  Optical coatings formed from amorphous oxide thin films have many
applications in precision measurements. The Advanced Laser Interferometer
Gravitational-Wave Observatory (LIGO) and Advanced Virgo use coatings of
SiO$_2$ (silica) and TiO$_2$:Ta$_2$O$_5$ (titania doped tantala) and
post-deposition annealing to 500$^\circ$C to achieve low thermal noise and low
optical absorption. Optical scattering by these coatings is a key limit to the
detectors' sensitivity. This paper describes optical scattering measurements
for single-layer ion-beam-sputtered thin films on fused silica substrates: two
samples of Ta$_2$O$_5$ and two of TiO$_2$:Ta$_2$O$_5$. Using an imaging
scatterometer at a fixed scattering angle of 12.8$^\circ$, in-situ changes in
the optical scatter of each sample were assessed during post-deposition
annealing to 500$^\circ$C in vacuum. The scatter of three of the four coated
optics was observed to decrease during the annealing process, by 25-30$\%$ for
tantala and up to 74$\%$ for titania-doped tantala, while scatter from the
fourth sample held constant. Angle-resolved scatter measurements performed
before and after vacuum annealing suggests some improvement in three of four
samples. These results demonstrate that post-deposition high-temperature
annealing of single-layer tantala and titania-doped tantala thin films in
vacuum does not lead to an increase in scatter, and may actually improve their
scatter.
