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We probe the optical mode structure of a two-dimensional triangular photonic crystal using a near-field scanning optical microscope (NSOM). Light from the subwavelength NSOM aperture was coupled through the nanochannel glass (NCG) array photonic crystal and collected in the far-field using a microscope objective. The optical intensity variations as a function of positions in a unit cell were recorded as the sample was moved with respect to the NSOM aperture. We varied the excitation frequency of the NSOM light and the collection objective numerical aperture (NA), in order to distinguish the relation between reciprocal lattice wave-vectors and the transverse momenta of the photons responsible for the NSOM image.