A Skysearch allows the user to select specific astronomical targets, either by giving their names, or by specifying a region of the sky.
The most common mode is to select records corresponding to a given astronomical target specified by its name. A given source is generally known under various names, identified in different catalogues or publications. Some sources may have more than 100 designations. For example, MESSIER 87, NGC 4486, and VCC 1316 are three designations of the same object. In addition, a given designation may accept different spellings: Some catalogue names can be abbreviated (though it is generally discouraged), and the embedded spaces may or not be mandatory. For example, MESSIER 87 and M87 are two spellings of the same designation.
The names are generally case insensitive, and the mandatory blanks can be replaced with a _ (undescore).
The interpretation of names needs to apply multiple rules which are called the name resolution algorithm (or nra). The nra takes a user's input, and generates a designation that is to be looked for in the target table. The nra is in principle defined by the service and is used both for decoding a name given by the user, and the name of the source when they are ingested in the table. The nra consists in a series of attempts (or possibly just one) to recognize a name. Three name resolving services may be used: HyperLeda, Simbad and NED, or the input name may be searched in the target table after no or a little massaging (for example to ignore the case or non-mandatory blanks).
A set of names can be passed as a comma, or extended searchs, using the wildcard character % can be used. For example: "ngc4486, ngc4476" specifies two objects, "NGC44%" searches all the objects whose names start with NGC44.
The syntax nearest:[position,maxrad], search the nearest object to celestial position position within a maximum distance maxrad. This maximum distance is given in arcsec. The celestial position may be a coordinate string, like J123049.2+122329 (a system prefix plus two coordinates in decimal degrees or sexagesimal; systems are B, J, G and SG), or a position resolved in NED or Simbad, like: simbad:51_peg or ned:m87.
Rather than giving a name, it is possible to search around a celestial position,
which can either be the position of an object specified by its name, or celestial coordinates.
If the position is a designation, it is first resolved into a position using the nra described above. Otherwise, celestial coordinates may be equatorial (B1950 or J2000), Galactic or SuperGalactic. The system has to be given as a prefix to the string written by the user, either B, J, G or SG (if no prefix is given, the coordinates are assumed to be B1950).
The coordinates string may be given in a variety of free formats, either sexagesimal or decimal.
The cone size and geometry has to be specified. A searh in a box (rather than a disk) is the fastest.
While a list of objects may be copied and pasted in the "object name of list" box, for long lists it is easier to upload a file. This file shall contain a list of object specifications (as described above) separated either with a comma or newline character.
The Skysearch constraints specified in this form may be combined with others, for example with individual field constraints. This combination is made with a logical AND.