Radial Velocity and Light-Time Corrections

From d54fe7c1f704a63824c5bfa0ece65245572e9b27 Mon Sep 17 00:00:00 2001 From: Joseph Hunkeler Date: Wed, 4 Mar 2015 21:21:30 -0500 Subject: Initial commit --- src/slalib/sun67.htx/node225.html | 143 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 143 insertions(+) create mode 100644 src/slalib/sun67.htx/node225.html (limited to 'src/slalib/sun67.htx/node225.html') diff --git a/src/slalib/sun67.htx/node225.html b/src/slalib/sun67.htx/node225.html new file mode 100644 index 0000000..ce716c7 --- /dev/null +++ b/src/slalib/sun67.htx/node225.html @@ -0,0 +1,143 @@ + + + + +Radial Velocity and Light-Time Corrections + + + + + + + + + + + + +

+ +

+
+ Next: Focal-Plane Astrometry +
+Up: EXPLANATION AND EXAMPLES +
+ Previous: Ephemerides +

+Radial Velocity and Light-Time Corrections +

+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. +

+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. +

+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 etc. omitted): +

+             :
+      *  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
+             :
+

+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. +

+To remove the intrinsic $\sim20$ km/s motion of the Sun relative +to other stars in the solar neighbourhood, +a velocity correction to a +local standard of rest (LSR) is required. There are +opportunities for mistakes here. There are two sorts of LSR, +dynamical and kinematical, 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. +

+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. +

+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. +

+ +

+
+ Next: Focal-Plane Astrometry +
+Up: EXPLANATION AND EXAMPLES +
+ Previous: Ephemerides +

+SLALIB --- Positional Astronomy Library
Starlink User Note 67
P. T. Wallace
12 October 1999
E-mail:ptw@star.rl.ac.uk +

+ + -- cgit