What is the Hubble Source Catalog (HSC)? What data does it contain?
The Hubble Source Catalog (HSC) combines the tens of thousands of visit-based, general-purpose source lists in the Hubble Legacy Archive (HLA). into a single master catalog.
In the current Beta 0.2 release, the HSC contains members of the ACS/WFC and WFPC2 Source Extractor source lists from HLA version DR6 that have valid detections (i.e., data with quality flag values less than 5; see HLA Source List FAQ for a definition of the flagging system for HLA source lists). Data from the ACS/HRC (High Resolution Camera) are NOT included in the HSC at the current time. Approximately 20% of the ACS/WFC and WFPC2 images from DR6 do not have source lists, and hence data from these images is not included in the HSC.
This is a Beta version of the HSC; you can expect some rough edges and a larger fraction of artifacts than will be present in the Version 1 release. In addition, the database may change without warning since we are working to make improvements. For the upcoming HSC Version 1 release (tentatively planned for fall 2013), we plan to include sources from WFC3 images and may also incorporate the DAOPHOT source lists.
What are five things you should know about the HSC?Click here for a 1-page, graphic description of all five of the following.
All HLA magnitudes are in the ABMAG system. Here is a discussion of the ABMAG, VEGAMAG and STMAG systems. A handy, though not exact conversion for ACS is provided in Sirianni et al. (2005) The Synphot package provides a more generic conversion mechanism for all HST instruments.
Where can I find examples of good and bad regions of the HSC?
What are the primary "known problems" with HSC Beta 0.2?
# 1 - Magnitudes for objects on the PC (Planetary Camera) portion of the WFPC2 may differ when compared to the same object when it is on the WF (Wide Filed) portion of the WFPC2. The primary reason for this is that the Point Spread Function (PSF) on the PC is different than on the WF. While the rebinning of the PC to match the pixel size of the WF largely takes care of this effect, differences still remain. We are currently evaluating how large this difference is for point sources and will include documentation on the effect HERE in the near future.
How are HLA images and source lists constructed?
The Hubble Source Catalog (HSC) is based on HLA Source Extractor Bertin & Arnouts 1996 source lists. To build these source lists, the HLA first constructs a "white light" or "detection" image by combining the different filter observations 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 checked for sources at the positions indicated by the detection image. If a valid source is detected at a given position. then its properties are entered into the HLA source list appropriate for the visit, detector, and filter,
Sources that are found in the white light detection image, but not in a particular filter used to make the white light image, are regarded as "filter-based nondetections". These can be examined by asking for level = 1 under Detection Options on the HSC Detailed Search form.
More details about how HLA images are constructed can be found at the HLA Images FAQ . More details about how HLA source lists are constructed can be found at the HLA Source List FAQ .
Note that corrections for Charge Transfer Efficiency (CTE) problems have been made to the WFPC2 HLA source lists, but not the ACS source lists. See the HLA CTE FAQ
How are "matches" defined in the HSC? What is the algorithm that combines the sources?
The detections (and nondetections) that correspond to
the same physical object (as determined by the algorithm defined in
are given a unique
MatchID number. Each member of the match, including nondetections,
also has an assigned MemID value and a unique position (MatchRA,
MatchDec). The procedure dramatically improves the
relative astrometry between visits, to a level of 10 mas or better
for the majority of the sources.
Each source detection and nondetection has some separation distance, D, from the match position. The D-Sigma value is the standard deviation of the D values. Visit-based nondetections are listed at the end of the results and have blank MatchID and MemID values.
Many of the detections in the Detailed Search form are in matches that involve a single visit and detector. These cases have D=0 and D-Sigma=0. Searches on the Detailed Search Form for only crossmatched detections are made by specifying the constraint D > 0 using the User-specified field and the Field Descriptions options.
How are HSC Levels 0, 1,and 2 for the Detection Options defined?
The output of the Detailed Search Form has the following options
available via the Detection Options field.
Level 0 - Includes all detections (no nondetections).
Level 1 - Includes detections and filter-based nondetections. Sources that are found in the white light detection image, but not in one or more of the filters included in the visit, are called "filter-based nondetections".
Only certain properties for filter-based nondetections are reported, including the image name, exposure time, and filter. Certain properties such as fluxes and magnitudes are indeterminate and are left blank on the search form.
Level 2 - Includes detections, filter-based nondetections, and visit-based nondetections. Visit-based nondetections are cases where the existence of an object is inferred from at least one Hubble observation, but there is nothing detected at that position in a particular visit, even though it overlaps.
These nondetections have image information, but have blank source positions, fluxes, and magnitudes.
Nondetections have "N" in the "Det" column, and are listed last on the form.
Note: Users studying nondetections may want to use the HLA image cutout Cutout FAQ capability to confirm them.
What is the difference between the HSC Summary and Detailed Search forms?
The HSC Summary Search Form includes results for all detections for a given object on a single row. The magnitudes for different visits are averaged together.
The HSC Detailed Search Form shows each individual detection for a given object on a single row.
The HSC Summary Search Form lists aggregate properties of each match such as the number of visits and filters. In addition, the average magnitude values and their associated RMS scatter (sigma), are listed for the most frequently used filters. More information about a match can be obtained by searching the Detailed Search Form for the appropriate MatchID value (under the User-specified field), or by clicking on the blue MatchID value in the first column when displaying the html version of the requested query. Frequently used filters are listed first; other filters are listed next using a different format
The HSC Detailed Search Form displays an entry for each individual detection in a match on a single row. Each object has a unique MatchID value. Each member of the match, including nondetections, also has an assigned MemID value, a unique position (MatchRA, MatchDec), and a separation distance D from the match position. See FAQ # 4 for a more detailed discussion of how matches are defined (see the next FAQ).
What are the frequently used filters listed in the Summary Search form? How are other filters listed?
Currently, the most frequently used filters listed in the HSC Summary Search form are:
A_F814W W2_F606W W2_F814W A_F775W W2_F300W
where A stands for ACS/WFC and W2 stands for WFPC2.
Note that values for the same filter, but different instruments (e.g., A_F814W and W2_814W), are NOT averaged together, since the same aperture size has not been used for different instruments. You can use the "Output Columns" feature to include only the filters you request, as described in FAQ # 7, and demonstrated in "A Basic Hubble Source Catalog Walkthrough".
Where can a definition of what all the fields mean be found?
Click on "Field Descriptions" in the upper right of the HSC Summary or Detailed Search forms. This provides a short description of the various parameters that can be used in a query. For more detailed information (for example how is the concentration index = CI defined ?), go to the HLA FAQ.
How can I customize the HSC output?
The output definition portions of the HSC Detailed and Summary Search forms can be used to change the output from the default values. The WFC3 sources lists are much better in this regard, but are not currently included in the HSC. The next generation of WFPC2 and ACS source lists, which will use the improved WFC3 algorithms, will be incorporated into the HSC when available (tentatively planned for fall 2013)
- False detections - Uncorrected cosmic rays are a common cause of blank sources. Users may want to use NumImage > 1 to help filter out this and other artifacts.
Another common cause of "false detections" is the attempt by the detection software to find large, diffuse sources. In some cases this is due to the algorithm being too agressive when looking for these objects and finding noise. In other cases the objects are real, but not obvious unless observed with the right contrast stretch and field-of-view.
- Bad images (and hence bad source lists) - Images taken when Hubble has lost lock on guide stars (generally after an earth occultation) are the primary cause of bad images. We attempt to remove these images from the HLA, but occasionally a bad image is missed and a corresponding bad source list is generated. A document showing these and other examples of potential bad images can be found at HLA Images FAQ. If you come across what you believe is a bad image please inform us at email@example.com
- Miscellaneous bad information - There is occassionally incorrect information about an object in the catalog due to a variety of errors ranging from incorrect information in the image header to undetected errors in the pipeline processing. If you suspect a problem, please send an e-mail describing the issue to the firstname.lastname@example.org so we can track it down.
Is there a summary of known anomalies in the ACS and WFPC2 data?
Yes - HLA Images FAQ. Here is a figure from the document showing a variety of artifacts associated with very bright objects.
How can I provide feedback?
Send a note to email@example.com. Please include enough information (e.g., a screen save of the problem) to make it possible to diagnose the issue.
What are the future plans for the HSC?
#1 - Write a PASP-type article explaining the basic workings of the HSC, demonstrating the quality that can be expected, and outlining plans for the future. A draft will be available for the Version 1 release.
#2 - Version 1 (Fall, 2013) - Include WFC3 HLA source lists.