FK54Z Subroutine

  1. astro_module.f90
  2. astro_module
  3. FK54Z

public subroutine FK54Z(r2000, d2000, bepoch, r1950, d1950, dr1950, dd1950)

Convert a J2000.0 FK5 star position to B1950.0 FK4, assuming zero proper motion in FK5 and parallax.

Status: support routine.

Notes

  1. In contrast to the FK524 routine, here the FK5 proper motions, the parallax and the radial velocity are presumed zero.

  2. This routine converts a star position from the IAU 1976 FK5 (Fricke) system to the former FK4 (Bessel-Newcomb) system, for cases such as distant radio sources where it is presumed there is zero parallax and no proper motion. Because of the E-terms of aberration, such objects have (in general) non-zero proper motion in FK4, and the present routine returns those fictitious proper motions.

  3. Conversion from B1950.0 FK4 to J2000.0 FK5 only is provided for. Conversions involving other equinoxes would require additional treatment for precession.

  4. The position returned by this routine is in the B1950.0 FK4 reference system but at Besselian epoch BEPOCH. For comparison with catalogs the BEPOCH argument will frequently be 1950D0. (In this context the distinction between Besselian and Julian epoch is insignificant.)

  5. The RA component of the returned (fictitious) proper motion is dRA/dt rather than cos(Dec)*dRA/dt.

History

  • IAU SOFA revision: 2018 January 11

Arguments

TypeIntentOptionalAttributesName
real(kind=wp), intent(in) :: r2000

J2000.0 FK5 RA (rad)
real(kind=wp), intent(in) :: d2000

J2000.0 FK5 Dec (rad)
real(kind=wp), intent(in) :: bepoch

Besselian epoch (e.g. 1950D0)
real(kind=wp), intent(out) :: r1950

B1950.0 FK4 RA (rad) at epoch BEPOCH
real(kind=wp), intent(out) :: d1950

B1950.0 FK4 Dec (rad) at epoch BEPOCH
real(kind=wp), intent(out) :: dr1950

B1950.0 FK4 proper motions (rad/trop.yr)
real(kind=wp), intent(out) :: dd1950

B1950.0 FK4 proper motions (rad/trop.yr)

Calls

proc~~fk54z~~CallsGraph proc~fk54z FK54Z proc~fk524 FK524 proc~fk54z->proc~fk524 proc~c2s C2S proc~fk54z->proc~c2s proc~anp ANP proc~fk54z->proc~anp proc~s2c S2C proc~fk54z->proc~s2c proc~fk524->proc~anp proc~pdp PDP proc~fk524->proc~pdp proc~sxp SXP proc~fk524->proc~sxp proc~pv2s PV2S proc~fk524->proc~pv2s proc~ppp PPP proc~fk524->proc~ppp proc~s2pv S2PV proc~fk524->proc~s2pv proc~pmp PMP proc~fk524->proc~pmp proc~pm PM proc~fk524->proc~pm

Contents

Source Code

FK54Z

Source Code

    subroutine FK54Z ( r2000, d2000, bepoch, &
                       r1950, d1950, dr1950, dd1950 )

    implicit none

    real(wp),intent(in) :: r2000 !! J2000.0 FK5 RA (rad)
    real(wp),intent(in) :: d2000 !! J2000.0 FK5 Dec (rad)
    real(wp),intent(in) :: bepoch !! Besselian epoch (e.g. 1950D0)
    real(wp),intent(out) :: r1950 !! B1950.0 FK4 RA (rad) at epoch BEPOCH
    real(wp),intent(out) :: d1950 !! B1950.0 FK4 Dec (rad) at epoch BEPOCH
    real(wp),intent(out) :: dr1950 !! B1950.0 FK4 proper motions (rad/trop.yr)
    real(wp),intent(out) :: dd1950 !! B1950.0 FK4 proper motions (rad/trop.yr)

    real(wp) :: r, d, pr, pd, px, rv, p(3), w, v(3)
    integer :: i

    !  FK5 equinox J2000.0 to FK4 equinox B1950.0.
    call FK524 ( r2000, d2000, 0.0_wp, 0.0_wp, 0.0_wp, 0.0_wp, &
                 r, d, pr, pd, px, rv )

    !  Spherical to Cartesian.
    call S2C ( r, d, p )

    !  Fictitious proper motion (radians per year).
    v(1) = - pr*p(2) - pd*cos(r)*sin(d)
    v(2) =   pr*p(1) - pd*sin(r)*sin(d)
    v(3) =             pd*cos(d)

    !  Apply the motion.
    w = bepoch - 1950.0_wp
    do i=1,3
       p(i) = p(i) + w*v(i)
    end do

    !  Cartesian to spherical.
    call C2S ( p, w, d1950 )
    r1950 = ANP ( w )

    !  Fictitious proper motion.
    dr1950 = pr
    dd1950 = pd

    end subroutine FK54Z