I. Introduction

4. Cautionary Notes

b. Source Catalog

Contents

1. Source Detection
2. Position Reconstruction
3. Source Photometry
4. Photometric Uncertainties
5. Photometrical Calibration and Color-Corrections
6. Unreliable Sources
7. Very Bright Sources
8. Artifact Flagging
9. Extended Sources
10. 2MASS Associations

i. Source Detection

1. The Depth the Preliminary Release Source Catalog Varies With Position on the Sky

The limiting magnitude of the objects in the Preliminary Release Source Catalog is regulated by the net exposure time accumulated by WISE and characteristics of the sky where source extraction takes place. Among the factors that dictate the achieved limiting depth are:

• Effective depth-of-coverage (VI.2), including survey coverage, data quality and frame and pixel outlier rejection,


• Background emission levels and structure (Zodiacal emission, diffuse and complex Galactic emission),


• Confusion noise in high source density regions, and


• Effective sky masking by very bright sources (i.e. scattered light halos, diffraction spikes, latent images, ghosts and glints) and extended saturated regions (e.g. the Galactic Center, Orion).

The bulk variation in depth of the Source Catalog around the sky is illustrated in Figures 1-4 that show 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 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.

Figure 1 - W1	Figure 2 - W2
Figure 3 - W3	Figure 4 - W4
False-color maps showing the spatial distribution and variation of the peak of the differential source count histograms computed in 0.2x0.2 deg spatial bins. The color scale in the images gives the peak magnitude color encoding for each band. The Maps are Hammer projections in ecliptic coordinates. The centers correspond to ecliptic coordinate 180°,0°. Ecliptic north is towards the top, and ecliptic longitude increases towards the right.

2. The Preliminary Release Source Catalog Has A Conservative Detection Threshold

To emphasize reliability of the Preliminary Release Catalog, a conservative source detection threshold was used during first-pass Multiframe processing. Sources were required to have SNR>5 on the multiband sigma images (IV.5.b for detection and characterization to construct the Working Database (WDB) of extractions. Furthermore, a WDB entry was required to have a SNR>7 in at least one detected band to be included in the Catalog, as described in V.3.b.

You will very likely be able to see faint sources on Atlas Images that do not have counterparts in the Preliminary Release Source Catalog as a result.


3. Suppressed Detections of Sources Around Very Bright Stars

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.

4. Extended Sources Split Into Multiple Components

As discussed below, Sources that are significantly larger than the WISE beam may be artificially split into multiple extractions. Hence, very large galaxies and complex nebulosity in the Galactic Plane may produce clusters of entries in the Source Catalog.


5. Sources Lost Due to Confusion in the Duplicate Resolution Processing

The duplicate source resolution process in Source Catalog generation identified duplicate detections in the Tile overlaps regions and selected one for inclusion in the Source Catalog using simple geometric criteria. The selected duplicate apparitions were then required to satisfy the final source selection criteria to be included in the Catalog. In a small number of cases, one member of a group of multiple detections may have been selected by the duplicate resolution test but did not satisfy the final source selection criteria whereas the apparition that was not selected by duplicate resolution would have met the final criteria. 1,923 objects are known to have been omitted from the Catalog because of this confusion.

ii. Position Reconstruction

1. Systematic Declination Errors

The positions of objects in the WISE Preliminary Release Source Catalog may be systematically offset from their try position by up to 1" in the declination direction.

Approximately 20% of sources fainter than W1~14.5 mag exhibit this bias, while an increasingly smaller fraction of the sources are biased up to at least W1=13.0 mag. The declination bias was caused by an error in the pipeline source extraction software that failed to apply a 0.5 Atlas Image pixel declination offset to some sources when performing simultaneous PSF fitting of multiple detections in close proximity (i.e. passive deblending). The sign of the declination bias depends on the scan direction and celestial hemisphere in which a source is location. The error does not affect right ascension measurements. See VI.5 for illustrations of the effects of this error and discussion of its impact.

The quoted position declination uncertainties in the Catalog, sigdec, have been inflated to reflect the presence of the bias. See the following section for a description.

Reliable positions may be extracted by measuring source positions on the Atlas Images. However, positions derived using flux-weighted centroid or peak finding and the WCS information in the Atlas Image headers is also biased relative to the Source Catalog by up to 0.5" in W1-W3 and up to 1.4" in W4. See II.3.g for a procedure to correct the Atlas Image derived source positions and image WCS.

2. Catalog Declination Uncertainties Have Been Inflated to Reflect the Declination Bias

Because there was insufficient time to correct the declination bias prior to the Preliminary Release, the Catalog declination uncertainties, sigdec, were inflated to reflect the presence of the bias. The uncertainties were adjusted by adding 0.5" in quadrature to the statistical declination measurement uncertainty (see VI.5.b). This yields a maximum absolute deviation for the distribution of uncertainties equal to the what would be produced if the declination error distribution was Gaussian.

Users who conduct studies based on the Preliminary Release Catalog positions should note that the distribution of true position residuals with respect to the 2MASS reference frame is not Gaussian, and will not scale as they would for a Gaussian (see VI.5.b).

3. Differential Aberration not Corrected in First-Pass Processing

Correction for differential aberration was not made during the first-pass processing. This can contributes a small (~0.1") increase in the observed positions residuals with respect to the 2MASS reference frame that correlates with position on the sky because it is related to the WISE relative orbital velocity. Correction for differential aberration will be included in second-pass processing.

iii. Source Photometry

1. Catalog Photometry is Optimized for Point Sources

For point-like objects, the profile-fit photometry measurements (w?mpro) provide the most accurate measurements for unresolved (to WISE) objects. The curve-of-growth corrected "standard-aperture" photometry can on average provide slightly better accuracy for sources with approximately SNR>30 because they not subject to PSF knowledge uncertainties like the profile-fit measurements. However, aperture measurements are more susceptible to contamination from aberrant and masked pixels within the measurement aperture.

The profile-fit and standard aperture photometry will systematically underestimate the brightness of objects that are extended with respect to the WISE PSF. See below for further discussion of extended objects.

2. A non-null entry in the WISE Source Catalog magnitude column for a given band does not necessarily mean a detection in that band.

If the associated measurement uncertainty in band is "null", then the quoted magnitude is computed from a 2σ brightness upper limit (see IV.5.c.iii.3). Care must be taken to not mis-interpret upper limits as detections.


3. Systematic Brightness Underestimation In Very High Source Density Regions

As discussed in VI.4.c, WISE source brightnesses are systematically underestimated in very high source density regions. This occurs because background levels in those regions are overestimated by contamination from surrounding stars. Improved background estimation algorithms have been employed for WISE second-pass processing.


4. Aperture Photometry Limitations

The Preliminary Release Source 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 measurements.

• The standard-aperture photometry in the Source Catalog has been curve-of-growth corrected.


• The W4 apertures for a given group of magnitudes are always a factor of two larger than the W1, W2 and W3 apertures


• Aperture photometry is subject to contamination by missing, masked or aberrant pixels in the measurement aperture, confusion with nearby objects and saturation. Use the aperture photometry measurement quality flags, w?flg, w?flg_1, w?flg_2, to assess if one or more pixels in the measurement aperture was contaminated.


• Aperture photometry will systematically underestimate the brightness of saturated sources.


• As described in VI.4.c, aperture photometry for sources fainter than 13 mag (W1 and W2) becomes systematically brighter than the profile-fit photometry because of confusion with faint sources within the measurement aperture. This impacts primarily W1 and W2 because source densities are much higher on average.
5. Erroneous Flux Measurements for Sources in Low or No-Coverage Regions

Sources that are extracted near regions with very low or no depth-of-coverage sometimes exhibit erroneous flux measurements because the background estimate has been compromised by the low coverage. In these cases, the background estimates can be aberrantly low, resulting in an apparent bright source 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.

6. Active Deblending Will Split Sources Into At Most Two Components

WISE profile-fitting photometry attempts to deblend a detection into two components if the single-PSF fit chi-squared goodness-of-fit value is >1.8. 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. For first-pass processing, at most one component can be added. Actively deblended sources in the Preliminary 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.

iv. Photometric Uncertainties

1. Several Different Measurements Of Photometric Uncertainty Are Provided in the Source Catalog



WISE Source Catalog entries contain several different measurements of photometric uncertainties.

• w?sigmpro - These are the profile-fit photometry measurement uncertainties in magnitude units. Similarly, w?msigm are the formal aperture photometry measurement uncertainties. Note that the magnitude uncertainties are null when the sources are not detected.


• w?snr - These are the ratios of the flux (in DN) to the flux uncertainty from the profile-fit photometry measurements. The profile-fit magnitude uncertainties are derived from these values if the SNR is >2. The SNR values are populated even for SNR<2 measurements.


• w?sigp1 and w?sigp2 - These are the standard deviation of the population and standard deviation of the mean of the fluxes measured on the individual frames used for the profile-fitting photometry, respectively. The former provides a measure of the characteristic measure of the scatter in the single-exposure measurements. These are useful as diagnostics with which to compare to the formal profile-fit measurement uncertainties, which come from PSF-fitting on all exposures simultaneously, but should not be used as a substitute for them.


2. Profile-fit Photometry Uncertainties Are Overestimated in High Source Density Regions (leads to systematically low chi-squared values)

The profile-fit photometry measurement uncertainties, w?sigmpro are systematically underestimated in high source density regions because the contribution of confusion noise was overestimated. Consequently, the profile-fitting chi-squared values, w?rchi2, are systematically underestimated in these regions which can have adversely affect decisions to deblend confused sources in crowded fields.


3. Profile-fit Measurement SNR Values (w?snr) Typically Do Not Exceed 40-60

The profile-fit photometry flux SNR values in the WISE Source Catalog, w?snr, do not usually exceed ~60 even for extremely bright sources. This occurs because the profile-fit measurement includes a PSF uncertainty component that dominates the measurement precision at high SNR levels. As a result, profile-fit measurement signal-to-noise ratios do not continue to increase arbitrarily with source brightness.

v. Photometric Calibration and Color Corrections

1. Photometry Presented in Natural WISE System

All Catalog source magnitudes represent in-band brightness measurements that are calibrated by comparison with WISE measurements of a network of standard stars of presumably known brightness within the WISE bands (see IV.3.g). The magnitudes are not monochromatic values.


2. Zero Point Stability and Calibration Accuracy

The WISE system photometric response was stable to <<1% over the life of the cryogenic survey (IV.3.g.vii). Therefore, single photometric instrumental zero points in each band are used to calibrate photometry. For first-pass processing, these zero points were defined using the first two months of on-orbit data.

Residuals biases with respect to the mean brightness of the standard stars is <0.01 magnitudes in W1, W2 and W4 in the Source Catalog. For W3, a there is a mean bias of approximately 0.022 magnitudes in the sense that the Catalog W3 magnitudes may be on average 0.022 mags fainter than their quoted values (e.g. add 0.022 mag to the Catalog W3 mags to get a better calibrated measurement).


3. Color-Corrections

The broad WISE bandpasses may necessitate significant color corrections when deriving monochromatic flux densities from the in-band magnitudes depending on the spectral energy distribution of the particular object. The nominal system flux zero points are defined for a f_ν∝ν^-2 spectrum through the WISE bandpasses. Color corrections for other spectral slopes are given in IV.3.g.vi and Jarrett et al. (2011 in press).

The reliability of Source Catalog is estimated to by >99.9% for sources brighter than W1<15.6, W2<14.3, W3<10.2, W4<6.5 mag in unconfused regions of the sky with 12 or more frame coverages (VI.7). The fractional reliability decreases for fainter objects and in regions where there is less coverage. Therefore, for a catalog the size of the WISE Source Catalog, there will be thousands of unreliable entries that do not correspond to an actual object on the sky.

The source selection criteria used to construct the Source Catalog were designed designed to construct a reliable Catalog from the superset of extractions in the Working Database. Exploit the WISE survey strategy of obtaining multiple, independent exposures of each region of the sky, conservative detection limits, flagging of artifacts in processing pipeline.

Here are some tips for selecting preferentially reliable entries in the WISE Preliminary Release Source Catalog.

• Select sources detected in more than one band. On average, sources detected in only one of the four WISE bands will be less reliable on average than those detected in multiple bands. This is not to imply that all single-band sources in the Catalog are unreliable. In fact, some of the most interesting objects are anticipated to be detected in only one band (e.g. brown dwarfs, highly embedded protostars, extreme ultraluminous galaxies), but they are statistically rare in the Catalog, constituting only 4.4% of the total. Use care when working with single-band sources.


• Select sources with a depth-of-coverage >5. Frame and pixel outlier rejection during the image coaddition processing is most efficient where the survey depth-of-coverage is highest. Effectively no outlier rejection was performed with depth <5 during image coaddition in Multiframe processing. Sources in lower coverage regions have a higher probability of being spurious detections of transient single-frame artifacts.


• Select sources with higher signal-to-noise ratios. - Sources with higher profile-fit photometry SNR values, w?snr, in one or more bands will be more reliable than those with very low SNR values. This is really an SNR limitation, not a flux threshold. An extraction with bright apparent flux, but with low SNR is just as likely to be unreliable.


• Select sources that are not flagged as spurious detections of image artifacts in any band. The cc_flags Catalog column identifies sources that were positionally associated with the predicted position of artifacts from bright sources. The Catalog source selection criteria require that a source not be flagged as a spurious detection of an artifact in at least one band (along with meeting other reliability criteria in that band). However, Catalog sources may be flagged as spurious detections in other bands, making it suspicious. To exclude such objects, select sources where cc_flags not matches '*[DHOP]*').

vii. Artifact Identification

1. Limitations to Artifact Flagging in the Preliminary Release

Source extractions that are positionally associated with the predicted locations of image artifacts produced by bright sources were identified and flagged in the Artifact Identification subsystem of the WSDS Multiframe Pipeline. Artifact locations are predicted using geometric models scaled with "parent" bright source magnitudes. These models and their parameters were based on analyses of IOC and approximately one month of survey data, so could not be fully optimized in the time necessary to begin first-pass processing. As a consequence, artifact identification and flagging in the Source Catalog is both overly conservative in some areas, and deficient in others.

2. Most Artifacts from Off-Frame Sources Not Flagged

With the exception of latent images, no attempt was made in first-pass processing to flag artifacts from sources detected on a given Atlas Image that were produced by bright sources outside the footprint of that Tile. This includes diffraction spikes, scattered light halos and optical ghost images. Therefore, contaminated sources and spurious detections from off-image sources may be unflagged in the Source Catalog.

3. Overly Conservative In First-Pass Processing

To emphasize the reliability of the Preliminary Release Source Catalog, the artifact flagging model parameters of artifacts were conservatively set in the sense that sources tended to be overflagged. This results in decreased Catalog completeness around bright stars (see VI.2.c.iii, and a large percentage of all sources being flagged as contaminated in very high source density regions.

4. Missed Flagging Due to Inaccurate Bright Star Photometry or Detection

Source brighter than approximately 1.0, 0.0, -2.0 and -6.0 in W1, W2, W3 and W4, respectively, were not reliably extracted in first-pass processing (VI.4.d.iv). Therefore, artifacts produced by these objects will not be flagged in the Source Catalog. For example, the star Aldebaran is in the Preliminary Release Catalog, but it has valid photometry only in W4 (-2.90 mag). It was not properly extracted in W1, W2 and W3, and therefore artifacts and spurious detections from Aldebaran's artifacts in those bands will be in the Source Catalog without appropriate artifact flagging.

viii. Very Bright Sources

1. Sources brighter than approximately 1.0, 0.0, -2.0 and -6.0 in W1, W2, W3 and W4, respectively, may not be in the Source Catalog

Source brighter then these limits were not reliably extracted in first-pass processing (VI.4.d.iv). 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.

2. Saturated Source Photometric Biases

Saturation begins to affect sources brighter than approximately 8.0, 6.7, 3.8 and -0.4 in W1, W2, W3 and W4. Profile-fit photometric measurements are extracted for saturated sources by fitting the PSF to the non-saturated pixels in their wings. Sources brighter than the saturation limit exhibit varying degrees of photometric bias, as described in VI.4.c.i. The most severe bias is seen in W2 where source brightness may be overestimated by nearly one magnitude approximately two magnitudes brighter than the saturation limit.

As discussed above, missing detections of bright stars can lead to the failure to flag spurious and contaminated to detections that may wind up in the Catalog.

ix. Extended Sources

1. The WISE Source Catalog contains both Point and Resolved Source

WISE does not have a separate extended source catalog. The WISE Source Catalog contains entries for all sources detected on the Atlas Images, both point-like and small resolved objects.

2. Profile-fit and Standard Aperture Photometry Underestimate the Brightness of Resolved Sources

The Profile-fit and standard aperture photometry in the Source Catalog are optimized for point-source characterization. They both systematically underestimate the brightness of extended sources (IV.5.c.vi). Refer to the ext_flg to identify Source Catalog entries that may be extended.

Large aperture photometry is provided for Catalog entries, and those measurement may provide better estimates for extended objects. For WISE sources that are associated with objects in the 2MASS Extended Source Catalog (XSC), measurements in elliptical apertures scaled from the object size and shape in the 2MASS XSC are provided in the WISE Catalog that better characterize the object flux.

3. Large Extended Objects May Be Broken Into Multiple Detections in the Source Catalog

The source detection and extraction steps in the WISE pipelines may split large, extended objects into multiple pieces. Any and all pieces that that satisfy the source selection criteria will appear in the Catalog. See IV.5.c.vi for examples that illustrated extended source splitting. Refer to the xscprox column to find WISE Catalog entries that may be sections of large galaxies identified by the 2MASS XSC. However, WISE is sensitive to large Galactic diffuse emission structures, and these objects will also be split into multiple components that will not be tracked by the xscprox column.

x. 2MASS PSC Associations

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 positions (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: 2011 May 4