The first steps are reported towards providing a flux library for spectroscopic observations of old stellar systems in the mid-ultraviolet, 2280 - 3120A, based on calculations from first principles. Model atmospheres and atomic and molecular line lists were input into the Kurucz program SYNTHE to generate mid-UV spectra of ten main-sequence turnoff stars spanning a metallicity range from 1/200 solar to twice solar. The calculated spectra were compared to spectra obtained with the Space Telescope Imaging Spectrograph (STIS), and line parameters (gf-values and occasionally damping constants) were adjusted to produce agreement. In the process, temperatures, iron abundances, and the relative abundances of key light elements such as magnesium were rederived from optical spectra. At metallicities higher than 1/30 solar, lines were added manually to reproduce features otherwise missing from the line list, and a chromosphere-like temperature rise near the surface of the model atmosphere was introduced to reduce discrepancies in the cores of strong lines and between strong and weak features of the OH molecule.

The calculations agree very well with observations at the metal-poor end, provided models are chosen in which convective overshoot has been turned off. The metal-poor spectra enable constraints to be placed on both the oxygen abundance and the abundances of several rare-earth elements. The agreement deteriorates toward the metal-rich end in several spectral regions where missing lines still dominate. The prognosis for improvements at and above solar metallicity will be discussed.