subroutine pitpha(minang)
c
c routine returns the spin phase angle where the minimum pitch angle
c is sampled.
c minang is returned in range -180 to +180 deg.
c minang is returned =-1000 is b field is invalid.
c
c v1.0 jfe johnson 9 sep 82
c
      real b(3),brot(3)
      integer*2 idat(2812)
      common/maf1/idat
c
      minang=-1000
      call tstbm1(kode)
      if(kode.ne.0)return                  ! bad b field values
c
c put the b field values for the first sample into real arrays
c
      do 1000 i=1,3
 1000      b(i)=float(idat(100+i))*float(idat(25))
c
      spang=float(idat(23))*0.1
      call axisrt(b,-spang,brot)            ! rotate to fixed frame
c determine phase angle at which we look, which is projected angle of
c rotated b field onto spin plane
      rminan=atan2(brot(2),brot(1))
      rminan=rminan*180.0/3.14159
      rminan=amod(rminan+360.0,360.0)
      if(rminan.gt.180.0)rminan=rminan-360.0
      minang=rminan
      return
      end