UV photometry for EXT8 (left-hand panels) and artificial star experiments (right-hand panels). Isochrones are shown for [Fe/H] = −3.2 and helium abundances and ages as indicated in the legend. The red symbols are sources that do not have a match in the input artificial star catalogue, most of which are spurious. The detection completeness is indicated by the coloured circles. Error bars are shown for stars with typical HB colours (−2 < M_F300X − M_F606W < +1).