Approximately 57.5, 130.5, 434.7, 300.5 sq.deg. of the sky in W1, W2, W3 and W4, respectively, are not covered by the All-Sky Release Source Catalog because of insufficient depth-of-coverage by individual frames with acceptable quality (V.2). Although WISE obtained multiple, independent single-exposures on 100% of the sky area, some single-exposure framesets were excluded from second-pass Multiframe Pipeline processing from which the All-Sky Release Atlas and Catalog were generated. Sources extracted during Multiframe processing were included in the Catalog only if they satisfied a number of reliability criteria including the requirement that they were extracted from regions where at least five independent single-exposures were available for measurement in at least one band. Areas with less than five frame depth-of-coverage will therefore not be covered by the All-Sky Release Catalog.
Survey depth-of-coverage is discussed in more detail in VI.2.
Source extractions from all Single-exposure Images obtained by WISE during the full cryogenic survey period are available in the Single-exposure Source Working Database, regardless of the quality of the Single-exposure framesets. Therefore, it may be possible to find measurements of brighter sources in the Catalog coverage gaps in the Single-exposure Working Database. However, the reliability and/or accuracy of measurements of entries in that database may be compromised. Please review the Cautionary Notes for using the Single-exposure Source Working Database.
Because the All-Sky Catalog source selection criteria (V.3) required extractions to be in a location with at least five independent exposures available, but the All-Sky Atlas can have real pixel data in regions with only one exposure coverage, you may see real sources on the Atlas Images that are not found in the Catalog.
Multiframe Pipeline processing source extractions within low coverage areas may be found in the All-Sky Release Reject Table. However, the reliability of extractions in low coverage areas may be compromised. Please read II.4.a.ii for suggestions on how to identify reliable detections in the Reject Table.
The All-Sky Release Source Catalog is not a "Point Source" Catalog. It contains detections of point-like objects, such as stars and unresolved galaxies, as well as resolved sources such as close multiple stars and galaxies, and detections of sections of large nearby galaxies and clumps or filaments in Galactic nebulosity, as long as they meet the Catalog selection criteria described in V.3.
WISE data processing did not produce a separate extended source catalog. However, Source Catalog entries that may be resolved relative to the WISE PSF in any band are identified by having an ext_flg value >0. 28,792,758, or 5.1% of the entries in the Source Catalog have non-zero ext_flg values. The profile-fitting (w?mpro) and curve-of-growth corrected aperture photometry (w?mag) are optimized for point sources and may underestimate the true brightness of extended sources. See I.4.b.iii for a discussion of alternative measurements for resolved objects in the Source Catalog.
The limiting magnitude of the objects in the All-Sky Release Source Catalog is regulated by the net exposure time accumulated by WISE and the characteristics of the sky where source extraction takes place. Among the factors that dictate the achieved limiting depth are:
The bulk variation in the Source Catalog depth around the sky are illustrated in Figures 9-12 in II.2.c which show sky maps of the peak of the Catalog source count histograms in each band. The impact on the effective depth of the Catalog by survey depth-of-coverage and decreasing Zodiacal background emission is manifested as the increasing depth from the ecliptic plane towards the poles. The loss of depth due to confusion in the Galactic Plane is most prominent in W1 and W2 where the source density is highest.
The are a number if circumstances in which objects that are clearly visible on the All-Sky Atlas Images may be missing from the All-Sky Release Catalog.
Source detection and extraction is performed simultaneously in all WISE bands, as described in IV.4.b and IV.4.c. Catalog objects that have relatively high SNR measurements in some bands may have very low SNR "detections" in other bands. Taken by themselves, the very low SNR bands might not have triggered detections. But because the Catalog selection criteria (V.3) required that a source have SNR>5 in only one band, there are many Catalog entries that have SNR<5 detection in other bands.
A by-product of the simultaneous multiband detection and extraction
and the very high density of W1 and W2 sources around the sky is that
short wavelength detections of real stars and galaxies that are superimposed
on structured diffuse emission in W3 and W4, such as nebulosity in the ISM,
zodiacal dust bands or cometary dust trails, will result in low SNR
"detections" in the longer wavelength bands even if there is not a discrete
source present. This effect accounts for the structure
seen at faint flux levels in the W3 and W4 Source Catalog count maps
(Figures 5 and 6 in II.2.c)
that traces the zodiacal dust bands and scattered light halos around
Very bright sources effectively mask the sky in their vicinity. Detections of fainter sources are suppressed because the local background levels are elevated from the scattered light halos, and because of confusion with diffractions spikes, latent images, and optical ghost images from the bright sources. Furthermore, even when fainter sources are detected near very bright objects, they may be flagged as contaminated and inadvertently filtered from the Catalog. The impact of masking by bright sources is discussed in VI.2.c.ii.
The reliability of Source Catalog is estimated to be >99.9% for sources brighter than SNR=20 in unconfused regions of the sky (VI.6). The fractional reliability decreases for fainter objects and in regions where there is less coverage. Even with this very high level of reliability, the size of the All-Sky Release Catalog means that it may contain thousands of entries that do not correspond to an actual source of infrared radiation on on the sky.
The source selection criteria used to construct the Source Catalog (V.3) were designed to select reliable extractions from the superset in the Working Database produced by Multiframe Pipeline processing. These criteria we intended to exploit the WISE survey strategy of obtaining multiple, independent exposures of each region of the sky, conservative detection limits and flagging of artifacts in processing pipeline.
When in doubt about the reliability of any entry in the WISE All-Sky Catalog, the very best way to verify that is is real is to look at its position in the WISE Atlas Images.
Here are some tips for selecting preferentially reliable entries in the WISE All-Sky Release Source Catalog.
As described in VI.3.c.2 the Catalog profile-fit photometry measurements are systematically fainter than measurements of the same objects using deeper and higher spatial resolution data from the Spitzer Space Telescope. The flux underestimation begins at approximately W1=14 mag and W2=13.5 mag, and increases with decreasing source brightness, reaching biases of as much as 0.3-0.4 mag at the WISE faint detection limit. The amplitude of the bias is larger in regions with higher source densities. This bias occurs because background levels were overestimated in the source extraction process.
There is a similar bias observed between WISE profile-fit and WISE aperture photometry for faint sources because the two are affected differently by source confusion and the background overestimation problem.
WISE Catalog W3 and W4 profile-fit photometry is not as strongly affected by these biases because of the lower source surface densities and higher background levels at those wavelengths.
The Catalog profile-fit photometry (w?mpro) and curve-of-growth corrected "standard-aperture" photometry (w?mag) are optimized for point sources. Profile-fitting provides the most accurate measurements for unresolved (to WISE) objects, and the standard aperture photometry provides slightly better accuracy for sources with approximately SNR>30 because they are not subject to PSF knowledge uncertainties like the profile-fit measurements. Because the All-Sky Release Catalog contains resolved sources as well as point-like ones, the profile-fit and standard aperture photometry will systematically underestimate the brightness of objects that are extended with respect to the WISE PSF. See the discussion of extended sources in I.4.b.x for a description of better photometry for resolved objects.
If the associated measurement uncertainty in band is "null", then the quoted magnitude is computed from a 2σ brightness upper limit (see IV.4.c.iii.3). Care must be taken to not mis-interpret upper limits as detections.
The All-Sky Release Catalog contains aperture photometry measurements made on the Atlas Images for apertures ranging in size from 5.5" to 24.75" (W1-W3) and 11.0" to 49.5" (W4). These measurements have not been curve-of-growth corrected like the standard-aperture magnitudes. The large-aperture photometry is useful for estimating the brightness of small, extended sources too large to be properly measured using the profile-fit and standard-aperture photometry.
Users should be aware of features and limitations of the aperture photometry measurements.
Sources that are extracted near regions with very low or no depth-of-coverage sometimes exhibit erroneous flux measurements because the background level has been grossly underestimated in the low coverage area. In these cases, an apparently bright source is reported where one does not exist. Many of these sources are suppressed by the minimum depth-of-coverage requirement for Catalog source selection (VI.3.b), but sources affected by this on one band may be pulled into the Catalog if they have better coverage in other bands.
Refer to the w?m and w?cov parameters in the Catalog to identify bands that may affected by low coverage.
WISE profile-fitting photometry attempts to actively deblend a single source detection into two components if the chi-squared goodness-of-fit to a single PSF is >1.5. If the resulting two-component fit reduces the chi-squared from the initial one, and the two-component chi-squared is <3 (from an initial value >3), the two sources are reported. In WISE second_pass processing, active deblending was limited to adding at most one component. Actively deblended sources in the All-Sky Release Source Catalog can be identified by having the parameter na=1.
Note that active deblending was not performed in the profile-fitting measurements for the Single-exposures during Scan/frame Pipeline processing.
WISE Source Catalog entries contain several different measurements of photometric uncertainties.
The profile-fit photometry measurement model includes a term that accounts for the uncertainty in the PSF template. The PSF uncertainty term dominates the measurement precision at high SNR levels. As a result, All-Sky Release Catalog w?snr values do not exceed ~60 even for extremely bright but non-saturated sources.
All Catalog source magnitudes represent in-band brightness measurements that are calibrated by comparison with WISE survey measurements of a network of standard stars of known brightness within the WISE bands. The magnitudes are not monochromatic values. See IV.4.h for a discussion on how to convert WISE magnitudes to flux densities.
Be aware of the systematic flux bias
towards fainter profile-fit magnitudes when converting those magnitudes
to flux densities.
The WISE system photometric response was stable to <<1% over the life of the full cryogenic survey (see IV.4.h.iii). Therefore, single photometric instrumental zero points in each band are used to calibrate photometry. Improved instrumental photometric zero points were derived using all cryogenic survey data for second-pass data processing.
Residuals biases with respect to the mean brightness of the standard
stars are <0.01 magnitudes in all bands.
The nominal WISE photometric system flux zero points are defined for a fν∝ν-2 spectrum through the WISE bandpasses. Because of the broad WISE bandpasses, color corrections may be necessary when converting magnitudes to flux densities depending on the spectral energy distribution of different objects. Color corrections for other spectral slopes are given in IV.4.h.vi and Jarrett et al. 2011, ApJ, 735, 112.
Position reconstruction for WISE was performed using a subset of well-measured objects in the 2MASS Point Source Catalog (see IV.4.d.i) as the astrometric reference. As discussed in VI.4, the astrometric residuals relative to the 2MASS frame are of order 0.2" RMS on each coordinate axis for non-saturated, high SNR sources, gradually increasing with decreasing source SNR. The residuals are uniform across the sky indicated that WISE astrometry is tied rigidly to the 2MASS frame.
Although the 2MASS reference frame is tied to the International Celestial Reference System (ICRS) via the Tycho Catalog, because of non-negligible proper motion of 2MASS PSC reference stars in the ten years separating the 2MASS and WISE epochs, the 2MASS reference frames has distorted slightly with respect to ICRS. 2MASS source proper motions therefore imprint small but systematic position biases of up to ~150 mas between the WISE reference frame and the ICRS that vary coherently over the sky. See VI.4.f for a more detailed discussion of the All-Sky Catalog astrometric performance relative to the ICRS.
The amplitude of the position uncertainties in the All-Sky Release Catalog are given by the sigra and sigdec parameters. The value of these parameters are underestimated and become unphysically small for heavily saturated sources that have W1<4 mag.
Source brighter then these limits were not reliably extracted in pipeline processing (VI.3.d.ii.3). If they are brighter in all bands, they may not be appear in the Source Catalog. Such objects may be in the Source Catalog if they were properly extracted in a fainter band. However, photometry will likely will be missing or badly inaccurate in the very bright bands.
Missed extractions and inaccurate photometry can result in spurious detections of and contamination by image artifacts produced by the bright sources failing to be flagged in the artifact identification subsystem of the WISE pipelines (IV.4.g).
Saturation begins to affect sources brighter than approximately 8.1, 6.7, 3.8 and -0.4 in W1, W2, W3 and W4, respectively. WISE profile-fit photometry extracts measurement of saturated sources by fitting the PSF to the non-saturated pixels in their wings. The accuracy of photometric measurements of saturated degrades with increasing brightness, and exhibits varying degrees of photometric bias, as described in VI.3.c.4. The most severe bias is seen in W2 where source brightness may be overestimated by nearly one magnitude at W2=3 mag, approximately 3.5 magnitudes brighter than the saturation limit.
The WISE pipeline source extraction algorithm (IV.4.c) relied on the masking of saturated pixels that was based on pixel encoding provided by the WISE payload Digital Electronics Board (DEB). The source detection and profile-fitting photometry routines excluded flagged saturated pixels to avoid biasing measurements. A small number of pixels in the WISE focal plane detections may have reached hard saturation before the threshold DN value used by the DEB to flag saturation was reached. When this occurred, the saturated pixels were incorrectly factored into the profile-fit and the resulting flux measurement was too low. A relatively small number of pixels suffered from this deficiency, and therefore the occurrence of flux underestimation is rare. In addition, the influence of this effect is mitigated in the Multiframe Pipeline processing because photometric measurements of an object are made on many frames at once, of which at most one frame may be contaminated by unflagged saturation.
Multiframe Pipeline source extractions that are positionally associated with the predicted locations of image artifacts produced by bright sources were identified and flagged by the Artifact Identification pipeline subsystem. Artifact locations were predicted using geometric models scaled with "parent" bright source magnitudes. For WISE second-pass data processing, the artifact prediction models were greatly improved through analysis of data from the full cryogenic survey. Improvements were also added to account for varying artifact contrast in response to different background emission levels, the changing size of artifacts with different survey depth-of-coverage, and to enable flagging artifacts from very bright sources that are off the field of on individual images. While the many improvements yielded excellent overall results, it was not possible to develop algorithms that could flag perfectly artifacts and contamination by artifacts under all conditions over the entire sky. As a result, there may be spurious detections of artifacts that were inadvertently included in the Catalog, and reliable sources that were omitted from the Catalog because of overly conservative flagging.
The known limitations to the artifact flagging algorithms are summarized in IV.4.g.vii.
The accuracy achieved by artifact flagging was dependent on how well the position and brightness of bright "parent" sources were known. The flagging of artifacts from bright sources off the Atlas Image fields-of-view or from sources too bright to have been reliably extracted in the WISE data was based on a prior Bright Source List (BSL). The BSL was constructed using a combination of WISE source extractions from first-pass processing, and external data sets such as the IRAS Faint Source Catalog and 2MASS Point Source Catalog to fill in for the very brightest sources.
The positions and fluxes of very bright objects in the BSL may be inaccurate because of proper motion, source variability, or errors introduced by having to estimate fluxes in the WISE bands using data at other wavelengths. As a result, the resulting flagging of artifacts from these objects may be similarly uncertain.
Although they moved during the time covered by the WISE Single-exposures, very bright asteroids and comets do not move sufficiently far to be cleanly eliminated from the Atlas Images by pixel outlier rejection. Consequently, they will trigger detections that do not have associated unique position or brightnesses in the Multiframe Pipeline processing. As a consequence, the efficiency of artifact flagging is compromised around these objects. An example of this is the asteroid 4 Vesta, whose residual images can be seen in the W3 and W4 Atlas Images in Tile 1504p196_ab41. There are an excess of spurious W3 and W4-only detections in the Catalog near the trail left by the asteroid.
Spurious detections of artifacts caused by Mars, Jupiter and Saturn were cleaned from the Catalog manually, as described in VI.3.b.iv.
The Profile-fit (w?mpro) and standard aperture (w?mag) photometry in the Source Catalog are optimized for point-source characterization. They both systematically underestimate the brightness of sources that are extended relative to the WISE PSF (IV.4.c.vi). Catalog sources suspected of being resolve are identified by having ext_flg values >0.
Large aperture photometry provided for Catalog entries may provide better brightness estimates for extended objects. In addition, for WISE Catalog sources that are associated with objects in the 2MASS Extended Source Catalog (XSC), photometric measurements made using elliptical apertures scaled from the object size and shape in the 2MASS XSC are provided in the Catalog.
The source detection and extraction steps in the WISE pipelines may split large, extended objects such as large galaxies and galactic nebulae into multiple pieces. Extractions of any and all pieces that that satisfy the source selection criteria will appear in the Catalog. See IV.4.c.vi for examples of extended source splitting. The Catalog ext_flg column value can be used to identify WISE sources that may be sections of large extended objects.
The photometry subsystem in the Multiframe Pipeline computes most likely flux of source using multiple single-exposure measurements. Objects whose flux levels varied during the time interval within which the source was measured in the WISE Single-exposure observations will have a mean brightness reported in the Catalog, and may have larger than expected flux uncertainties. The probability of flux variability in each band is tabulated in the Catalogs var_flg parameter for sources with a minimum of size available single exposure measurements. A detailed description of the algorithm used to estimate the probability of variability, known limitations of the estimation, and examples of how to use the var_flg parameter in Catalog queries are given in IV.4.c.iii.4.
Objects that are resolved with respect to the WISE PSF or that are contaminated by image artifacts will exhibit more intrinsic scatter in repeated Single-exposure measurements than clean, isolated point sources. As a result, the probability of variability encoded in the Catalog var_flg may be overestimated for extended and/or contaminated objects. Extended sources can be identified by having ext_flg>0. Source contaminated by image artifacts have cc_flags!='0' in any band that is contaminated.
The WISE Source Catalog entries include photometry for the closest entry from the 2MASS Point Source Catalog that falls within 3" of the WISE source position (IV.7). These are associations not identifications. Although the position accuracy of the two Catalogs is very good, there is a non-zero probability of chance associations between physically unrelated objects, as well as missed associations between the Catalogs. This is particularly true for objects with high proper motions, and those in regions with large foreground extinction. You should always confirm associations by reviewing carefully the entries from both Catalogs.
Last update: 2012 December 27