idlastro / Astronomical Utilities: JPRECESS

[Source code]

NAME
JPRECESS
PURPOSE
Precess astronomical coordinates from B1950 to J2000
EXPLANATION
Calculate the mean place of a star at J2000.0 on the FK5 system from the
mean place at B1950.0 on the FK4 system.
Use BPRECESS for the reverse direction J2000 ==> B1950
CALLING SEQUENCE
jprecess, ra, dec, ra_2000, dec_2000, [ MU_RADEC = , PARALLAX = 
         RAD_VEL =, EPOCH =   ]
INPUTS
RA,DEC - input B1950 right ascension and declination in *degrees*.
         Scalar or vector
OUTPUTS
RA_2000, DEC_2000 - the corresponding J2000 right ascension and 
         declination in *degrees*.   Same number of elements as RA,DEC
         but always double precision. 
OPTIONAL INPUT-OUTPUT KEYWORDS
MU_RADEC - 2xN element double precision vector containing the proper 
            motion in seconds of arc per tropical *century* in right 
            ascension and declination.
PARALLAX - N_element vector giving stellar parallax (seconds of arc)
RAD_VEL  - N_element vector giving radial velocity in km/s
 The values of MU_RADEC, PARALLAX, and RADVEL will all be modified
 upon output to contain the values of these quantities in the
 J2000 system.    Values will also be converted to double precision.  
 The parallax and radial velocity will have a very minor influence on 
 the J2000 position.
 EPOCH - scalar giving epoch of original observations, default 1950.0d
     This keyword value is only used if the MU_RADEC keyword is not set.
S:
 The algorithm is taken from the Explanatory Supplement to the 
 Astronomical Almanac 1992, page 184.
 Also see Aoki et al (1983), A&A, 128,263
 JPRECESS distinguishes between the following two cases:
 (1) The proper motion is known and non-zero
 (2) the proper motion is unknown or known to be exactly zero (i.e.
         extragalactic radio sources).   In this case, the algorithm
         in Appendix 2 of Aoki et al. (1983) is used to ensure that
         the output proper motion is  exactly zero.    Better precision
         can be achieved in this case by inputting the EPOCH of the
         original observations.
 The error in using the IDL procedure PRECESS for converting between
 B1950 and J2000 can be up to 12", mainly in right ascension.   If
 better accuracy than this is needed then JPRECESS should be used.
EXAMPLE
The SAO catalogue gives the B1950 position and proper motion for the 
star HD 119288.   Find the J2000 position. 
   RA(1950) = 13h 39m 44.526s      Dec(1950) = 8d 38' 28.63''  
   Mu(RA) = -.0259 s/yr      Mu(Dec) = -.093 ''/yr
IDL> mu_radec = 100D* [ -15D*.0259, -0.093 ]
IDL> ra = ten(13,39,44.526)*15.D 
IDL> dec = ten(8,38,28.63)
IDL> jprecess, ra, dec, ra2000, dec2000, mu_radec = mu_radec
IDL> print, adstring(ra2000, dec2000,2)
        ===> 13h 42m 12.740s    +08d 23' 17.69"
RESTRICTIONS
"When transferring individual observations, as opposed to catalog mean
 place, the safest method is to tranform the observations back to the
 epoch of the observation, on the FK4 system (or in the system that was
 used to to produce the observed mean place), convert to the FK5 system,
 and transform to the the epoch and equinox of J2000.0" -- from the
 Explanatory Supplement (1992), p. 180
REVISION HISTORY
Written,    W. Landsman                September, 1992
Corrected a couple of typos in M matrix   October, 1992
Vectorized, W. Landsman                   February, 1994
Implement Appendix 2 of Aoki et al. (1983) for case where proper
motion unknown or exactly zero     W. Landsman    November, 1994
Converted to IDL V5.0   W. Landsman   September 1997
Fixed typo in updating proper motion   W. Landsman   April 1999
Make sure proper motion is floating point  W. Landsman December 2000
Use V6.0 notation  W. Landsman Mar 2011
ake sure at least float
n radians
n arc seconds per century
elocity vector
Remove the effects of the E-terms of aberration to form r1 and r1_dot.
Make output scalar if input was scalar