How old is the Galaxy? The White Dwarf Luminosity Function:
The age of the Galaxy's disk and halo can be measured from the shape of the faint end of the white dwarf luminosity function (Log [L/Lsun] < -4.5). Current estimates range from 6 to 12 Gyrs, but these are based on ~5 white dwarfs. To determine the age within 1 Gyr at the 95% confidence level requires ~100 white dwarfs at this faint luminosity, where the density on the sky is 1 per sq. deg., thereby requiring ~50 pointings.
Spectra are required to distinguish the WDs from contaminants, which after photometric sifting, still represent 90% of the objects. There will be about 1000 WD candidates per pointing; all but ~1 of the ~100 true WDs will be relatively luminous ones. Requires spectra of ~50,000 stars down to 25th mag at low dispersion.
This image illustrates the problem of finding the critical white dwarfs. Many objects have colors of white dwarfs, but it takes spectra to distinguish the real ones in an unbiased way.