  We use the Pristine survey CaHK narrow-band photometry, combined with the
SDSS ugr photometry, to provide a cleaner sample of blue horizontal branch
stars in the Galactic halo out to large distances. We demonstrate a
completeness of 91% and a purity of 93% with respect to available spectroscopic
classifications. We subsequently use our new clean sample of these standard
candles to investigate the substructure in the Galactic halo over the Pristine
footprint. Among other features, this allows for a careful tracing of multiple
parts of the Sagittarius stream, providing a measurement independent from other
tracers used and reaching larger distances. Moreover, we demonstrate with this
clean and complete sample that the halo follows a density profile with a
negative power-law slope of 3.5 - 4.0. As the relatively shallow SDSS u-band is
the limiting factor in this technique, we foresee large potential for combining
Pristine survey photometry with the much deeper u-band photometry from the
Canada-France-Imaging Survey.
