  Deep fields are unique probes of the Galactic halo. With a limiting magnitude
of, e.g., I = 30 mag, all stars down to the hydrogen-burning limit are detected
out to distances of 10 kpc, while stars with 0.5 solar masses could be traced
out to distances of 400 kpc. Thus deep fields provide an opportunity to study
both the structure of the Galactic halo and the mass function of population
(Pop) II and Pop III stars. I summarize previous work on the Galactic stellar
halo utilizing the Hubble Deep Fields North and South, supplemented by a
preliminary analysis of the recently released Hubble Ultra-Deep Field.

Astrophysics