BARYCEN

      Convert geocentric modified Julian date (MJD) to helio bary centric Julian
      date.

      This procedure correct for the extra light travel time between the Earth
      and the bary center of the Sun.

       bary_time = BARYCEN( date, ra, dec, /B1950, /TIME_DIFF,
                            ephemfile=ephemfile, orbitfile=orbitfile)

       date -  Modified Julian Date (= JD - 2400000.5), scalar or vector, MUST
               be double precision.
               The date describes an event at the geocentric time.
               The format of date can be a vector which is recommended for
               computing several dates because ephemeris file will be
               accessed only once.
      ra,dec - scalars giving right ascension and declination of the
               observed object in DEGREES
               Equinox is J2000 unless the /B1950 keyword is set.
               The object direction is needed to take into account
               the time delay due light travelling from center to
               earth to the common solar bary center.

       bary_time - solar bary centric Modified Julian Date.
                   If /TIME_DIFF is set, then BARYCEN() instead
                   returns the time difference in seconds of
                   barycentric-geocentric Julian date.

   ephemfile - (string) The file which contains JPL ephemeris
               information. It must have the JPLEPHREAD specific
               format. Public available are
               - JPLEPH.200 - JPL-DE200 which is the older but well
                               known ephemeris data file.
               - JPLEPH.405 - JPL-DE405 is more recent and precise.
               It is possible to build one's own ephemeris
               file. Refer JPLEPHREAD procedure.
               This file is not needed if the pinfo and pdata input
               variables are set.
               If this file is not given BARYCEN looks for the
               environment variable "ASTRO_DATA" describing a
               directory which must contain the file "JPLEPH.405".
               If the ephemeris file is not found an error is raised.
  orbit      - 2-dim array of double containing the orbit position
               information in respect of the geocenter. The position
               must be given for all dates given with the DATE
               input. Position must be given in FK5 coordintates (or
               ICRF if using JPLEPH.405) in units of km.
               Dimensions must be: array(3,n_elements(date)).
               If orbit is not given the correction will be performed
               at the center of the earth.
               Note: as mentioned by the different authors the use of
               the input date to define the orbit (and earth) vector
               uses an circular argumentation; but the error due to
               the small contribution of this correction is small.

                Main steps of the correction are taken from
                C. Marquardts description in
                http://lheawww.gsfc.nasa.gov/users/craigm/bary/
                which gives a good overview of the relevant aspects.
                BARYCEN does not perform a dispersion time
                correction.
                The procedure first reads the jpl ephemeris file
                taking the part which is needed for all dates to
                convert. Then the coordinates of the earth for all
                dates are computed within the FK5 (or ICRF when using
                DE405 ephemerides which is mostly the same like FK5)
                coordinate system, origin located at the solar system
                bary center.
                Additionally a unit vector is derived from the RA/DEC
                position which is converted also to the FK5
                coordinate system (J2000 equinox).
                Finally the projection of this vector at the
                coordinates of the earth yield the distance the light
                must travel; ivided by the speed of light this gives the
                time delay we are looking for.
                We should mention that JPL DE200 ephemeris are
                defined for the FK5 (J2000) coordinate system while
                the DE405 is defined for the ICRS coordinate
                system. They are defined close together but not
                exact.
                For the algorithm we always use FK5 coordinates
                (i.e. computing all in J2000 and not e.g. in the
                Hipparcos IRCS J1991.25 system). The difference lies
                in the region of ~50-80mas.
                Care must be taken when using the orbit input because
                errors in the orbit position directly influence the
                correction in a non-neglectable manner.
                Internally all computations are performed in units of
                km and seconds.

       /B1950 - if set, then input coordinates are assumed to be in equinox
                B1950 coordinates.
                Default is the J2000 equinox.
       /TIME_DIFF - if set, then HELIO_JD() returns the time difference
                (helio bary centric JD - geocentric JD ) in seconds.

       What is the barycentric Julian date of an observation of V402 Cygni
       (J2000: RA = 20 9 7.8, Dec = 37 09 07) taken June 15, 1973 at 11:40 UT?
       IDL> juldate, [1973,6,15,11,40], jd      ; Get geocentric
                                                ; reduced Julian date
       IDL> bjd = barycen( jd-0.5D0,  $         ; convert to bary
                           ten(20,9,7.8)*15., $ ; center MJD
                           ten(37,9,7) )
       ==> bjd =  41848.988072569 (MJD)

       Reading of orbit files could be performed with the fits
       accessing function READORBIT().
       !!! Care must be taken for the units of the result of the
       !!! readorbit procedure. Satellite orbit files usually contain
       !!! the spacecraft position in meters while here we are
       !!! using kilometers!.

       jplephread, jplephinterp,
       jprecess, tdb2tdt

       Processed RXTE lightcurves of GX301 (thank you, Ingo) with
       barycen and checked result against fxbary product.
       Maximal difference was proven to be in the range of
       1.2572855e-06 sec while the averaged difference was
       9.5243109e-08 sec. (17-09-2002, E. Goehler)

       $Log: barycen.pro,v $
       Revision 1.5  2003/07/09 08:34:24  goehler
       Major change: date input now MJD instead RJD
       Revision 1.4  2003/04/29 09:34:48  goehler
       added warning concerning units in km when using the readorbit() function
       Revision 1.3  2002/09/17 12:49:17  goehler
       Fix of wrong used JPLEPHINTERP function in respect of velocities.
       Tests yielded difference to fxbary in the range of less than 1usec.
       Revision 1.2  2002/09/13 16:14:43  goehler
       preliminary tests give good results (~10msec) for geocentric case. Use of
       orbit files still bad.
       Revision 1.1  2002/09/12 14:13:58  goehler
       first version,
       !!! still not tested !!!

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