Abstract: The invention relates to an arrangement for holding an instrument aligned onto a movable reflector, which may have the form of a cube corner prism. The instrument has a transmitter which transmits a slightly divergent beam of electromagnetic radiation onto the reflector, and a receiver which receives the radiation reflected by the reflector. An optical periphery modulator is placed downstream of the transmitter in the proximity of a lens or in a collimated beam path. The modulator modulates differently various peripheral parts of the transmitted beam of radiation, and if the instrument is not correctly aligned with the prism, the modulator introduces a modulation of low frequency of the radiation received by the receiver in a manner such that the phase position of the modulation is dependent on the direction in which the transmitted beam of radiation deviates from optimal alignment with the prism, and such that the amplitude of the modulation signal is dependent on the extent of this deviation.