HELIO_RV

     Return the heliocentric radial velocity of a spectroscopic binary

    This function will return the heliocentric radial velocity of a
    spectroscopic binary star at a given heliocentric Julian date (HJD)
    given its orbit.

  Result = HELIO_RV ( Reduced_HJD ,T ,Period ,Gamma , K, [,e ,Omega ] )

 Reduced_HJD   - Reduced_HJD of observation
 T             - Reduced_HJD of periastron passage (max. +ve velocity
                 for circular orbits)
 Period        - the period in days
 Gamma         - systemic velocity
 K             - velocity semi-amplitude in the same units as Gamma.
 e             - eccentricity of the orbit, default is 0.
 Omega         - longitude of periastron in degrees. Must be specified for
                 eccentric orbits.

  The predicted heliocentric radial velocity in the same units as Gamma
  for the date(s) specified by Reduced_HJD.

  To ensure consistency with the routines JULDATE and HELIO_JD, the
  reduced HJD must be used throughtout.

 Example 1
  What was the heliocentric radial velocity of the primary component of HU Tau
 at 1730 UT 25 Oct 1994?
 IDL> juldate ,[94,10,25,17,30],JD                 ;Get Geocentric julian date
 IDL> hjd = helio_jd(jd,ten(04,38,16)*15.,ten(20,41,05)) ; Convert to HJD
 IDL> print, helio_rv(hjd,46487.5303D,2.0563056D,-6.0,59.3)
      -63.661180
 NB. 1. The routines JULDATE and HELIO_JD return a reduced HJD (HJD - 2400000)
        and so T and P must be specified in the same fashion.
     2. The user should be careful to use double precision format to specify
        T and P to sufficient precision where necessary.
 Example 2
  Plot two cycles of an eccentric orbit, e=0.6, omega=45 for both
  components of a binary star
 IDL> phi=findgen(100)/50.0             ; Generates 100 phase points
 IDL> plot, phi,helio_rv(phi,0,1,0,100,0.6,45),yrange=[-100,150]
 IDL> oplot, phi,helio_rv(phi,0,1,0,50,0.6,45+180)
 This illustrates both the use of arrays to perform multiple calculations
 and generating radial velocities for a given phase by setting T=0 and P=1.
 Note also that omega has been changed by 180 degrees for the orbit of the
 second component  (the same 'trick' can be used for circular orbits).

  Written by:  Pierre Maxted CUOBS, October, 1994
  Circular orbits handled by setting e=0 and omega=0 to allow
  binary orbits to be handled using omega and omega+180.
                                                      Pierre Maxted,Feb 95
  BUG - omega was altered by the routine - corrected Feb 95,Pierre Maxted
  Iteration for E changed to that  given by Reidel , Feb 95,Pierre Maxted
  /SINGLE keyword removed.                           May 96,Pierre Maxted
       Converted to IDL V5.0   W. Landsman   September 1997

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