The error in using the IDL procedure PRECESS for converting betweenī1950 and J1950 can be up to 12”, mainly in right ascension. Precision can be achieved in this case by inputting the EPOCH (1983) is used toĮnsure that the output proper motion is exactly zero. The algorithm in Appendix 2 of Aoki et al. (2) the proper motion is unknown or known to be exactly zero (i.e.Įxtragalactic radio sources). (1) The proper motion is known and non-zero The algorithm is taken from the Explanatory Supplement to theīPRECESS distinguishes between the following two cases: EPOCH - scalar giving epoch of original observations, default 2000.0d This keyword value is only used if the MU_RADEC keyword is not set.
The parallax and radial velocity will have a very Upon output to contain the values of these quantities in theī1950 system. The values of MU_RADEC, PARALLAX, and RADVEL will all be modified RAD_VEL - N_element vector giving radial velocity in km/s PARALLAX - N_element vector giving stellar parallax (seconds of arc)
OPTIONAL INPUT-OUTPUT KEYWORDS MU_RADEC - 2xN element double precision vector containing the proper motion in seconds of arc per tropical century in right Scalar or N element vector OUTPUTS: RA_1950, DEC_1950 - The corresponding B1950 right ascension and declination in degrees. CALLING SEQUENCE: bprecess, ra, dec, ra_1950, dec_1950, INPUTS: RA,DEC - Input J2000 right ascension and declination in degrees. The mean place at J2000.0 on the FK5 system. bprecess ( ra0, dec0, mu_radec=None, parallax=None, rad_vel=None, epoch=None ) ¶ NAME: BPRECESS PURPOSE: Precess positions from J2000.0 (FK5) to B1950.0 (FK4) EXPLANATION: Calculates the mean place of a star at B1950.0 on the FK4 system from Landsman September 1997ĭocumentation update W. Landsman, Cleaned up program sent by Chris McCarthy (SfSU) June 1994Ĭonverted to IDL V5.0 W. Vb*cos(dec)*sin(ra) + vb*sin(dec) REVISION HISTORY: Jeff Valenti, U.C.
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