A rotating support for a large astronomical mirror has been implemented using an opposed surface planar orifice air bearing. This bearing needs to provide rotation as free as possible of static and dynamic friction, as well as maximizing the resistance to sudden angular deviations produced by wind gusts. Information on the design of traditional bearings, namely, a cavity supplied with air under pressure through an orifice or diffuser flow restriction and closed by the moving member, has been published in detail. These traditional bearings are not suitable for the application described, due to their slow speed of response to transient changes in forces, a considerable volume of air being needed to change the pressure in the cavity. The air bearing used in this application has no cavity but consists of two flat surfaces in close proximity with air under pressure introduced at the center of one of the surfaces. The volume of air in the bearing is therefore minimized, improving the response to load change transients. The load capacity of this type bearing is reduced as the air between the bearing surfaces is not at constant pressure, but is expanding from the center point of injection to its escape at the edges. This paper indicates a quantitative method of determining the size of the air supply orifices to achieve the maximum rigidity orthogonal to the direction of motion. The approach described can also be applied to optimizing the geometry of other non-cavity air bearings.