27
4.13
TRANSFORM_CAT
void transform_cat (short int option, double date_incat,
cat_entry *incat, double date_newcat,
char newcat_id[4],
cat_entry *newcat)
PURPOSE:
To transform a star's catalog quantities for a change of epoch
and/or equator and equinox.
INPUT
ARGUMENTS:
option (short int)
Transformation option
= 1 ... change epoch; same equator and equinox
= 2 ... change equator and equinox; same epoch
= 3 ... change equator and equinox and epoch
date_incat (double)
TT Julian date, or year, of input catalog data.
*incat (struct cat_entry)
An entry from the input catalog (struct defined in novas.h).
date_newcat (double)
TT Julian date, or year, of transformed catalog data.
Newcat_id[4] (char)
Three-character abbreviated name of the transformed catalog.
OUTPUT
ARGUMENTS:
newcat (struct cat_entry)
The transformed catalog entry (struct defined in novas.h).
RETURNED
VALUE:
None.
Discussion:
Function
transform_cat
performs mean place to mean place transformations on
star catalog data. Only reference data, not observables, are involved. Two dates must be
specified: the input data is associated with the first date, and the output data is associated
with the second date. Two basic transformations are available:
1. The star's data is updated to account for the star's space motion between
the first and second dates, within a fixed reference frame. That is, the epoch
of the data is changed, but not the equator and equinox.
2. The reference frame within which the star's coordinates and proper motion
are expressed is rotated corresponding to precession between the first and
second dates. The star's position in space is not changed. That is, the equator
and equinox of the data are changed, but not the epoch.
