Preliminary
1 The needs
2 Already existing tools
3 Prerogatives for the SOFA site
4 Interface

Preliminary

Having experienced various observational techniques in various wavelenght domains, I have started this web site, called SOFA (Standard Objects For Astronomy) allowing to easily search standard objects.
For standard objects, I mean stars, galaxies, etc. with physical properties (magnitude, spectrum,...) typical of their class, that the user observes to calibrate his/her data, or that could be element of comparison.

1 The needs

• astronomical observations have to be calibrated. The artificial means (thorium lamp, flight-fields with white light, polarizer,...) are not sufficient: astronoms usualy need real objects in the sky, well known, as references:

• calibrators to analyse wavefront in AO;
• objects close to the target in interferometry (constrained from the delay line, instrumental visibility);
• polarized or unpolarized stars;
• binaries with well established separation and orientation;
• bright extended objects;
• spectroscopic standards;
• astrometric references;
• etc.

• a vast number of catalogues already exist on the web: their use is generally easy, but on the other hand,  it is impossible to cross-interrogate them while making a selection among different criteria;

• these selection criteria vary according to the observational technique, the site or the aimed sky region: it is rather cumbersome to have to consult several catalogues individually for reference objects;

• the large telescopes will offer a higher detection sensibility, and it is not always easy to find faint reference sources to observe; these objects will be obtained only after observations and the need to archive and to catalogue them wil arise.

2 Already existing tools

A few tools exist to interrogate catalogues on web. Let us quote for example:

• AstroBrowse, Starcast: intesting because it undertakes to interrogate itself the existing data bases dispersed on the web. But to retrieve the information, the user has then to navigate through each selected database;
• Aladin, Skyview: allow to obtain images of the sky at any wavelenght;  powerful, but for a long list of targets, it is tedious to choose for each of them a close reference star;
• Vizier: the most complete library of catalogues; allow to select stars by specifying constraints, but by working only on one catalogue at one time.

3 Prerogatives for the SOFA site

Of course, it is out of the question to copy on local disks the catalogues already available on the web. The use of the GLU system (Générateur de Liens Uniformes, settled at CDS), could allow to interrogate different selected catalogues.

Only one list of interesting objects, that do not belong to already published catalogues, should be kept: the necessary space will be very reasonable at the beginning (about ten megabits of memory on the disk).

SOFA is obviously not meant to copy the VizieR service: the interface should
return the query results in an homogeneous form, whatever the type of the searched catalogues. Furthermore, the output should be customable by the user in order to let him retrieve files ready to use by his own programs (e.g. to create OB (Observation Blocks) for phase P2PP (Phase II Proposal Preparation) with the VLT or spatial telescopes).

The site update could be done at various stage:

• regular addition of links towards new catalogues (available at VizieR for example);

• monthly watch of publications, to include in SOFA new interesting objects: binaries, stellar diameters, high resolution IR spectra,...

• regular consulting of observatories archives (HST, ESO, CFH,...) whern the object type - calibrator, standard - is known;

• update from the users voluntary input, following their own observational results: astronomers will be invited to submit their new data in an interactive way and thus significantly contribuating to keep update the SOFA site.

4 Interface: belouw is a proposed interface which could be used to search for standard objects: