Getting Started with GALEX

On April 28, 2003 NASA launched the GALEX satellite as part of its Explorer class series. The mission's purpose is to survey galaxies out to a redshift of z = 2 and to study the evolution of star-forming galaxies. GALEX conducts this work by means of UV sky surveys and deep sky searches in the imaging mode and partial sky surveys. All observations are made through a beam-splitter allowing light to be recorded by far-UV and near-UV detectors in either direct-imaging and spectroscopic (grism) modes. The wavelength bandpasses of these detectors are approximately 1400--1800 Angstroms and 1800--2800 Angstroms, respectively. Further technical information can be obtained at the GALEX/Caltech website or through a FAQ page periodically updated by Caltech and maintained at MAST. Eventually, the All Sky Survey (direct imaging) will cover some 3/4 of the sky (generally in single short visits). Users should be aware that some regions of the Galactic Plane, the Magellanic Clouds, and bright stars (m(AB) ~ 10) will be avoided in order to protect the detectors from saturation and damage.

The Caltech GALEX project delivers its data to MAST as periodic public releases. These have included the Early Release Observations ERO in early 2004, the first Public Release GR1 in early 2005, and the second Public Release GR2 in spring of 2006. At this writing, the direct images for the GR2, consisting of the Medium Imaging, Deep Imaging, Nearby Galaxy, and All Sky Surveys (MIS, DIS, NGS, and AIS) have been delivered to MAST and staged for public release. The grism data (spectroscopic surveys) will be released in early summer of 2006. As implied by its name, there are no proprietary rights on public release data.

Beginning in 2005, Guest Investigator (GI) data were delivered to MAST. Pursuant to NASA policy, these data are proprietary to the GI for a nominal period of six months after date of ingest at MAST. The Guest Investigator GI office at Goddard Space Flight Center administers this program and coordinates with MAST to communicate release dates as well as instructions to the GIs. At the end of a proprietary period, MAST opens the permissions to a GI dataset and allows them to be accessed by the general users at its public GI Programs entry point. To date, solicitations for GI proposals have been made annually by NASA's Science Mission Directorate through a Research Opportunities for Research in Space and Earth Sciences (``ROSES"). Parties interested in writing proposals should consult the GI website.

Novice users may want to familiarize themselves with the site by using the GALEX site tutorial. Users can also navigate to internal sites from any page by clicking on the top menu or left gutter. Each of the pages contains a number of hot links in the form of pop-up windows or tool tips.

Data Searches:
Data selection, browsing, and retrieval proceeds by making queries in the "Search and Retrieval" tab in the left gutter. In general, users have several ways of searching for data from the GR releases. The first and most effective way of executing a Search is via the SQL search form . This form allows one to find data for one or more objects within a sky region known as a "tile" or data for all objects within the tiles satisfying the query. Search queries are written by SQL syntax, and a text box is provided for this purpose. SQL-challenged users may consult a drop-down menu and submit customized requests on objects or sky regions (tiles) by modifying any of these programmed queries. Clicking on "Execute" returns a table embedded in the search results form; the user is free to choose the format of this table as html (default), Comma Separated Value, or VOTable.

A second way to discover General Release data is to use the more familiar Main search form. Both of these forms (as of May, 2006) serve the need to search on individual objects or tiles. Whenever objects are requested, users may click on the "Explore" entry in the search results table. This action takes the user to the Explore page, which provides much information about the object and the observation and also exhibits direct-images cut-outs and, if spectra have been observed, plots of 1- and 2-dimensional spectra. Users may click on the Download button to obtain object products as FITS files.

GR data discovery may be excecuted in a third manner by clicking on one of the survey links in the Search and Retrieval tab. The user is then free either to enter coordinates for a search or to load a full table of all tiles in the survey -- note this may take several seconds to load. The resulting tile list permits one to browse the far-UV, near-UV jpeg images of a tile of interest and/or to download individual products for the tile. In the latter case, the user may decide whether to download only selected data products (FITS format), a designated set of "minimum recommended files" suggested by the project, all files, or only a catalog of the files. The user should be cautioned that the products downloaded will include those for all objects in the sky tile.

A fourth way to discover data is to search on former-GI program (now public) program data by clicking on the separate Guest Investigators tab in the left gutter. This request mode operates similarly as for those of the GR surveys. Note that as with the surveys, one cannot drill down to the level of individual objects for the released GI program data. One difference in this respect is that in order to download released GI data, one must download a short ftp script and then execute it on one's computer. The project anticipates that old GI data will be subsumed within a future GR release.

"search * from fileDescriptions"

into the SQL text box, and click "Execute." The results page will list the full descriptions for all files. MAST will soon simplify this procedure by placing links on the short descriptions on the Files Available pages to these fuller descriptions. More often, the user may be interested in knowing which files should be downloaded for routine research purposes. Such a list is called the"minimum required products, which is available as an download option in the Files Available for Download page, which is returned when the user selects the Retrieve Imaging or Spectra (FITS) column in the Survey Search. By early spring of 2007, MAST expects to facilitate the automatic download of this select group of files by prepackaging them in zipped tar format and providing a download button on the Files Available pages.

Determining header (keyword) definitions:
The columns in the database that the user sees on the Files Available for download page are taken from the FITS header keywords of the datafiles. These keywords are redefined as column names in the Database Info/Tables pages (sometimes with names to make them more intuitive), and often in more than one Table. The definition of these columns can be obtained either by looking in the FITS keyword headers in the datafiles or by entering an appropriate "select" query the SQL Search Form, and clicking "Execute." This will return a list of Tables containing the column name of interest. For example, to search on Tables containing the column "RA", type the query

"select tablename,name,unit,description from dbcolumns where name like 'ra' order by tablename,name".

The 'ra' field may be wildcarded with a % symbol within the single quotes. To obtain the field's description the user can then note the Table names returned, navigate to the left menu tab under Documentation/Tables, and click on any of the Tables noted.

All comments concerning GALEX data should be sent to MAST or the GI office and not to the Caltech project office directly. Questions that MAST can not or should not answer will be directed to the Project or GI office, as appropriate. Also, users should be aware that the site has some browser/platform incompatibilities. For example, at the moment one cannot download data in VOTable using the Internet Explorer browser.