A Catalogue of Locus Algorithm Pointings for Optimal Differential
  Photometry for 23,779 Quasars
  This paper presents a catalogue of optimised pointings for differential
photometry of 23,779 quasars extracted from the Sloan Digital Sky Survey (SDSS)
Catalogue and a score for each indicating the quality of the Field of View
(FoV) associated with that pointing. Observation of millimagnitude variability
on a timescale of minutes typically requires differential observations with
reference to an ensemble of reference stars. For optimal performance, these
reference stars should have similar colour and magnitude to the target quasar.
In addition, the greatest quantity and quality of suitable reference stars may
be found by using a telescope pointing which offsets the target object from the
centre of the field of view. By comparing each quasar with the stars which
appear close to it on the sky in the SDSS Catalogue, an optimum pointing can be
calculated, and a figure of merit, referred to as the "score" calculated for
that pointing. Highly flexible software has been developed to enable this
process to be automated and implemented in a distributed computing paradigm,
which enables the creation of catalogues of pointings given a set of input
targets. Applying this technique to a sample of 40,000 targets from the 4th
SDSS quasar catalogue resulted in the production of pointings and scores for
23,779 quasars. This catalogue is a useful resource for observers planning
differential photometry studies and surveys of quasars to select those which
have many suitable celestial neighbours for differential photometry
