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<H2><A NAME="SECTION000491000000000000000">SLA_FK425 - FK4 to FK5</A>
<A NAME="xref_SLA_FK425"> </A><A NAME="SLA_FK425"> </A>
</H2>
<DL>
<DT><STRONG>ACTION:</STRONG>
<DD>Convert B1950.0 FK4 star data to J2000.0 FK5.
This routine converts stars from the old, Bessel-Newcomb, FK4
system to the new, IAU 1976, FK5, Fricke system. The precepts
of Smith <I>et al.</I> (see reference 1) are followed,
using the implementation
by Yallop <I>et al.</I> (reference 2) of a matrix method
due to Standish.
Kinoshita's development of Andoyer's post-Newcomb precession is
used. The numerical constants from
Seidelmann <I>et al.</I> (reference 3) are used canonically.
<P> <DT><STRONG>CALL:</STRONG>
<DD><TT>CALL sla_FK425 (
R1950,D1950,DR1950,DD1950,P1950,V1950,
R2000,D2000,DR2000,DD2000,P2000,V2000)</TT>
<P> </DL>
<P> <DL>
<DT><STRONG>GIVEN:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>R1950</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP>B1950.0 <IMG WIDTH="13" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img24.gif"
ALT="$\alpha$"> (radians)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>D1950</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>B1950.0 <IMG WIDTH="10" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img23.gif"
ALT="$\delta$"> (radians)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>DR1950</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>B1950.0 proper motion in <IMG WIDTH="13" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img24.gif"
ALT="$\alpha$">(radians per tropical year)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>DD1950</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>B1950.0 proper motion in <IMG WIDTH="10" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img23.gif"
ALT="$\delta$">(radians per tropical year)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>P1950</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>B1950.0 parallax (arcsec)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>V1950</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>B1950.0 radial velocity (km s<SUP>-1</SUP>, +ve = moving away)</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>RETURNED:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>R2000</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP>J2000.0 <IMG WIDTH="13" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img24.gif"
ALT="$\alpha$"> (radians)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>D2000</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>J2000.0 <IMG WIDTH="10" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img23.gif"
ALT="$\delta$"> (radians)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>DR2000</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>J2000.0 proper motion in <IMG WIDTH="13" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img24.gif"
ALT="$\alpha$">(radians per Julian year)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>DD2000</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>J2000.0 proper motion in <IMG WIDTH="10" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img23.gif"
ALT="$\delta$">(radians per Julian year)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>P2000</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>J2000.0 parallax (arcsec)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>V2000</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>J2000.0 radial velocity (km s<SUP>-1</SUP>, +ve = moving away)</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>NOTES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>The <IMG WIDTH="13" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img24.gif"
ALT="$\alpha$"> proper motions are <IMG WIDTH="13" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img115.gif"
ALT="$\dot{\alpha}$"> rather than
<IMG WIDTH="48" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img116.gif"
ALT="$\dot{\alpha}\cos\delta$">, and are per year rather than per century.
<DT>2.
<DD>Conversion from Besselian epoch 1950.0 to Julian epoch
2000.0 only is provided for. Conversions involving other
epochs will require use of the appropriate precession,
proper motion, and E-terms routines before and/or after FK425
is called.
<DT>3.
<DD>In the FK4 catalogue the proper motions of stars within
<IMG WIDTH="26" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img117.gif"
ALT="$10^{\circ}$"> of the poles do not include the <I>differential
E-terms</I> effect and should, strictly speaking, be handled
in a different manner from stars outside these regions.
However, given the general lack of homogeneity of the star
data available for routine astrometry, the difficulties of
handling positions that may have been determined from
astrometric fields spanning the polar and non-polar regions,
the likelihood that the differential E-terms effect was not
taken into account when allowing for proper motion in past
astrometry, and the undesirability of a discontinuity in
the algorithm, the decision has been made in this routine to
include the effect of differential E-terms on the proper
motions for all stars, whether polar or not. At epoch J2000,
and measuring on the sky rather than in terms of <IMG WIDTH="27" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img118.gif"
ALT="$\Delta\alpha$">, the errors resulting from this simplification are less than
1 milliarcsecond in position and 1 milliarcsecond per
century in proper motion.
<DT>4.
<DD>See also sla_FK45Z, sla_FK524, sla_FK54Z.
</DL></DL>
<P> <DL>
<DT><STRONG>REFERENCES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>Smith, C.A. <I>et al.</I>, 1989. <I>Astr.J.</I> <B>97</B>, 265.
<DT>2.
<DD>Yallop, B.D. <I>et al.</I>, 1989. <I>Astr.J.</I> <B>97</B>, 274.
<DT>3.
<DD>Seidelmann, P.K. (ed), 1992. <I>Explanatory
Supplement to the Astronomical Almanac,</I> ISBN 0-935702-68-7.
</DL></DL>
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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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