17 Converting between celestial coordinate systems

17.1 Running catcoord

CURSA contains some limited facilities for converting between different celestial coordinate systems. Application catcoord can convert mean equatorial coordinates for a given equinox and epoch to mean equatorial coordinates for a new equinox and epoch, to Galactic coordinates9, or to de Vaucouleurs’ supergalactic coordinates. catcoord uses the Starlink subroutine library SLA to convert between coordinate systems. The manual for this library, SUN/67[32], contains a brief introduction to the various celestial coordinate systems. Further details can be found in standard textbooks on spherical astronomy (see, for example, Spherical Astronomy by R.M. Green[15]).

catcoord creates a copy of the catalogue with the new coordinates added. It operates on a target list (see Section 7). That is, it requires that the input catalogue contains columns of coordinates which it can interpret. The input catalogue must contain columns of Right Ascension and Declination for some equinox and epoch. Optionally it can also contain columns of proper motion in Right Ascension and Declination, parallax and radial velocity which permit more accurate conversions. It is not necessary that all four additional columns be present in order to use them. For example, if only columns of proper motion are present they can be used in isolation. These additional columns are usually only available in catalogues of relatively nearby and well-observed stars. In most catalogues the coordinates will simply comprise a Right Ascension and Declination for some equinox and epoch.

The coordinates computed by catcoord are suitable for plotting, display, pairing etc. However, for accurate work they are not suitable for further subsequent conversions to another equinox and epoch. This limitation arises because only new coordinates are computed; the proper motions etc. are not revised for the new equinox and epoch. Thus, in accurate work, new coordinates should always be computed from the original coordinates in the target list, not from intermediate coordinates created with catcoord. However, this caveat is only important when accurate coordinates are being computed.

catcoord offers only a limited set of conversions (converting mean equatorial coordinates to a new equinox and epoch, to Galactic coordinates or to supergalactic coordinates). Additional conversions, such as converting mean equatorial coordinates for some equinox and epoch to apparent coordinates, are available using the Starlink package COCO (see SUN/56[31]). To use COCO first use xcatview (see Section 11) to save the coordinates as a text file in a suitable format and them import them into COCO.

17.1 Running catcoord

To run catcoord in its simplest mode type:

catcoord

By default catcoord simply reads columns of Right Ascension and Declination from the input target list and computes equatorial coordinates for some new equinox and epoch. To compute more accurate coordinates using columns of proper motion, parallax etc. in the input catalogue type:

catcoord full=true

Similarly, to compute Galactic rather than equatorial coordinates type:

catcoord  coords=galactic

or for supergalactic coordinates:

catcoord  coords=supergalactic

These options may be combined. Thus, to compute Galactic coordinates from accurate input coordinates type:

catcoord  full=true  coords=galactic

The amount of textual information written to the output catalogue is controlled using the command line mechanism described in Section 10.1.

You then answer a series of prompts to define the required conversion. All the possible prompts are listed below, identified by the corresponding ADAM parameter name. All the prompts will not appear in a given run. For example, catcoord tries to obtain the equinox and epoch of the input coordinates from the input target list and will only prompt you if it cannot find them.

The new coordinates may either be written to the same columns as the original input coordinates (thus replacing them) or to new columns (in which case both sets of coordinates will continue to be available). All the other columns and parameters in the catalogue are simply copied.

CATIN
Enter the name of the input catalogue (which must be a target list, see Section 7).
CATOUT
Enter the name of the output catalogue to contain the new coordinates.
EPOCHI
Specify the epoch of the input catalogue, for example, ‘J2000’ or ‘B1950’.
EQUINI
Specify the equinox of the input catalogue, for example, ‘J2000’ or ‘B1950’.
RAIN
Enter the name of the column containing Right Ascension in the input catalogue.
DECIN
Enter the name of the column containing Declination in the input catalogue.
PMRA
Enter the name of the column containing the proper motion in Right Ascension in the input catalogue. Enter ‘NONE’ if no column is available.
PMDE
Enter the name of the column containing the proper motion in Declination in the input catalogue. Enter ‘NONE’ if no column is available.
PLX
Enter the name of the column containing the parallax in the input catalogue. Enter ‘NONE’ if no column is available.
RV
Enter the name of the column containing the radial velocity in the input catalogue. Enter ‘NONE’ if no column is available.
EPOCHO
Specify the epoch of the output catalogue, for example, ‘J2000’ or ‘B1950’.
EQUINO
Specify the equinox of the output catalogue, for example, ‘J2000’ or ‘B1950’.
RAOUT
Enter the name of the column containing the new Right Ascension in the output catalogue.
DECOUT
Enter the name of the column containing the new Declination in the output catalogue.
L
Enter the name of the column containing the new Galactic longitude in the output catalogue.
B
Enter the name of the column containing the new Galactic latitude in the output catalogue.
SGL
Enter the name of the column containing the new supergalactic longitude in the output catalogue.
SGB
Enter the name of the column containing the new supergalactic latitude in the output catalogue.

9catcoord calculates ‘new’ (IAU 1958) Galactic longitude and latitude, conventionally denoted $l,b$. Previously these coordinates were often denoted ${l}^{II},{b}^{II}$ in order to differentiate them from ‘old’ (pre-1958) Galactic coordinates, ${l}^{I},{b}^{I}$. Old Galactic coordinates are now rarely used.