  We describe the design, construction, and characterization of a
medium-finesse Fabry-P\'erot cavity for simultaneous frequency stabilization of
two lasers operating at 960 and 780 nm wavelengths, respectively. The lasers
are applied in experiments with ultracold rubidium Rydberg atoms, for which a
combined laser linewidth similar to the natural Rydberg linewidth
(approximately 10 kHz) is desired. The cavity, with a finesse of approximately
1500, is used to reduce the linewidth of the lasers to below this level. By
using a spacer made of ultra low expansion (ULE) glass with active temperature
stabilization, the residual frequency drift is limited to 1 MHz/day. The design
optimizes for ease of construction, robustness, and affordability.
