4 Usage scenarios

In this section we will focus on describing the usage of the ALADIN Java interface, as it is available now (November 1999).

4.1 Access

The ALADIN home page is available through the CDS Web server at the following address: http://aladin.u-strasbg.fr/

This site provides access to ALADIN documentation, including scientific reports, recent publications, etc.

4.2 Query modes

The typical usage scenario starts with a request of the digitized image for an area of the sky defined by its central position or name of central object (to be resolved by SIMBAD). The size of the sky field is determined by the photographic survey used: it is $14.1\hbox{$^\prime$ }$ in the case of the DSS-I.

Astrometric information comes from the FITS header of the DSS image, and is generally accurate to the arcsecond (with deviations up to several arcsec. in exceptional cases, on plate edges).

Figure 1: Example of Images/Data ALADIN query panel

Figure 2: Example of ALADIN window, with an image centered on NGC 7436, and objects from NED and 2MASS marked by symbols

In a subsequent step, the interface, illustrated by Figs. 1 and 2, allows the user to stack several information planes related to the same sky field, to superimpose the corresponding data from catalogues and databases, and to obtain interactive access to the original data.

The possible information planes are the following:

• Image pixels from the ALADIN database of digitized photographic plates (DSS-I, MAMA, DSS-II); functionalities include zooming capabilities, inverse video, modification of the color table;

• Information from the SIMBAD database (Wenger et al. [2000]); objects referenced in SIMBAD are visualized by color symbols overlaid on top of the black and white image; the shape and color of the symbols can be modified on request, and written labels can be added for explicit identification of the objects; these features are also available for all the other information planes;

• Records from the CDS library of catalogues or tables ( VIZIER, Ochsenbein et al. [2000]); the user can select the desired catalogue from a preselected list including the major reference catalogues such as the Tycho Reference Catalogue (ESA [1997]; Høg et al. [1998]), GSC (Lasker et al. [1990]), IRAS Point Source Catalog, or USNO A2.0 (Monet [1998]); the user can alternatively select the catalogues for which entries may be available in the corresponding sky field, using the VIZIER query mechanism by position (see Sect. 4.3), catalogue name or keyword;

• Information from the NED database: objects referenced in the NASA/IPAC Extragalactic Database (Helou et al. [1991]) can also be visualized through queries submitted to the NED server at IPAC;

• Archive images will gradually become available through the corresponding mission logs: Hubble Space Telescope images are currently available (see Fig. 3 for an example), and more archives will follow;

  
  

  Figure 3: Example of ALADIN display of a famous image from the HST archive (WFPC2) featuring the Eagle Nebula (M 16). Objects present in Simbad, GSC, and USNO A2.0 are flagged with different symbols. Field size is 2.6 arcmin (full image, right) and 1.6 arcmin (left)

  
  

• Local, user data files can also be overlaid, but, because of current limitations of the Java applications, this feature is only available in the stand-alone version, or in ALADIN X.

The stack of images and graphics is made visible to the user (under the eye icon, on the right of Fig. 2) so that each plane can be turned on and off. The status of queries is also easily visualized.

For all information planes ( SIMBAD, VIZIER, NED) links are provided to the original data. This is done in the following way: when selecting an object on the image, with mouse and cursor, it is possible to call for the corresponding information which will appear in a separate window on the Internet browser. It is also possible to select with the mouse and cursor all objects in a rectangular area: the corresponding objects are listed in a panel on the bottom of the ALADIN window; this list includes basic information (name, position and, when applicable, number of bibliographical references) and anchors pointing to the original catalogue or database.

At any moment the position of the cursor is translated in terms of right ascension and declination on the sky and visualized in the top panel of the ALADIN window. Additional features are available, such as a tool for computing angular distance between marked objects.

The standalone version gives access to additional facilities, not available through the Web, such as printing and saving the images and data.

   
4.3 The catalogue server

The ability to access all VIZIER catalogues and tables directly from ALADIN is a unique feature which makes it an extremely powerful tool for any cross-identification or classification work.

The "Select around target'' request relies on a special feature - the genie of the lamp: this is the ability to decide which catalogues, among the database of (currently) over 2 600 catalogues or tables, contain data records for astronomical objects lying in the selected sky area. In order to do that, an index map of VIZIER catalogues is produced (and kept up-to-date), on the basis of about ten pixels per square degree: for each such "pixel'' the index gives the list of all catalogues and tables which have entries in the field.

When a user hits the button "Select around target'', this index is queried and the list of useful catalogues is returned. It is possible, at this stage, either to list all catalogues, or to produce a subset selected on the basis of keywords. Note that, as the index "pixels'' generally match an area larger than the current sky field, there is simply a good chance, but not 100%, to actually obtain entries in the field when querying one of the selected tables.

4.4 Cache

The images of the 30 000 most cited objects in SIMBAD are pre-computed and available on a cache on magnetic disk. For these objects, the image is served much faster than for other objects where the image has to be extracted from the Digitized Sky Survey.

4.5 Usage statistics

As the newest service developed by CDS, ALADIN has not yet been widely publicized, and its usage is in a steeply growing phase. Currently about 10 000 queries are processed monthly, generating the extraction of more than 5 000 images.