Part I: Introduction to Reducing the HST Data

1.3 Getting Data with the World Wide Web

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HDA datasets can be searched for, previewed and retrieved via the World Wide Web in very much the same way as with StarView. As noted in Section 1.1, StarView offers more capabilities for this process, including cross-qualification, the use of VTT, and more information about instrument calibration files. However, Web retrievals may be preferable in some cases, particularly when information on calibration files is not needed, and the hypertext on the Results pages makes it easy to access all the information they contain. The starting point for Web-based searches of the HDA is the MAST web site at: http://archive.stsci.edu¹

This web page is shown in Figure 1.6. A powerful feature of MAST is that all of its mission archives, including the HDA, can be searched simultaneously. This is done with the Quick Target Search option shown on the MAST home page. This search will return all datasets for all missions available for a given object or coordinates, according to the search constraints specified by the user (based on the wavelength region of interest), and will provide hypertext links to these datasets. If only HST datasets are desired, they can be accessed separately by clicking "HST" on the MAST home page from the "Missions" pull-down menu. Searches of the HDA by object class can also be made with the VizieR Catalog Search tool at http://archive.stsci.edu/vizier.php.

The HST section of MAST offers tutorials about the HDA as well as a FAQ page and HDA news. It also provides links to HST "Prepared" datasets such as the Ultra Deep Field and the Hubble Deep Field images. Clicking on the "Main Search Form" option of the Search and Retrieval menu in the HST section brings up the page shown in Figure 1.7. Here the user is queried for the same search parameters as requested by StarView, e.g., Object Name, Instrument and Proposal ID Once these are entered, clicking the Search button returns a page listing the datasets found, which can then be selectively marked for retrieval. The data type and retrieval options remain the same as those for StarView. Previews of GIF files of most datasets are also available by clicking on the image name. Datasets marked for retrieval can be retrieved via ftp, sftp, or can be placed on the Archive staging disk. The preferred format for ftp is compressed as gzip. Data placed on the staging disk (the preferred method) can be retrieved using ftp with the archive user name and password.

 
Figure 1.6: MAST Home Page
 

 
Figure 1.7: HST Archive Web search Form
 

1.3.1 WFPC2 Associations

Since November 2002, MAST contains WFPC2 associations, more than 15000 combined WFPC2 images stacked by CADC and ST-ECF (http://archive.stsci.edu/hst/wfpc2/about.html). These combined images are the products of the basic registration and averaging of related sets of WFPC2 images, referred to as associations, that is usually performed by archival researchers after the retrieval of individual images. The WFPC2 constituent images all have the same orientation on the sky and hence no rotation procedure is used to stack them. They form an important new set of high-quality astronomical data that should be useful for a wide range of investigations. In addition, they provide better previews of a given field than the individual WFPC2 images. The web page also provides a project description and the pipeline procedure applied.

Figure 1.8 shows the query form that can be used to search for WFPC2 associations in MAST. Entering a target like M87 and clicking on the search button will return you to a screen with the respective datasets in FITS format. The data can be downloaded directly from this page.

 
Figure 1.8: HST Archive Web search FormWFPC2 Associations Screen
 

1.3.2 High Level Science Products

MAST also contains a number of High Level Science Products (HLSP), which are accessible at http://archive.stsci.edu/prep_ds.html. High-Level Science Products are fully processed (reduced, co-added, cosmic-ray cleaned, etc.) images and spectra that are ready for scientific analysis. HLSP also include files such as object catalogs, spectral atlases, and README files describing a given set of data. The data originate from the Treasury, Archival Legacy and Large Programs (TALL) from cycle 11 onward, but contain contributions from smaller HST programs and other MAST missions. An example of an HLSP product from an early FOS program, a composite quasar spectrum, is shown in Figure 1.9. Users who are interested in contributing to the HLSP, are referred to the Guidelines for Contributing High-Level Science Products to MAST (http://archive.stsci.edu/hlsp/hlsp_guidelines.html, please make sure to get the latest version). Furthermore, they are asked to contact the archive scientist involved as soon as they start working on the data.

 
Figure 1.9: Example High Level Science Product
 
¹ European archive users should generally use the ST-ECF Archive at http://archive.eso.org. Canadian users should request public archival data through the CADC web site at http://cadcwww.dao.nrc.ca. Proprietary data are only available through STScI.

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