59,731 survey scans that were selected for the All-Sky Release, the PSRT and XSRT contain only those source extractions not included in the All-Sky Release PSC and XSC. Thus, they do not contain most reliable detections of astrophysical sources. Instead, they contain:
- Low SNR extractions of both faint astrophysical sources and noise excursions, both of which were below the thresholds used to select the highly reliable catalogs.
- Spurious extractions of image artifacts produced by bright stars and transient events such as cosmic ray hits, meteor trails and intermittent noisy detector pixels.
- In the case of the XSRT, detections of single and multiple stars that are not truly extended.
- Bright, reliable source detections near scan edges that were excluded from the Catalogs during the multiple detection resolution process.
- In the 10,981 survey scans that were not used for the All-Sky Release, the PSRT and XSRT contain the reliable detections of astrophysical sources that are analogous to those found in the All-Sky Release Catalogs. However, they also contain the spurious detections of low SNR noise events, image artifacts and transients described above.
- Low Quality Scans
Because all 2MASS survey data were obtained with well-characterized facilities and reduced with the same processing software that incorporates multiple levels of quality assurance, the overall characteristics of the 2MASS Source Reject Tables and Full Image Atlas are expected to be comparable to the those from the All-Sky Release. However, the Survey Reject Tables and Full Survey Atlas contain, in part, extracted source data and images that do not meet the strict standards of reliability and uniformity required for the primary 2MASS products, and they were therefore not included in the All-Sky Release. In addition, the Survey Reject Tables and Full Atlas have not been as carefully analyzed the All-Sky Release Catalogs prior to their release, so all areas of performance have not been characterized and validated. Users should therefore use these products with care and follow these recommendations:
The effective sky coverage of the 2MASS PSRT, XSRT and Full Image Atlas is complex and non-uniform. Consequently, the PSRT and XSRT are not suitable for extracting statistically complete samples of point or extended sources.
Although the Survey PSRT and XSRT and Full Image Atlas are drawn from the 70,712 survey mode scans that covered 99.998% of the sky under photometric conditions, the content and depth-of-coverage of these products differ depending on whether a scan was used in the 2MASS All-Sky Release.
Approximately 30% of the sky was observed
more than once during 2MASS survey operations
(see A2.1.a.i). Half of that area
is contained in the overlap region between survey
tiles, and half is contained in the
8,266 tiles that were repeatedly scanned.
One measurement of a source that was observed and detected multiple times
in survey scans is generally found in the All-Sky Release
PSC or XSC. The remaining detections are contained in the
PSRT or XSRT. Objects observed and detected three or
more times may have multiple, independent measurements
in the PSRT or XSRT. The Full Survey Atlas contains multiple,
independent images of all regions that were observed multiple times.
The Survey PSRT and XSRT and Full Image Atlas contain data from scans that may not meet all of the survey's quality requirements, or that exhibit other anomalies that excluded them from the All-Sky Release.
The calibrated images and source lists extracted from each 2MASS survey scan were subjected to a rigorous and uniform Quality Assurance process during survey data reduction. This process resulted in the assignment of a numerical quality score for each scan indicating the likelihood that the data satisfy the survey's Level 1 Requirements. Scans of tiles that have a 100% chance of meeting these requirements were assigned quality=10, and those with the lowest probability of meeting the requirements but were still taken under photometric conditions were assigned quality="1". Quality="0" was reserved for scans that were known to fail the requirements, usually because of non-photometric weather. Only data from survey scans with quality>0 were loaded into the Survey Working Databases and Full Image Atlas. The quality score for the 70,712 survey scans is give in the qual column in the Survey Scan Information Table.
During the course of the 2MASS survey observations, tiles with scans having poor quality scores were scheduled for reobservation, and of these were successfully observed under improved conditions. The scan of each tile with the best relative quality was selected for the All-Sky Release Catalogs and Image Atlas. However, the Survey Reject Tables and Full Image Atlas contain the extracted source data and calibrated images from all survey scans, including those with quality that may fall below various thresholds and requirements imposed on the All-Sky Release.
The 2MASS All-Sky Release data products were required to achieve SNR>10 at J>15.8, H>15.1 and Ks>14.3 mag. The estimated magnitude at which point source SNR=10 was achieved in each survey scan is given by the j_msnr10, h_msnr10 and k_msnr10 columns in the Survey Scan Information Table.
The distributions of estimated J, H and Ks SNR=10 levels in all 2MASS are shown in Figure 7 of A2.2. There are 4,303 survey scans that have SNR=10 levels brighter than the required limits in at least one band. The low sensitivity typically occurs in the H-band (4,097 scans) because of elevated sky background levels due to atmospheric airglow emission.
Expect to find elevated photometric uncertainties for fainter sources and brighter completeness limits than in the All-Sky Catalogs in scans of tiles with low sensitivities. Because most scans with poor sensitivity were reobserved under better conditions, it is often possible to find data from alternative scans of the same tile with better sensitivity in the All-Sky Catalogs or Survey Reject Tables.
Although source and image data were supposed to be loaded into the Survey Working Database and Image Atlas only if they were from scans taken under nominally conditions, there are data from 24 scans that were found to show some evidence for clouds during detailed validation after data processing was completed. These scans are identified by having a value of cld=1 in the Survey Scan Information Table.
The clouds in these scans were usually diagnosed from examination of frame background levels or comparison with photometry of the same sources in scans of the same tile or adjacent tiles observed at different times. They were not severe enough to degrade the photometric solutions for the nights. The impact on photometry in these scans is generally small and transient, affecting data in only a small portion of the scan. There are often alternative scans available of the same region of sky that are not effected by clouds. However, scans of seven tiles that did not have alternates are included in the All-Sky Release. The two most severe cases are discussed in I.6.a.iii.
Select sources and images from scans having cld=0 to avoid any possible degradation in photometry due to clouds.
The Survey Scan Information Table contains several other flags that identify scans that were found to exhibit anomalies or peculiarities during the the Quality Assurance process. These anomalies and the number of occurrences among the full set of survey scans are summarized in Table 1. Scans that are effected by one or more of these anomalies will have a relevant flag value of "1". Scans with flag values of "0" are not effected.
Use caution when utilizing source and image data from any scan in which the quality anomaly flags listed in Table 1 have a value of "1". There may be data from alternative scans of the same regions of sky available in the All-Sky Catalogs or Survey Reject Tables.
| Flag | Meaning | Number of Scans |
|---|---|---|
| cld | Clouds may effect data from a small portion of scan | 24 |
| xph | Non-cloud related photometric anomaly in scan | 37 |
| anom | Unusual problem found in Atlas Images such as residual meteor trails, satellite trails, insect trails, and unusual reflections or glints from bright stars. Examples of these are shown in the gallery of Image Anomalies. | 399 |
| hgl | Elevated "noise-glitch" may affect one or more J or H band images in scan | 19 |
ii. Survey Point Source Reject Table (PSRT)
- Please see Cautionary Notes for the 2MASS All-Sky Release Point Source Catalog (I.6.b)
- Nature of the Survey PSRT
- Low Reliability Extractions in the PSRT - The Reliability Flag (rel)
- The Default Magnitude
As was the case for the All-Sky PSC, PSRT 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 1 below contains the possible values of rd_flg, a description of the photometry algorithm corresponding to each value, and the number of instances of each value, by band, in the Survey PSRT.
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
- Origin and Quality of Photometry - Survey Point Source Reject Table Quality Flags
- Flux Overestimation for Faint Sources
- Multiple Detections in the Survey PSRT
- Solar System Objects
- Optical Associations
The Survey PSRT is not a "Catalog" in the same sense as the highly reliable, uniform All-Sky PSC.
The 2MASS Survey PSRT contains a complex mixture of measurements of real astrophysical sources, and spurious detections of faint, low SNR noise excursions, image artifacts and transient events such as cosmic ray strikes and meteor trails. It is not a uniform, high reliability representation of the near infrared sky like the 2MASS All-Sky PSC. The Survey PSRT is not suitable for statistical investigations of complete samples of sources.
The Survey PSRT is a resource for supplementing source information found in the All-Sky PSC or other external sources. For example, the PSRT contains one or more independent measurements of 150,569,237 objects found in the All-Sky PSC that can be used to improve photometric and astrometric knowledge, or to search for photometric variability and/or proper motion. Detection statistics of sources observed multiple times found in the the PSRT can also provide an additional source reliability indicator. Finally, the PSRT may also contain faint source detections excluded from the All-Sky PSC because they fall below the SNR threshold for reliability. These faint source extractions must be used cautiously, though, because they are subject to flux overestimation biases and have a much lower reliability than brighter objects.
To help discriminate between real source detections and the many spurious extractions of noise and artifacts in the Survey PSRT, each database 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 Survey PSRT sources having a reliability flag value of rel="A" to minimize the number of spurious extractions. Caution should be exercised when using any PSRT 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 Survey Image Atlas.
Table 2 contains a breakdown of the Survey PSRT according to reliability flag value, and the probability of reliability associated with each value of rel.
| rel Value | Number (Survey PSRT) | Probability of Reliability |
|---|---|---|
| A | 243892807 | >90% |
| B | 37304125 | 80-90% |
| C | 25303048 | 70-80% |
| D | 32227208 | 50-70% |
| E | 72557964 | 30-50% |
| F | 432703745 | <30% |
In general, unreliable database and catalog entries exhibit one or more of the characteristics described in Table 3. 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 Survey PSRT entry whose reliability may be in question.
| 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 origin and general quality of the default magnitude photometry listed in the Survey PSRT 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 Survey 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.
rd_flg - Origin of Point Source Photometry
The rd_flg denotes the origin of and algorithm used to derive the quoted default magnitudes for each 2MASS PSRT extraction. Table 4 contains the possible values of rd_flg, their interpretation and the number of instances of each value in each band in the PSRT.
Only rd_flg values of "1", "2", "3" or "4" indicate that the default magnitude represents a valid detection of flux at the position of the extraction. Rd_flg values of "0" or "6" indicate that the default magnitudes are upper limits or that the source is unmeasurable, and should not be interpreted as valid detections.
| rd_flg Value (1 per band) | Source of Photometry | 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 Survey 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. | 339592855 | 424765757 | 491006221 |
| 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. | 713316 | 1129207 | 1045839 |
| 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 Survey PSRT, 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. | 497158802 | 409842999 | 342637943 |
| 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. | 9087 | 13246 | 11998 |
| 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. | 244771 | 218717 | 173555 |
| 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 Survey 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. | 5701970 | 7469288 | 8580999 |
| 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 Survey 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. | 568096 | 549683 | 532342 |
ph_qual - Photometric Quality
The photometric quality flag, ph_qual, provides a concise guide to the quality of the default point source photometry that is based on signal to noise ratio, measurement quality, detection statistics, etc. Sources with ph_qual="A" in a band, have <10% measurement uncertainties, and generally have the highest quality characterization. Measurement quality decreases with alphabetically increasing values of ph_qual. The values of "U" and "X" are reserved for non-detections/inconsistent deblends or otherwise unmeasurable detections in a band. The value "Z" denotes extractions that are believed to be spurious detections of image artifacts. The ph_qual value is set in each band according to the precedence listed in Table 5 below. For example, a measurement in a band that satisfies the conditions for "Z" is not tested for other conditions. Table 5 also gives the number of sources in each band that have the corresponding quality value in the Survey PSRT.
| ph_qual Value (1 per band) | Meaning | 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". | 30676294 | 21949069 | 17229804 |
| 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. | 2223 | 21947 | 5067 |
| 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. | 345033384 | 431661389 | 499030264 |
| 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. | 16702 | 22019 | 30628 |
| 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. |
1288459 | 2091764 | 1949241 |
| A | Detections in any brightness regime where valid measurements
were made (rd_flg=1,2 or 3) with [jhk]_snr10
AND [jhk]_msig0.10857. |
142814326 | 117494936 | 94313292 |
| B | Detections in any brightness regime where valid measurements
were made (rd_flg=1,2 or 3) with [jhk]_snr7
AND [jhk]_msig0.15510. |
37873789 | 35234860 | 27552940 |
| C | Detections in any brightness regime where valid measurements
were made (rd_flg=1,2 or 3) with [jhk]_snr5
AND [jhk]_msig0.21714. |
95934403 | 58711758 | 34725554 |
| 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. | 190349317 | 176801155 | 169152107 |
In the 2MASS Survey PSRT, 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 scan data processing are discussed in IV.7. The encoding of the cc_flg is summarized in Table 6 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").
Survey PSRT entries with a cc_flg value other than "0" in
any band 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 an artifact.
PSRT 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) | Meaning | N(J) | N(H) | N(Ks) |
|---|---|---|---|---|
| 0 | Real source unaffected by artifacts, or not detected in that band | 729075940 | 765678666 | 788474127 |
| p | Real source whose photometry may be contaminated by a persistence (latent) image of a bright star | 4993547 | 3108461 | 2140845 |
| c | Real source whose photometry may be biased because of confusion with nearby brighter source | 77097787 | 51079497 | 33623727 |
| d | Real source whose photometry may be contaminated by a diffraction spike | 83844 | 247307 | 348507 |
| s | Real source whose photometry may be contaminated by a horizontal "stripe" (electronic cross-talk) due to a bright star | 2021035 | 1883719 | 2127606 |
| b | Real source that is confused in the bandmerging process because of a nearby source | 40450 | 42178 | 44281 |
| P | Spurious detection of a Persistence (Latent) image of a bright star | 19467284 | 13918101 | 8146996 |
| C | Spurious detection in the Confusion "halo" around a bright star | 1910380 | 1614173 | 1301506 |
| D | Spurious detection on a Diffraction spike from a nearby bright star | 4079483 | 5311416 | 4167825 |
| G | Spurious detection of a dichroic Glint | 5219147 | 1105379 | 3613477 |
Flux measurements of faint source extractions with SNR<8-9 in the Survey PSRT 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 Survey PSRT (as well as the 6x and Calibration Scan PSWDBs) 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 is shown in Figure 1. Measurements from the calibration scans of the 90067 field are used as a proxy for survey measurements because they have the same exposure time and therefore the same sensitivity. In the top panels of Figure 1, 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 1 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 1 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 1 - 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 Survey PSRT contains a number of repeated, independent measurements of sources that fall in regions observed multiple times during 2MASS survey operations. This "overcompleteness" is another reason that the PSRT is not suitable for studies based on the statistics of pure source counts. Use the 2MASS All-Sky PSC for this type of inquiry.
The duplicate detections in the Survey PSRT is also one of its virtues. Photometric and positional measurements of sources detected many time can be combined to provide photometry and astrometry well beyond what is possible from single observations. Information about potential source variability and proper motion is provided by statistics on the distribution of flux and position measurements of multiply-detected sources. Finally, source detection statistics provide an independent assessment of reliability; spurious detections are less likely to be repeatedly detected.
The Survey Merged Point Source
Information Table provides combined brightness
and position measurements for multiply-detected point sources
in the survey 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 Survey PSRT: 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.
No attempt was made to remove detections of non-inertially fixed source extractions listed in the Survey PSRT (or All-Sky PSC). Sources that are near the positions of known asteroids, comets, planets or planetary satellites at the time of the survey 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.
The complete lists of asteroids, comets, planets and planetary
satellites associated with 2MASS all survey point source extractions
are provided in A1.6.
All point source extractions in the 2MASS survey scans were positionally correlated with the Tycho 2 and USNO-A2.0 optical catalogs.
2MASS survey sources that have optical catalog counterparts within ~5´´ have listed in the Survey PSRT 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, Tycho 2 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´´
A software error in setting the optical catalog identifier flag,
a, resulted in an incorrect
value of a="0" being assigned to optical associations that have
dist_opt5´´.
There are 525,487 sources in the PSRT 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 Survey PSRT sources.
Missed Optical Associations Near the Equatorial Poles
An error in the positional correlation procedure between 2MASS and USNO-A2.0 sources resulted in associations being systematically missed in the vicinity of the north and south equatorial poles. This error is discussed in I.6.b.xvi.
For sources with |dec|>86°, the optical association information is not reliable. The absence of optical association data in the PSRT record does not necessarily mean that there is not an optical source within 5´´ of the 2MASS position.
iii. Survey Extended Source Reject Table (XSRT)
- Please see Cautionary Notes for the 2MASS All-Sky Release Extended Source Catalog (I.6.c)
- Nature of the Survey Extended Source Reject Table
- Low Reliability Extractions in the XSRT - The "Reliability" Flag (rel)
- Visual Examination: the "vc" flag
- Artifacts and Source Contamination: The cc_flg Flag
- Multiple Detections in the Survey XSRT
- Large Galaxies
The Survey XSRT is not a "Catalog" in the same sense as the highly reliable, uniform All-Sky XSC.
The Survey XSRT contains all extractions of sources believed to be extended relative to the instantaneous point-spread-function (PSF) in all 2MASS survey observations that were not used for the All-Sky XSC. 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.
A distinguishing feature of the Survey XSRT is that it contains multiple, independent extended source measurements in the survey tiles. For example, the XSRT contains independent measurements of 349,184 sources found in the All-Sky XSC. Thus, the XSRT 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 survey extended source processing (see IV.5.d) attempts to identify all source detections that are not well-fit by a single PSF. Because the classification algorithms are not perfect, the XSRT contains detections of both true extended sources such as as galaxies and nebulae, as well as detections of close multiple and single 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 Survey XSRT 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 7 contains a breakdown of the Survey XSRT according to reliability flag value, and the probability of reliability associated with each value of rel.
Users should
select XSRT sources having a reliability flag value of
rel="A" to minimize the number of false-extended sources
and spurious extractions.
Caution should be exercised when using any XSRT 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 Survey Image Atlas.
| rel Value | Number (XSRT) | Probability of Reliability |
|---|---|---|
| A | 682720 | >90% |
| B | 65024 | 80-90% |
| C | 17791 | 70-80% |
| D | 43277 | 50-80% |
| E | 69890 | 30-50% |
| F | 64739 | <30% |
The images of over 260,000 extended source extractions in the Survey XSRT have been visually inspected and classified. Virtually 100% of those brighter than Ks=12.5 mag have been examined. The results of these examinations are encoded in the visual classification score (vc) parameter. The possible values of vc and frequency of occurrence in the Survey XSRT are listed in Table 8.
| vc | Number of Sources | Comment |
|---|---|---|
| 1 | 190925 | Galaxy or Galactic Extended Source - sources that have shapes and surface brightnesses that are associated with external galaxies or galactic nebulae. The latter are highly confined to the Galactic Plane (|b|<3°). |
| 2 | 49550 | Star/point sources or Artifact - sources that are unresolved or barely resolved with respect to the instantaneous point spread function, or marginally resolved double and triple stars. Most of these are foreground (Milky Way) stars. Also includes spurious extractions of image artifacts usually produced by nearby bright stars. |
| -1 | 682355 | Not Classified - extractions that have not been visually classified. |
| -2 | 20611 | Unknown - Objects that are not reliably classified. They tend to be faint and compact, but are most likely extragalactic in nature. |
The cc_flg is used to denote Survey XSRT 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.
The "contamination and confusion flag" (cc_flg) indicates whether or not an extended source extraction is associated with an image artifact caused by nearby bright stars, or is a segment of a large galaxy. Unlike the All-Sky XSC, the Survey XSRT does not contain entries whose positions, photometry and shape information is from the 2MASS Large Galaxy Atlas.
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-XSWDB are summarized in Table 9. Note that the calibration scan cc_flg encoding is slightly different than used for the All-Sky XSC. Artifacts are denoted in the Cal-X SWDB 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 | Number of Sources | Comment |
|---|---|---|
| 0 | 892821 | nominal |
| A | 49550 | artifact |
| z | 425 | large galaxy "chaff" |
| X | 645 | associated with large galaxy |
The Survey XSRT contains a number of repeated, independent measurements of sources that fall in regions observed multiple times during 2MASS survey operations. This "overcompleteness" is another reason that the XSRT is not suitable for studies based on the statistics of pure extended source counts. Use the 2MASS All-Sky XSC for this type of inquiry.
The repeated source detections in the Survey XSRT are one of its virtues. Photometric measurements of sources detected many times can be combined to provide photometric accuracy well beyond what is possible from single observations. Information about potential source variability is provided by statistics on the distribution of flux of multiply-detected sources. Finally, source detection statistics provide an independent assessment of reliability; spurious detections and non-extended sources are less likely to be repeatedly detected or classified as extended.
The Survey Merged Extended Source Information Table provides combined position, brightness and shape information for multiply-detected extended sources in the survey 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 Survey XSRT: the spos and sdet columns in the XSRT 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.
2MASS Atlas Images within each survey scan were constructed with 54 pixels (54") of declination overlap between adjacent images. Because extended source processing operated on each individual Atlas Image, objects that fell into the overlap region between images or that span more than one image within a scan may have been detected more than once. All such "in-scan duplicate" detections were loaded into the Survey XSWDB. These duplicates differ from objects detected in regions that were scanned multiple times during the survey because the measurements are not independent.
The Multiple Source Detection Resolution processing used to prepare the All-Sky Release XSC selected only one unique apparition of of an extended source for inclusion in the Catalog. However, the in-scan duplicates remain in the Survey XSRT. In-scan duplicate detections can be identified as two extended source entries with 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 Survey XSRT does not contain the results of the special Large Galaxy processing that characterizes extended objects that may span more than one survey Atlas Image. Users should consult the 2MASS All-Sky XSC or Large Galaxy Atlas for measurements of galaxies that lie within approximately 1' of any edges of the calibration scans, or that are larger than ~50" in diameter.
Extended source pipeline data processing operated on single 2MASS Atlas Images. As a result, the flux and/or position of objects with large angular extent that intersected the edge of an Atlas Image were usually inaccurate. Flux was missed from the sections of the object not on the image, and the background removal process was compromised by the small area of the image relative to the size of the object. A special Large Galaxy Atlas (LGA) was produced that contains source measurements made from mosaics of Atlas Images large enough to fully contain the large objects. The LGA entries were added to the All-Sky XSC, but are not found in the Survey XSRT.
The Survey XSRT does contain "detections" of large galaxies. However, these detections usually correspond to the sections of the galaxies found on the individual images. Very large sources were often split into a number of fragments, all of which were independently listed in the XSRT. Such fragments are flagged in the XSRT by having cc_flg values of "z" or "X".
- Please see Cautionary Notes for the 2MASS All-Sky Release Image Atlas (I.6.d)
- Backgrounds
- Saturation
- Multiple Images at a Specific Location
The survey 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 survey 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.
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 survey scan data pipeline processing.
Therefore, users are advised to defer to the All-Sky
PSC and XSC and Survey PSRT and XSRT
for photometry of bright point and extended sources.
Because regions of the sky were observed more than once during survey observations, the Full Survey Image Atlas may contain more than one independent images covering a particular location on the sky. When examining the image of a particular source selected from the All-Sky PSC or XSC or Survey PSRT or XSRT, 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 Survey Atlas. If these are not specified, you may not receive the image from the correct scan.
[Last Updated: 2006 November 13; by R. Cutri]
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