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<H2><A NAME="SECTION0004177000000000000000">SLA_V2TP - Direction Cosines to Tangent Plane</A>
<A NAME="xref_SLA_V2TP"> </A><A NAME="SLA_V2TP"> </A>
</H2>
<DL>
<DT><STRONG>ACTION:</STRONG>
<DD>Given the direction cosines of a star and of the tangent point,
determine the star's tangent-plane coordinates
(single precision).
<P> <DT><STRONG>CALL:</STRONG>
<DD><TT>CALL sla_V2TP (V, V0, XI, ETA, J)</TT>
<P> </DL>
<P> <DL>
<DT><STRONG>GIVEN:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>V</EM></TD>
<TH ALIGN="LEFT"><B>R(3)</B></TH>
<TD ALIGN="LEFT" NOWRAP>direction cosines of star</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>V0</EM></TD>
<TD ALIGN="LEFT"><B>R(3)</B></TD>
<TD ALIGN="LEFT" NOWRAP>direction cosines of tangent point</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>RETURNED:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>XI,ETA</EM></TD>
<TH ALIGN="LEFT"><B>R</B></TH>
<TD ALIGN="LEFT" NOWRAP>tangent plane coordinates (radians)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>J</EM></TD>
<TD ALIGN="LEFT"><B>I</B></TD>
<TD ALIGN="LEFT" NOWRAP>status:</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> 0 = OK, star on tangent plane</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> 1 = error, star too far from axis</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> 2 = error, antistar on tangent plane</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> 3 = error, antistar too far from axis</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>NOTES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>If vector V0 is not of unit length, or if vector V is of zero
length, the results will be wrong.
<DT>2.
<DD>If V0 points at a pole, the returned <IMG WIDTH="26" HEIGHT="27" ALIGN="MIDDLE" BORDER="0"
SRC="img90.gif"
ALT="$\xi,\eta$"> will be based on the
arbitrary assumption that <IMG WIDTH="43" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img89.gif"
ALT="$\alpha=0$"> at the tangent point.
<DT>3.
<DD>The projection is called the <I>gnomonic</I> projection; the
Cartesian coordinates <IMG WIDTH="41" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img85.gif"
ALT="$[\,\xi,\eta\,]$"> are called
<I>standard coordinates.</I> The latter
are in units of the distance from the tangent plane to the projection
point, <I>i.e.</I> radians near the origin.
<DT>4.
<DD>This routine is the Cartesian equivalent of the routine sla_S2TP.
</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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