THE FAINTEST STARS - FROM SCHMIDT PLATES TO LUMINOSITY FUNCTIONS

We describe the construction of photometric catalog from scans of IIIaF and IVN plate material in 11 fields of the UKSRC and POSSII surveys. The procedures used and quality checks applied are described in detail, and should be considered as illustrative for those planning scientific programs with the forthcoming scans of these surveys. We find our plate material is complete to I almost-equal-to 18 and R almost-equal-to 20.5 with photometric uncertainties of +/- 0.20 and +/- 0.25 magnitudes (respectively) at those limits. These data are used to construct luminosity functions for stars within 150 pc of the Sun in four distinct directions. We find no significant evidence for variations in the form of the luminosity function in different locations within the Galactic disk. Approximately 10%-20% variations are seen in the normalization of the luminosity function. We find that we reproduce the steeply increasing luminosity function seen by previous studies for M(Bol) > 13, however we show that this increase is completely spurious. It is an artifact introduced by: (1) the small number of real stars detected in our survey at these luminosities, combined with the large number of spurious sources; and, (2) the fact that optical colors are a poor luminosity estimator for very faint stars. In light of this we show how our catalogs can be used to construct samples of very low mass (M less than or similar to 0.2 M.) stars for further observation in the infrared.