PRECESS

PRECESS converts radio source coordinates from one coordinate system to another. It reads source catalogs in the KEYIN free format (see SCHED help file), which contain source positions in either the B1950 or the J2000 systems, and computes any of the following:

mean RA, Dec of B1950.0 (old FK4 system)
mean RA, Dec of J2000.0 (new FK5 system)
geocentric apparent RA, Dec at any epoch (new system)
ecliptic longitude and latitude (new system, mean of date)
galactic latitude and longitude (IAU 1958 system)

The program assumes that the sources have negligible proper motion or parallax. If this is not true, you can use the Starlink program COCO to do rigorous coordinate conversions (see Starlink User Note 56).

Example

The following example reads a source catalog and computes the J2000 and galactic source coordinates, and the geocentric apparent position for two dates.

$ precess
  Srcfile    = mnt:[tjp.survey]sources.cat
  Listfile   = precess.lis
  System     = J2000
  Precisi    = 2
  /
  System     = GAL
  /
  Date       = 90:05:31
  System     = APP
  /
  Date       = 90:06:01
  /
  EXIT
  /

Parameters

Parameters are supplied in standard KEYIN format. Parameter names may be abbreviated. End each group of parameters with a slash (/). One group of parameters is specified for each source catalog or coordinate system requested. Parameters that are not specified are inherited from the previous group. To exit from the program, type EXIT followed by a slash, or end-of-file (control-Z in VMS, control-D in Unix).

SRCFILE: the file name of the source catalog. The default is "VLB_SOURCES" which can be a logical name in VMS.
LISTFILE: a file name for the listing (a text file). The default is "precess.lis". The listing is not formatted with column headings, page-dividers etc.; they could be added if required.
CATFORMAT: if this parameter is specified with no value (or zero or positive value) the output file specified by LISTFILE will be written in the standard catalog format (similar to SRCFILE). Otherwise the listing contains both input and output coordinates in a columnar format. (CATFORMAT is ignored unless SYSTEM=J2000 or B1950.)
PRECISION: specifies the precision to which the coordinates are printed in the listing; it is the number of decimal places in the seconds field of declination (right ascension is printed with one more decimal place). Values 0, 1, 2, 3 are allowed.
SYSTEM: specifies the coordinate system for which the coordinates are to be computed. Choices are "J2000" (mean coordinates of equinox and epoch J2000), "B1950" (mean coordinates of equinox B1950 and epoch specified by BEPOCH), "APP" (geocentric apparent coordinates of date), "GAL" (galactic longitude and latitude), "ECLIP" (mean ecliptic longitude and latitude of date).
DATE: UT date on the Gregorian calendar, format yy:mm:dd, eg DATE = 1981:3:21 (March 21, 1981). The default date is today's date. This is used for "APP" and "ECLIP" coordinate systems. The century may be omitted (e.g., 81:3:21); if the year is 00--49, it is interpreted as 2000--2049; if it is 50--99, it is interpreted as 1950--1999; otherwise it is taken literally. Positions are computed for 0h UT (strictly, TDB or barycentric coordinate time).
BEPOCH: the epoch of observation for sources in the B1950 system (see Notes). The default value of 1975.0 will be adequate for all but the most demanding applications.
EXIT: specify EXIT to exit from the program.

Notes

The program uses the SLALIB subroutine library by Patrick Wallace. For more detailed information, see Starlink User Note 67.

Input coordinates: Sources may be specified in the catalog with EPOCH="B1950" or EPOCH="J2000". Other values for EPOCH cause the source to be ignored. The B1950 coordinate system has some subtleties; for accurate work, obtain an accurate J2000 position if you can. One complication is that B1950 positions are sometimes specified omitting the "E-terms of aberration". Another is that B1950 is not an inertial coordinate system. Sources (such as quasars) with zero proper motion in the J2000 frame have non-zero fictitious proper motion in the B1950 frame. For this reason, parameter BEPOCH is provided to specify the epoch at which the B1950 coordinates are intended to apply. This should be the date of the observations from which the position was obtained.
Geocentric apparent coordinates: these are the coordinates at which the source would be observed by a perfect telescope at the center of the earth in the absence of an atmosphere, at the given instant. For pointing real telescopes, they must be corrected for parallax (dependent on the telescope location, but negligible for extragalactic objects) and for atmospheric refraction.
Mean coordinates: mean coordinates are fictitious, and correspond to a "smoothed" position of the source omitting the rapidly-varying terms of nutation, etc. PRECESS does not provide a means to compute mean coordinates except B1950.0 and J2000.0, but it could be added if needed. Mean coordinates should not be used to point a telescope!
Galactic coordinates: these are (l,b) in the [new] IAU 1958 system.
Ecliptic coordinates: these are the mean ecliptic longitude and latitude of date, in the IAU 1980 theory.

History

Version 2.0: 1990 May 31 - new program (T.J. Pearson).
Version 2.1: 1994 Aug 8 - option to write source-catalog file (TJP).
Version 2.2: 1994 Nov 17 - 12-character source names (TJP).

Tim Pearson, California Institute of Technology
tjp·astro.caltech.edu