1. Introduction

The MAST database can be searched for object classes by relying on the target descriptor put in by the program PI. However, this information can be very crude and sometimes even meaningless (in the case of parallels). For that reason cross correlation of MAST data with positions from published object catalogs is a very powerful tool to discover data and use it in ways different from those anticipated by the PI for scientific research.

There are currently two major archive centers for catalogs and object classification, VizieR and NED. MAST built web-based cross correlation services for both of them.

The VizieR cross correlation relies on positions (RA and DEC) and assumes implicitly that the catalog positions refer to the Epoch 2000. The NED cross correlation is built along the lines of their advanced all sky search for objects by using redshifts and/or object classes.

• Use a source catalog to find if there are any HST observations available in the archive.
• Search the MAST archive for a particular object classification

2. VizieR cross correlation

VizieR is a service run by the Centre de Données astronomiques de Strasbourg and provides access to the most complete library of published astronomical catalogues and data tables that are available on line. Figure 1 shows the MAST search interface for VizieR catalogs. You can either enter your favorite catalog name --- in this case the Milky Way Globular Cluster Catalog from Harris (1997) --- or use other information such as wavelength range, or object class to shop for possible VizieR catalogs (see Figure 2). Once you select your catalog, you get to the next screen, where you can decide if you simply want to search the catalog (S) or do a cross correlation with MAST data. Assuming that you know the content of the selected catalog, you would click on CC for cross correlation. This brings you to the next step: selecting the MAST missions and search criteria.

You can choose between different formats for the search result such as an HTML table, an Excel spreadsheet, comma separated values (CSV), or a VO table.

If you had chosen any other HST entry such as ACS for example, you would be able to see the preview by clicking on the dataset name. Figure 7 summarizes the steps needed to retrieve the data through DADS. The pipeline processed data is generated on-the-fly and depending on the volume, requests on average take about one hour.

3. NED cross correlation

The second example illustrates how to search the MAST archive for a particular object class using NASA's Extragalactic Database (NED). The MAST form for cross correlation with NED shows the same capabilities as NED's Advanced All Sky search interface for object types.

Figure 8 shows the MAST NED cross correlation interface. Suppose, we are interested in high redshift, z>6, quasars that have ACS observations. Use the redshift pull-down menu to select "larger than" and enter 6.0 in the first numeric field. Further down select "QSO's" from the list of extragalactic objects to be included. At the bottom of the form you find --- similar to the Vizier cross correlation --- a list of all MAST missions to cross correlate with. We select now ACS and hit the "Search NED and cross correlate" button to execute the NED search and a subsequent cone search of ACS data.

The query returns 10 high redshift quasars that have one or more ACS observations taken within a radius of 3 arcmin (Figure 9). Clicking on "See results from NED" brings up a new screen with NED's summary of its catalog metadata on all 10 quasars (Figure 10). Clicking on a dataset name on the results screen (Figure 9) gives access to a preview image whenever available. Loading the preview into Aladin gives additional functionality such as overlaying catalogs (e.g. NED, Simbad), loading additional images from other archive centers or from disk, overplotting VizieR catalogs and image manipulation (see Figure 11).