About the HEASARC: HEASARC Overview: The Archive > Science Results Using HEASARC Data

The Archive

The philosophy of the HEASARC from the beginning has been to provide immediate access to the latest version of any data file from any particular mission. This means making every file directly available via FTP and the Web. This rapid access to the data has been a major factor in enabling large survey projects, where access to a significant fraction of the archive is required. This instant access was achieved with re-writable optical disk juke-boxes mounted as a Unix file system. These juke-boxes proved to be a very cost effective way of providing online access to the entire HEASARC data holding. As technology has advanced the HEASARC has been able to keep up with the growth in the archive. Over the last year we have replaced the optical juke-boxes with a magnetic RAID (Redundant Array of Inexpensive Disks), which provide even better performance for a similar price.

As of April 2010 the HEASARC Archive contains > 26 Terabytes (TB) of data, compared to 1.6 TB in April 2000. Files are compressed to save space and speed up network transfers. The data volume would be ~ 3 times larger uncompressed. Electronic retrieval is the main method of data distribution. Current network bandwidth appears to be adequate to satisfy users' needs. From 1991 to 1996 the HEASARC made CD-ROMs of ROSAT, CGRO, EXOSAT, and Einstein data. While these were popular give-aways, network access to the archive has made them redundant. Mirror archive sites have been established in Europe and Japan to improve access for astronomers worldwide in exchange for data archives from international missions. The HEASARC site itself sees data down-loads of order 3 TB /month (as of 2010), so the equivalent of the entire archive is retrieved every nine months. (Interestingly, in 2000 the ratio of the archive size to the download rate was similar). As projects have gained confidence in the HEASARC capabilities, they have elected to directly archive and distribute data through the HEASARC. During the proprietary period the data files are PGP-encrypted to ensure security and the PI is sent the key to decode their data.

The HEASARC archive interface helps the user find the data and also provides advanced catalog search capabilities. This was originally based on the Browse system developed in the late 1980s for the EXOSAT project in Europe, using a home-grown database system. The initial access, via a telnet account, provided a command line search-and-retrieval system and data analysis software. The HEASARC quickly recognized the power of the Web and in 1993 became one of the first astronomy archive sites to provide a browser-based search-and-retrieval facility. The database system has migrated to Sybase, which is better able to keep up with the increasing demand. The Web-based Browse interface is now the main access point to the archive. Through an AISRP-funded effort called Hera (link to this), the HEASARC plans to make data analysis capability available over the Web.

In the interests of increasing the availability of high energy data to a broader community it is the philosophy of the HEASARC to encourage other archive centers and services to provide access to the HEASARC data holding via their interfaces. The final disposition of the EUVE archive provided an opportunity to put this philosophy into practice. The EUVE data are stored in the HEASARC FTP area and accessed both from the HEASARC Browse and STScI/MAST user interfaces. Following this example the ROSAT archive has also been made available via MAST. These are simple examples of the trend to move away from the proliferation of archive interfaces and toward an open archive access layer and fewer custom interfaces. As another step along this path the HEASARC, in collaboration with STScI and CDS, developed the Astrobrowse facility to demonstrate how Web technology can be used to provide search facilities across many sites at minimal effort. The HEASARC is now working within a larger consortium to design a system that will extend the capabilities to more fully integrate Web services.

The HEASARC also provides another interface to the archive called SkyView. This provides an image of the sky in different wavebands (optical, IR, X-ray, Gamma-ray, EUV, and radio) in any projection or equinox system. The Data Archive page provides a central point where all the HEASARC Web services can be accessed.



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Last modified: Wednesday, 13-Oct-2010 14:24:27 EDT