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<H2><A NAME="SECTION000519000000000000000">
Radial Velocity and Light-Time Corrections</A>
</H2>
When publishing high-resolution spectral observations
it is necessary to refer them to a specified standard of rest.
This involves knowing the component in the direction of the
source of the velocity of the observer. SLALIB provides a number
of routines for this purpose, allowing observations to be
referred to the Earth's centre, the Sun, a Local Standard of Rest
(either dynamical or kinematical), the centre of the Galaxy, and
the mean motion of the Local Group.
<P>
The routine
sla_RVEROT
corrects for the diurnal rotation of
the observer around the Earth's axis. This is always less than 0.5 km/s.
<P>
No specific routine is provided to correct a radial velocity
from geocentric to heliocentric, but this can easily be done by calling
sla_EVP
as follows (array declarations <I>etc</I>. omitted):
<P><PRE>
:
* Star vector, J2000
CALL sla_DCS2C(RM,DM,V)
* Earth/Sun velocity and position, J2000
CALL sla_EVP(TDB,2000D0,DVB,DPB,DVH,DPH)
* Radial velocity correction due to Earth orbit (km/s)
VCORB = -sla_DVDV(V,DVH)*149.597870D6
:
</PRE>
<P>
The maximum value of this correction is the Earth's orbital speed
of about 30 km/s. A related routine,
sla_ECOR,
computes the light-time correction with respect to the Sun. It
would be used when reducing observations of a rapid variable-star
for instance.
Note, however, that the accuracy objectives for pulsar work are
beyond the scope of these SLALIB routines, and even the superior
sla_EVP
routine is unsuitable for arrival-time calculations of better
than 25 millisecond accuracy.
<P>
To remove the intrinsic <IMG WIDTH="36" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img324.gif"
ALT="$\sim20$"> km/s motion of the Sun relative
to other stars in the solar neighbourhood,
a velocity correction to a
<I>local standard of rest</I> (LSR) is required. There are
opportunities for mistakes here. There are two sorts of LSR,
<I>dynamical</I> and <I>kinematical</I>, and
multiple definitions exist for the latter. The
dynamical LSR is a point near the Sun which is in a circular
orbit around the Galactic centre; the Sun has a ``peculiar''
motion relative to the dynamical LSR. A kinematical LSR is
the mean standard of rest of specified star catalogues or stellar
populations, and its precise definition depends on which
catalogues or populations were used and how the analysis was
carried out. The Sun's motion with respect to a kinematical
LSR is called the ``standard'' solar motion. Radial
velocity corrections to the dynamical LSR are produced by the routine
sla_RVLSRD
and to the adopted kinematical LSR by
sla_RVLSRK.
See the individual specifications for these routines for the
precise definition of the LSR in each case.
<P>
For extragalactic sources, the centre of the Galaxy can be used as
a standard of rest. The radial velocity correction from the
dynamical LSR to the Galactic centre can be obtained by calling
sla_RVGALC.
Its maximum value is 220 km/s.
<P>
For very distant sources it is appropriate to work relative
to the mean motion of the Local Group. The routine for
computing the radial velocity correction in this case is
sla_RVLG.
Note that in this case the correction is with respect to the
dynamical LSR, not the Galactic centre as might be expected.
This conforms to the IAU definition, and confers immunity from
revisions of the Galactic rotation speed.
<P>
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<ADDRESS>
<I>SLALIB --- Positional Astronomy Library<BR>Starlink User Note 67<BR>P. T. Wallace<BR>12 October 1999<BR>E-mail:ptw@star.rl.ac.uk</I>
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