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Beta Version 1.0 of the
Hubble Source Catalog
Search with Summary Form now (one row per match)
Search with Detailed Form now (one row per source)
The Hubble Source Catalog contains information about sources detected in many of the ACS and WFPC2 images. It is based on the positional crossmatching of source lists in the Hubble Legacy Archive (HLA) across HST visits (pointings) and/or detectors.
The catalog contains all members of the ACS/WFC and WFPC2 Source Extractor source lists in the HLA version DR6 that are considered to be valid detections because they have flag values less than 5. The catalog describes which sources crossmatch together. The crossmatching process involves adjusting the relative astrometry of overlapping images so as to minimize positional offsets between closely aligned sources in different images. After correction, the astrometric residuals of crossmatched sources are significantly reduced to typically less than 10 mas. In addition, the catalog describes source nondetections. The crossmatching algorithms and the properties of the catalog are described in Budavari & Lubow (2012) .
Interfaces
There are two forms interfaces for querying the catalog. They follow the standards of MAST and provide a convenient way to browse the catalog. The Detailed Search Form provides a view of the source catalog that displays an entry for each source. The Summary Search Form displays an entry for each set of matching sources. In addition, we will be providing a more general SQL query interface for more complex and longer running searches through a CasJobs system.
HLA Source Lists
The catalog is based on HLA Source Extractor source lists. To build source lists, the HLA first constructs a white light detection image for each detector within each visit. This filter-combined drizzled image provides added depth. Source Extractor is run on the white light detection image to identify each source and its position. Next, the combined drizzled image for each filter used in the detection image is searched for a source at each position indicated by the detection image. If a valid source is detected, then its properties are entered into an HLA source list for the visit, detector, and filter with a flag value less than 5. Sources that are found in the white light detection image, but not in some filter with value flag less than 5 are regarded as nondetections or color dropouts for that filter. Some of the visits involve a single filter. In that case, the white light image is the same as the filter (color) image and no color dropouts are present.
Limitations
The catalog is preliminary and some of its limitations are listed below. We are working to make improvements on these issues.
- About 10% of values of magnitude values are offset from the correct values by typically about 0.5 mag and as much as 2 mag. The flux values (FluxAper2) in the Detailed Search Form are reliable.
- Some source entries in the catalog are false detections. In some cases, they are due to cosmic rays. In nearly all cases, they do not crossmatch with a source in another visit or detector.
- There is other incorrect information about the sources in the catalog, such as exposure start times and positions, due to errors in the source lists. These cases are rare.
- About 20% of the ACS/WFC and WFPC2 images do not have source lists and therefore do not have sources in the catalog.
- About 40% of the ACS/WFC and WFPC2 source lists could not be cross matched because they do not have enough sources in common with another source list. These non-crossmatched sources are included in the catalog.
- The catalog searching is currently optimized to be primarily based on position. Non-positional searches are not currently supported, but will be in the future.
- Although nondetections are provided in Level 1 or 2 searches on the Detailed Search Form, the significance of the nondetection varies with the exposure parameters. We do not yet provide a limiting magnitude for nondetections, but plan to do so in the future.
- The catalog has undergone some testing. But, it is new and there could be some errors.
Detailed Search Form
The Detailed Search Form provides a view of the source catalog that displays an entry for each source. Some of its properties are listed below.
Forms Search Levels
The output of the Detailed Search Form has the following options.
- Level 0 searches provide source detections only (no nondetections).
- Level 1 searches provide source detections and color dropouts (nondetections). For the color dropouts described above, we report certain properties of the nondetection, including the image name, exposure time, and filter. The source position of the dropout is taken as the white light detection image source position. Certain properties such as fluxes and magnitudes are indeterminate and are left blank on the search form (represented in the database as null). The detection threshold for color dropouts varies with the depth of the exposure. In some cases, the threshold is not very deep. Users studying dropouts may want to use the HLA image cutout capability to confirm them.
- Level 2 searches provide source detections, color dropouts, and white light nondetections. We infer white light image nondetections by searching for images with source lists that cover the position search being made, but are not found in the results of a Level 1 search. These nondetections have image information, but have blank (null) source positions, fluxes, and magnitudes.
Matches
The source detections and color dropouts that we determine to be describing the same physical object are classified as belonging to the same match. Matches of sources are represented on the Detailed Search Form output as entries that have the same MatchID value. Each member of the match, including color dropouts, has a MemID value that is unique within the match. The default output sort order on the search form groups together entries that have the same MatchID with ascending values of MemID. Detected sources (Det value 'Y') within a match always have lower MemID values than color dropouts (Det value 'N'). Each match has a certain position associated with it (MatchRA, MatchDec) that is determined by its source positions. This position is considered to be the location of the object described by the matched sources. Each source detection and color dropout has some separation distance D from the match position. The D Sigma value is the standard deviation of the D values for the independent source positions (white light source positions) within a match. The default sort order of matches is in increasing distance of the match position from the user specified search position. White light nondetections are listed at the end of the results and have blank MatchID and MemID values. They are not sorted due a limitation of the user interface.
Many of the sources in the Detailed Search Form are in matches that involve a single visit and detector (single white light source). These cases have D=0 and D Sigma=0. For some of these cases, the source list containing the D=0 source was successfully crossmatched, but the source did not match any of the sources in images from other visits or detectors. For other cases, the source could not be crossmatched because the image containing the source did not overlap sufficiently with an image from another visit or detector. Searches on the Detailed Search Form for only crossmatched sources are made by specifying the constraint D > 0.
Summary Search Form
The Summary Search Form displays
an entry for each set of matching sources.
Each row has a unique MatchID value as defined above.
More information about a match with some MatchID value can be obtained from
by searching the Detailed Search Form for the same MatchID value.
The Detailed Search Form lists aggregate properties of each match such as the number
of visits and filters. In addition, the average magnitude values (average ApMag values
on the Detailed Search Form) within the match are listed
for the most frequently used filters.
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