Context. The orbital angular momentum (OAM) of the photon is a property of light from astronomical objects that has not yet been measured. We propose a method of measuring the OAM spectrum of light from an extended natural source, the Sun. Relating the OAM spectrum of different solar areas to its structures could lead to a novel remote sensing technique. Aims: We present a method for measuring the OAM spectrum of solar photons. Methods: The THEMIS (Télescope Héliographique pour l'Étude du Magnetisme et les Instabilités Solaires is a 0.9 m solar telescope property of the French CNRS-INSU at the Spanish Observatorio del Teide.) telescope is being used with a novel phase-diversity technique. A spatial light modulator is placed on one pupil image, and an ad-hoc optical setup allows the measurement of two simultaneous phase-diverse images in the same CCD, with equal optical paths. Results: Preliminary results show that very good seeing is mandatory for this kind of observation. The method works in the laboratory, and good seeing conditions in the 2010 campaign are being awaited.