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White dwarf stars are tracers of the age and evolution of the local Galactic disk through the white dwarf luminosity function (LF). The abrupt drop-off in the white dwarf luminosity function at $\log(L/L\sun) \approx -4.5$ has been used to derive the age of the galactic disk and to infer the age of the universe. However, uncertainties in the current LF prevent us from reaping the vast amount of information it contains; we must define it better. To this end we have recently commissioned a 46-arcminute field prime focus CCD imaging camera (PFC) on the McDonald 0.8-m reflector. We have begun a deep, large-area multi-color survey to discover cool white dwarf stars thus better define the cool end of the white dwarf luminosity function.
We have developed a new photometric technique that allows us to separate cool degenerate stars from other field stars having similar surface temperature, minimizing the need for individual spectroscopic followup. Initial selection of white dwarf candidates from $UBVRI$ photometry is followed by a refinement of the selection based on the strengths of the MgH and CaH bands, observed through $\sim400\AA~$ wide filters. These data remove most of the confusion in the colors of metal poor subdwarfs and bonafide degenerate white dwarfs in the K and M spectral range. We present photometry of a representative sample of 60 cool white dwarfs and metal poor subdwarfs demonstrating our technique.
If a separate population of white dwarfs exists in the galactic halo we can use their LF to determine if the halo age differs significantly from the age of the Galactic disk. Because of the limited volume searched by the proper motion surveys at low luminosities, the region in the WDLF fainter M$_{\rm v} \sim 18$ remains unexplored, hence we know very little about the the LF for halo white dwarfs. With the technique presented here we not only expect to find many disk white dwarfs but we will also be sensitive to white dwarfs which might be present in the halo.
