The fluorescent rod technique is a rather new and sensitive method of experimentally mapping vacuum magnetic surfaces in toroidal confinement devices. It was applied to the modular low shear W7-AS stellarator before starting plasma experiments. Magnetic island widths down to less than 5% of the plasma radius could be recorded, these being caused by relative perturbation field amplitudes of down to 10-5. The almost complete Fourier spectrum of the resonant perturbation field components is derived for values of the rotational transform iota between 1/4 and 2/3. The results obtained on W7-AS demonstrate once more that the existence of magnetic islands is a common phenomenon in toroidal confinement devices, and that the assumption of closed, nested flux surfaces in transport theories may not always be justified. On the basis of the measured perturbation field spectrum, the enhancement of the radial transport caused by the vacuum field islands is estimated with a simple transport model. The predictions of the model are compared with the experimental dependence of the diamagnetic energy content on iota in W7-AS
m = 0 perturbations of the magnetic surfaces in an RFP