The restframe mid-ultraviolet spectral region ( 2000–3200 Å ) is important in analyzing the stellar populations of the “ red envelope ” systems observed at high redshifts .
Here , we explore the usefulness of the mid-UV spectral region for determining ages and abundances of old populations .
We work with a theoretical set of low resolution spectra and broad-band colors because tests show that these are presently more realistic than high resolution models .

A mid-UV to optical/IR wavelength baseline provides good separation of population components because the main sequence turnoff dominates the integrated light between 2500 and 4000 Å. Mid-UV spectral features are not sensitive to the dwarf/giant mixture in the population , unlike those in the optical region .
We find a six magnitude difference in the mid-UV continuum level ( normalized at V ) over the metallicity range -1.5 < \log Z / Z _ { \odot } < +0.5 and a comparable difference ( per unit \log t ) for ages in the range 4-16 Gyr .
Logarithmic derivatives of mid-UV colors with respect to age or metal abundance are 3-10 times larger than for the UBV region .
Most of the spectral information on old populations therefore resides below 4000 Å .
Measurement of a single mid-UV color is capable of placing a strong lower bound on the mean metallicity of an old population .
We investigate the capability of UBV and mid-UV broad-band colors to separately determine age and abundance , taking into account precision in the color measurements .
We find that the mid-UV improves resolution in \log t , \log Z space by about a factor of 3 for a given observational precision .

Contamination by hot , post-He-flash evolutionary phases can seriously affect the mid-UV spectra of old populations .
A simple estimate shows that contamination can reach over 80 % in some cases .
However , this is straightforward to remove as long as far-UV measurements are available .
We find that extinction should have relatively small effects on parameters derived for old populations from the mid-UV .

Finally , we show that a 4 Gyr , solar abundance model based on empirical spectra for nearby stars provides an excellent fit to the mid-UV spectrum of the Local Group elliptical galaxy M32 .
This indicates that the poorer results obtained from theoretical spectra do arise from limitations of the synthesis models for individual stars .