  The Ultraviolet (UV) continuum slope beta, typically observed at z=7 in
Hubble Space Telescope (HST) WFC3/IR bands via the J-H colour, is a useful
indicator of the age, metallicity, and dust content of high-redshift stellar
populations. Recent studies have shown that the redward evolution of beta with
cosmic time from redshift 7 to 4 can be largely explained by a build up of
dust. However, initial claims that faint z=7 galaxies in the Hubble Ultra Deep
Field WFC3/IR imaging (HUDF09) were blue enough to require stellar populations
of zero reddening, low metallicity and young ages, hitherto unseen in
star-forming galaxies, have since been refuted and revised. Here we revisit the
question of how best to measure the UV slope of z=7 galaxies through source
recovery simulations, within the context of present and future ultra-deep
imaging from HST. We consider how source detection, selection and colour
measurement have each biased the measurement of beta in previous studies. After
finding a robust method for measuring beta in the simulations (via a power law
fit to all the available photometry), we remeasure the UV slopes of a sample of
previously published low luminosity z=7 galaxy candidates. The mean UV slope of
faint galaxies in this sample appears consistent with an intrinsic distribution
of normal star-forming galaxies with beta=-2, although properly decoding the
underlying distribution will require further imaging from the ongoing HUDF12
programme. We therefore go on to consider strategies for obtaining better
constraints on the underlying distribution of UV slopes at z=7 from these new
data, which will benefit particularly from the addition of imaging in a second
J-band filter: F140W. We find that a precise and unbiased measurement of beta
should then be possible.
