pro Bprecess, ra, dec, ra_1950, dec_1950, MU_RADEC = mu_radec, $ PARALLAX = parallax, RAD_VEL = rad_vel, EPOCH = epoch ;+ ; 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 ; the mean place at J2000.0 on the FK5 system. ; ; CALLING SEQUENCE: ; bprecess, ra, dec, ra_1950, dec_1950, [ MU_RADEC = , PARALLAX = ; RAD_VEL =, EPOCH = ] ; ; INPUTS: ; RA,DEC - Input J2000 right ascension and declination in *degrees*. ; Scalar or N element vector ; ; OUTPUTS: ; RA_1950, DEC_1950 - The corresponding B1950 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 ; B1950 system. The parallax and radial velocity will have a very ; minor influence on the B1950 position. ; ; EPOCH - scalar giving epoch of original observations, default 2000.0d ; This keyword value is only used if the MU_RADEC keyword is not set. ; NOTES: ; The algorithm is taken from the Explanatory Supplement to the ; Astronomical Almanac 1992, page 186. ; Also see Aoki et al (1983), A&A, 128,263 ; ; BPRECESS 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 reverse of ; 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 J1950 can be up to 1.5", mainly in right ascension. If ; better accuracy than this is needed then BPRECESS should be used. ; ; An unsystematic comparison of BPRECESS with the IPAC precession ; routine (http://nedwww.ipac.caltech.edu/forms/calculator.html) always ; gives differences less than 0.15". ; EXAMPLE: ; The SAO2000 catalogue gives the J2000 position and proper motion for ; the star HD 119288. Find the B1950 position. ; ; RA(2000) = 13h 42m 12.740s Dec(2000) = 8d 23' 17.69'' ; Mu(RA) = -.0257 s/yr Mu(Dec) = -.090 ''/yr ; ; IDL> mu_radec = 100D* [ -15D*.0257, -0.090 ] ; IDL> ra = ten(13, 42, 12.740)*15.D ; IDL> dec = ten(8, 23, 17.69) ; IDL> bprecess, ra, dec, ra1950, dec1950, mu_radec = mu_radec ; IDL> print, adstring(ra1950, dec1950,2) ; ===> 13h 39m 44.526s +08d 38' 28.63" ; ; REVISION HISTORY: ; Written, W. Landsman October, 1992 ; Vectorized, W. Landsman February, 1994 ; Treat case where proper motion not known or exactly zero November 1994 ; Handling of arrays larger than 32767 Lars L. Christensen, march, 1995 ; Converted to IDL V5.0 W. Landsman September 1997 ; Fixed bug where A term not initialized for vector input ; W. Landsman February 2000 ; ;- On_error,2 if N_params() LT 4 then begin print,'Syntax - BPRECESS, ra,dec, ra_1950, dec_1950, [MU_RADEC =' print,' PARALLAX = , RAD_VEL = ]' print,' Input RA and Dec should be given in DEGREES for J2000' print,' Proper motion, MU_RADEC, (optional) in arc seconds per *century*' print,' Parallax (optional) in arc seconds' print,' Radial Velocity (optional) in km/s' return endif N = N_elements( ra ) if N EQ 0 then message,'ERROR - First parameter (RA vector) is undefined' if not keyword_set( RAD_VEL) then rad_vel = dblarr(N) else begin rad_vel = rad_vel*1. if N_elements( RAD_VEL) NE N then message, $ 'ERROR - RAD_VEL keyword vector must contain ' + strtrim(N,2) +' values' endelse if keyword_set( MU_RADEC) then begin if (N_elements( mu_radec) NE 2*N ) then message, $ 'ERROR - MU_RADEC keyword (proper motion) be dimensioned (2,' + $ strtrim(N,2) + ')' mu_radec = mu_radec*1. endif if not keyword_set( Parallax) then parallax = dblarr(N) else $ parallax = parallax*1. if not keyword_set(Epoch) then epoch = 2000.0d0 radeg = 180.D/!DPI sec_to_radian = 1.d0/radeg/3600.d0 M = [ [+0.9999256795D, -0.0111814828D, -0.0048590040D, $ -0.000551D, -0.238560D, +0.435730D ], $ [ +0.0111814828D, +0.9999374849D, -0.0000271557D, $ +0.238509D, -0.002667D, -0.008541D ], $ [ +0.0048590039D, -0.0000271771D, +0.9999881946D , $ -0.435614D, +0.012254D, +0.002117D ], $ [ -0.00000242389840D, +0.00000002710544D, +0.00000001177742D, $ +0.99990432D, -0.01118145D, -0.00485852D ], $ [ -0.00000002710544D, -0.00000242392702D, +0.00000000006585D, $ +0.01118145D, +0.99991613D, -0.00002716D ], $ [ -0.00000001177742D, +0.00000000006585D,-0.00000242404995D, $ +0.00485852D, -0.00002717D, +0.99996684D] ] A_dot = 1D-3*[1.244D, -1.579D, -0.660D ] ;in arc seconds per century ra_rad = ra/radeg & dec_rad = dec/radeg cosra = cos( ra_rad ) & sinra = sin( ra_rad ) cosdec = cos( dec_rad ) & sindec = sin( dec_rad ) dec_1950 = dec*0. ra_1950 = ra*0. for i = 0L, N-1 do begin ; Following statement moved inside loop in Feb 2000. A = 1D-6*[ -1.62557D, -0.31919D, -0.13843D] ;in radians r0 = [ cosra[i]*cosdec[i], sinra[i]*cosdec[i], sindec[i] ] if keyword_set(mu_radec) then begin mu_a = mu_radec[ 0, i ] mu_d = mu_radec[ 1, i ] r0_dot = [ -mu_a*sinra[i]*cosdec[i] - mu_d*cosra[i]*sindec[i] , $ ;Velocity vector mu_a*cosra[i]*cosdec[i] - mu_d*sinra[i]*sindec[i] , $ mu_d*cosdec[i] ] + 21.095d * rad_vel[i] * parallax[i] * r0 endif else r0_dot = [0.0d0, 0.0d0, 0.0d0] R_0 = [ r0, r0_dot ] R_1 = M # R_0 ; Include the effects of the E-terms of aberration to form r and r_dot. r1 = R_1[0:2] r1_dot = R_1[3:5] if not keyword_set(Mu_radec) then begin r1 = r1 + sec_to_radian * r1_dot * (epoch - 1950.0d)/100. A = A + sec_to_radian * A_dot * (epoch - 1950.0d)/100. endif x1 = R_1[0] & y1 = R_1[1] & z1 = R_1[2] rmag = sqrt( x1^2 + y1^2 + z1^2 ) s1 = r1/rmag & s1_dot = r1_dot/rmag s = s1 for j = 0,2 do begin r = s1 + A - (total(s * A))*s s = r/rmag endfor x = r[0] & y = r[1] & z = r[2] r2 = x^2 + y^2 + z^2 rmag = sqrt( r2 ) if keyword_set(Mu_radec) then begin r_dot = s1_dot + A_dot - ( total( s * A_dot))*s x_dot = r_dot[0] & y_dot= r_dot[1] & z_dot = r_dot[2] mu_radec[0,i] = ( x*y_dot - y*x_dot) / ( x^2 + y^2) mu_radec[1,i] = ( z_dot* (x^2 + y^2) - z*(x*x_dot + y*y_dot) ) / $ ( r2*sqrt( x^2 + y^2) ) endif dec_1950[i] = asin( z / rmag) ra_1950[i] = atan( y, x) if parallax[i] GT 0. then begin rad_vel[i] = ( x*x_dot + y*y_dot + z*z_dot )/ (21.095*Parallax[i]*rmag) parallax[i] = parallax[i] / rmag endif endfor neg = where( ra_1950 LT 0, NNeg ) if Nneg GT 0 then ra_1950[neg] = ra_1950[neg] + 2.D*!DPI ra_1950 = ra_1950*radeg & dec_1950 = dec_1950*radeg ; Make output scalar if input was scalar sz = size(ra) if sz[0] EQ 0 then begin ra_1950 = ra_1950[0] & dec_1950 = dec_1950[0] endif return end