Institut für Astronomie und Astrophysik
Abteilung AstronomieSand 1, D-72076 Tübingen, Germany
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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