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<H3><A NAME="SECTION000513200000000000000">
Efficiency considerations</A>
</H3>
The complete apparent place to observed place transformation
can be carried out by calling
sla_AOP.
For improved efficiency
in cases of more than one star or a sequence of times, the
target-independent calculations can be done once by
calling
sla_AOPPA,
the time can be updated by calling
sla_AOPPAT,
and
sla_AOPQK
can then be used to perform the
apparent-to-observed transformation. The reverse transformation
is available through
sla_OAP
and
sla_OAPQK.
(<I>n.b.</I> These routines use accurate but computationally-expensive
refraction algorithms for zenith distances beyond about <IMG WIDTH="26" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img33.gif"
ALT="$76^\circ$">.For many purposes, in-line code tailored to the accuracy requirements
of the application will be preferable, for example ignoring UT1-UTC,
omitting diurnal aberration and using
sla_REFZ
to apply the refraction.)
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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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