  We present phase resolved optical spectroscopy and photometry of V4580
Sagittarii, the optical counterpart to the accretion powered millisecond pulsar
SAX J1808.4-3658, obtained during the 2008 September/October outburst. Doppler
tomography of the N III 4640.64 Bowen blend emission line reveals a focused
spot of emission at a location consistent with the secondary star. The velocity
of this emission occurs at 324 +/- 15 km/s; applying a "K-correction", we find
the velocity of the secondary star projected onto the line of sight to be 370
+/- 40 km/s. Based on existing pulse timing measurements, this constrains the
mass ratio of the system to be 0.044^{+0.005}_{-0.004}, and the mass function
for the pulsar to be 0.44^{+0.16}_{-0.13} Msun. Combining this mass function
with various inclination estimates from other authors, we find no evidence to
suggest that the neutron star in SAX J1808.4-3658 is more massive than the
canonical value of 1.4 Msun. Our optical light curves exhibit a possible
superhump modulation, expected for a system with such a low mass ratio. The
equivalent width of the Ca II H and K interstellar absorption lines suggest
that the distance to the source is ~2.5 kpc. This is consistent with previous
distance estimates based on type-I X-ray bursts which assume cosmic abundances
of hydrogen, but lower than more recent estimates which assume helium-rich
bursts.
