7 Target lists

A target list is a catalogue which contains celestial coordinates in a restricted format which CURSA can interpret. The applications for converting between celestial coordinate systems, catcoord (see Section 17) and plotting finding charts, catchart (see Section 18) read target lists. The applications for importing a region from the HST Guide Star Catalog, catgscin (see Section 24) and extracting a subset from a remote catalogue, catremote (see Section 25) produce target lists. Similarly, the catalogues generated with catcdsin (see Section 23) from the text version of CDS catalogues are usually target lists. Though a target list places restrictions on the names and units of the celestial coordinates, the catalogue itself can be in any of the formats supported by CURSA: FITS table, TST or STL (see Appendix C).

A target list must contain columns of Right Ascension and Declination, for some equinox and epoch, called respectively RA and DEC. These coordinates must be stored in radians in a format which CURSA can interpret (see Section 6, above and Appendix B).

Additional optional columns allow the proper motion, parallax and radial velocity to be specified. These quantities are used for accurate conversions between celestial coordinate systems. All the columns which can be used to specify coordinates in a target list are listed in Table 3. The columns marked with a bullet (‘’) in the ‘Mandatory’ column must be present. The other columns are optional. However, if they are present they must be used as described. The names of the columns are chosen to be consistent with the recommendations of the CDS (see Astronomical Catalogues at CDS: Adopted Standards by F. Ochsenbein[21], p14).

Description Name Units Mandatory?

Right Ascension RA Radians
Declination DEC Radians
Annual proper motion in Right Ascension PMRA Radians
Annual Proper motion in Declination PMDE Radians
Parallax PLX Radians
Radial velocity RV Km/sec

Table 3: Columns defining celestial coordinates in a target list

The proper motions are specified per year rather than per century. Also the proper motion in Right Ascension is simply the rate of change of Right Ascension, α̇ (leading to large values for stars close to the poles), not the angle on the sky, α̇ cos δ. The latter quantity is tabulated in some catalogues. Similarly some catalogues give the proper motion as a position angle and size. In both these cases the tabulated values must be converted to the required form before they can be used in a target list.

The usual astronomical sign convention for radial velocity is used: objects which are receding should have a positive radial velocity.

A target list can also contain two optional parameters: EQUINOX and EPOCH. These parameters respectively contain the equinox and epoch of the coordinates. Both parameters are of data type CHARACTER.

The value of both parameters is a Besselian or Julian epoch4 expressed in years. The numeric value may optionally be preceded by a letter ‘B’ or ‘J’ to indicate a Besselian or Julian epoch respectively. If this preceding letter is omitted then values before 1984.0 are assumed to be Besselian and subsequent values to be Julian. This behaviour is consistent with the relevant IAU recommendations. Table 4 lists some examples of valid equinoxes and epochs.

Example Notes

B1950 often used in older catalogues
J2000 often used in modern catalogues
1992.37 interpreted as J1992.37 (because after 1984.0)
1943 interpreted as B1943.0 (because before 1984.0)
B1987 B’ is necessary here (because after 1984.0)

Table 4: Example equinoxes and epochs

An example of a simple target list is available as file /star/share/cursa/simple.TXT. In this target list the coordinates simply comprise Right Ascension and Declination. A more complicated example where the coordinates include proper motions etc. is available as file /star/share/cursa/propmotn.TXT. Note that though CURSA must interpret the columns of proper motions etc. as having units of radians they can be tabulated in an STL format catalogue in seconds of arc by using the TBLFMT option in the column definitions, as in this example. This option will often be convenient when creating target lists.

4In this context an epoch is simply an instant of time.