We report on the implementation of a versatile dynamic mode apertureless scanning near field optical microscope (aSNOM) for nanoscopic investigations of optical properties at surfaces and interfaces. The newly developed modular aSNOM optomechanical unit is essentially integrable with a multitude of laser sources, homemade scanning probe microscopes (SPMs) as well as commercially available SPMs as demonstrated here. The instrument is especially designed to image opaque surfaces without a restriction to transparent substrates. In the description of the instrument we draw frequent attention to various possible artifact mechanisms, how to overcome them, and we present effective checks to ensure true near field optical contrast. Lateral optical contrast in optical amplitude and phase images below 10 nm is demonstrated. (c) 2006 American Institute of Physics.