Version 3 of the Hubble Source Catalog

The Hubble Source Catalog (HSC) is designed to optimize science from the Hubble Space Telescope by combining the tens of thousands of visit-based source lists in the Hubble Legacy Archive (HLA) into a single master catalog.

Version 3 was released on 2018 July 5 and includes many improvements in the photometry and astrometry along with a significant increase in the number of objects and measurements.

Highlights from Version 3 include:

• 25% more ACS/WFC images and more than twice as many WFC3 images compared with the HSC v2 release. ACS and WFC3 data that were public as of 2017 October 1 are included. See notes on the HLA DR10 data release and on the current HLA holdings for details.
• Improved photometric quality in the source lists due both to the alignment algorithm used to match exposures and filters in the HLA image processing and to improved algorithms for Source Extractor photometry (particularly near the edges of images).
• Improved astrometric calibration based on the Gaia DR1 catalog.
• The scatter in magnitudes is measured using the median absolute deviation (MAD) of the differences from the median magnitude. This is more robust and less sensitive to outliers than the RMS used in previous versions.
• Cross-matching between HSC sources and spectroscopic COS, STIS, FOS, and GHRS observations.

The HSC v3 contains members of the WFPC2, ACS/WFC, WFC3/UVIS and WFC3/IR Source Extractor source lists from HLA version DR10 (data release 10). The cross-matching 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 cross-matched sources are significantly reduced, with median errors less than 8 mas. The absolute astrometry is calibrated using Gaia DR1, Pan-STARRS, SDSS, and 2MASS as the astrometric backbone for initial corrections. In addition, the catalog includes source nondetections. The cross-matching algorithms and the properties of the initial (Beta 0.1) catalog are described in Budavari & Lubow (2012), and the Version 1 catalog is described in Whitmore et al. (2016).

Interfaces

There are currently four ways to access the HSC as described below. We are working towards having these interfaces consolidated into one primary interface, the MAST Discovery Portal.

• The MAST Discovery Portal provides a one-stop web access to a wide variety of astronomical data. Select "Hubble Source Catalog v3" in the Select Collection dropdown menu, enter your search target, click Search, and you are on your way. Please try the Use Case Using the Discovery Portal to Query the HSC.
• The HSC CasJobs interface permits you to run large and complex queries, expressed in the Structured Query Language (SQL).
• The HSC TAP service provides database query access to the HSC through the Virtual Observatory Table Access Protocol (TAP). It is described in more detail in an article in the May 2018 MAST Newsletter. If interested in accessing the older HSC Version 2 via TAP, you may do so using this TAP service.
• HSC search form:
  • The HSC Summary Search displays a single row entry for each object, as defined by a set of detections that have been cross-matched and hence are believed to be a single object. Median values for magnitudes, positions, and other relevant parameters are provided.
  • The HSC Detailed Search displays an entry for each separate detection (or nondetection if nothing is found at that position) using all the relevant Hubble observations for a given object (i.e., different filters, detectors, separate visits).

Five things you should know about the HSC

1. Detailed Use Cases and Videos are available.

2. Sky coverage can be very non-uniform (unlike surveys such as SDSS) due to the highly variable HST observing patterns.

3. Current WFPC2, ACS/WFC and WFC3 source lists are of variable quality. In particular, the WFPC2 source lists are of poorer quality than the ACS and WFC3 source lists.

4. The default is to show all HSC objects. This may include a large number of artifacts. You can request NumImages > 1 (or more) to filter out many artifacts.

5. The default is to use mag_aper2 Source Extractor values for magnitudes (in the ABMAG system) . You can request mag_auto if desired.

Current Limitations

• About 30% of the ACS/WFC, WFC3, and WFPC2 images are not included in the catalog due to image quality and other issues (e.g., moving targets, grism observations, cosmic ray contamination, etc.)
• About 6% of the source lists (white-light images) in the HSC do not have an astrometric correction from either an external catalog (e.g. Gaia) or from overlapping HST images. (Note this is much improved from the previous version.)
• About 19% of the sources could not be cross-matched with sources in other visits. Such single sources are nonetheless included in the catalog.

Help and Acknowledgment

More detailed information can be found on the Frequently Asked Questions page. Several Use Cases are available to give you step by step instructions. There are also videos available.

We are interested in your feedback. If you have questions, or items to bring to our attention, please send them to: archive help.

HSC Data Use Policy: The HSC is based on data from the Hubble Legacy Archive (HLA). Refereed publications making use of the HSC should therefore include this footnote in the acknowledgements:

Based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESAC/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA).

Authors are also asked to acknowledge the "Hubble Source Catalog" in the text of the paper, and consider a reference to the Whitmore et al.(2016) paper describing the HSC.