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.
To run catcoord
in its simplest mode type:
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:
Similarly, to compute Galactic rather than equatorial coordinates type:
or for supergalactic coordinates:
These options may be combined. Thus, to compute Galactic coordinates from accurate input coordinates type:
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
CATOUT
EPOCHI
J2000
’ or ‘B1950
’.
EQUINI
J2000
’ or ‘B1950
’.
RAIN
DECIN
PMRA
NONE
’ if no column is available.
PMDE
NONE
’ if no column is available.
PLX
NONE
’
if no column is available.
RV
NONE
’ if no column is available.
EPOCHO
J2000
’ or ‘B1950
’.
EQUINO
J2000
’ or ‘B1950
’.
RAOUT
DECOUT
L
B
SGL
SGB
9catcoord
calculates ‘new’ (IAU 1958) Galactic longitude and latitude, conventionally denoted
. Previously these coordinates
were often denoted
in order to differentiate them from ‘old’ (pre-1958) Galactic coordinates,
. Old
Galactic coordinates are now rarely used.