Efficiency considerations

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. (n.b. These routines use accurate but computationally-expensive refraction algorithms for zenith distances beyond about $76^\circ$. For many purposes, in-line code tailored to the accuracy requirements of the application will be preferable, for example ignoring polar motion, omitting diurnal aberration and using sla_REFZ to apply the refraction.)