A new approach to detecting the spatial and energetic distribution of photodissociation products is demonstrated, in which an inhomogeneous electric field is used for spatially selective field ionization of Rydberg molecules. State-selected NO fragments from NO2 photodissociation in a supersonic beam are excited to high-n Rydberg states and allowed to expand through the inhomogeneous field provided by an octopole. The field in the octopole results in the field ionization of those fragments that have moved to large radial distances (similar to5 mm) from the beam axis, and therefore this device transmits only those Rydberg molecules with a low velocity component in the direction perpendicular to the beam. A detailed characterization of the properties of this "steradiancy analyzer" is carried out and factors limiting the energy resolution (currently approximately 100 cm(-1) at an excess energy of 250 cm(-1)), along with possible improvements, are discussed in detail.