I built a low-finesse confocal cavity with aluminum mirrors for use in the imaging of ultra-cold calcium ion plasmas. The confocal cavity provides stability at large mirror separations, a condition necessary for achieving both low finesse and narrow peak widths. Additionally, the finesse in the case of coherent input is effectively doubled compared to multi-mode resonance. The cavity was tested using a GaN diode laser input at 397nm. The transmission obtained indicated that the finesse of the cavity was slightly lower than the prediction based on measurements of the mirror reflectivities and the assumption of incoherent input.