% @(#)computprec.hlq	14.1.1.1 (ESO-IPG) 09/16/99 09:25:30 
%++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
%.COPYRIGHT  (c)  1990 European Southern Observatory
%.IDENT      computprec.hlq
%.AUTHOR     KB, IPG/ESO
%.KEYWORDS   MIDAS, help files, COMPUTE/PRECESSION
%.PURPOSE    On-line help file for the command: COMPUTE/PRECESSION
%.VERSION    1.0  26-FEB-1987 : Creation, KB
%.VERSION    1.1  12-NOV-1993 : Update, KB
%----------------------------------------------------------------
\se
SECTION./PREC
\es\co
COMPUTE/PRECESSION					12-NOV-1993  KB
\oc\su
COMPUTE/PRECESSION alpha delta equinox0 equinox1
COMPUTE/PRECESSION table.tbl equinox0 equinox1
	precess equatorial coordinates from one epoch to another
\us\pu
Purpose: 
          Precess equatorial coordinates from one epoch to another.
\up\sy
Syntax: 
          COMPUTE/PRECESSION alpha delta equinox0 equinox1
          COMPUTE/PRECESSION table.tbl equinox0 equinox1
\ys\pa
          alpha = original right ascension (hour,min,sec)
\ap\pa
          delta = original declination (degree,min,sec)
\ap\pa
          equinox0 = original equinox (year,month,day)
\ap\pa
          equinox1 = equinox of precessed coordinates (year,month,day)
\ap\pa
          table = name of table with the following columns:
\\
              :AHR, :AMIN, :ASEC     right ascension (hour,min,sec)
\\
              :DDEG, :DMIN, :DSEC    declination (degree, min,sec)
\\
              if `equinox0' is set to TAB, different original equinoxes apply
              to the coordinates and must be given in columns :EYEAR, :EMONTH,
              and :EDATE 
\ap\sa
See also:
          COMPUTE/ST, COMPUTE/UT, COMPUTE/TABLE
\as\no
Note:  
          The parameters may also be referenced via:
\\
          ALPHA0=  DELTA0=  EQUINOX0=  EQUINOX1=
\\
          Dates and coordinates are of type REAL so that, e.g., the date can
          also be entered as 1982.787,0,0 or just 1982.787 . However,
          parameters may not be combined. That is, in the example given, the
          fraction of the year may contain only the number of elapsed full days
          not also UT.  
\\
          The same applies to table input, however all input columns must be
          present even if they contain only zeros. The minus sign for negative
          values may occur in any field.
\\
          Output will be written to the following keywords:
\\
          OUTPUTR/R/1/3     precessed right ascension (hour,min,sec)
\\
          OUTPUTR/R/4/3     precessed declination (degree,min,sec)
\\
          OUTPUTD/D/1/2     original and new epoch
\\
          For negative RA or DEC the following convention applies:
\\
          In order not to "loose the minus sign" the first non-zero field is
          set negative. E.g. for DEC=-0:1:10 we set OUTPUTR(4,5,6) = 0,-1,10 .
\\
          With the table option the following table columns will be created or
          updated:
\\
          :PAHR, :PAMIN :PASEC     precessed right ascension (hour,min,sec)
\\
          :PDDEG, :PDMIN, :PDSEC  precessed declination (degree,min,sec)
\\
          Also the descriptor NEW_EPOCH will contain the new equinox.
\\
          If parameter P2 was set to TAB, a descriptor OLD_EPOCH will contain
          the original equinox.
\\
          (Note that the table option is rather slow.)
\on\exs
Examples:
\ex
          COMPUTE/PRECESS  13,24,32.0  -11,5,47  1987,7,1.333  2000.0
            Precess the coordinates of Spica from 1987 July 1 8:00:00 UT
            to 2000.0
\xe\ex
          COMPUTE/PRECESS  stars.tbl  1987,7,1.333  1993.671
            Precess coordinates contained in table `stars.tbl' from the equinox
            1987 July 1 8:00:00 UT to the equinox 1993.671.
\xe\ex
          COMPUTE/PRECESS  stars.tbl  tab  1993.671
            Precess coordinates contained in table `stars.tbl' to the common
            equinox 1993.671. All input coordinates may relate to different
            equinoxes which in any case are given in columns :EYEAR, :EMONTH,
            and :EDATE of table `stars.tbl'.
\xe \sxe