The PSF model also shows a relationship between the FWHM of the surface deflection and the distance h mat -z 0 of the absorber from the surface. The further the absorber is from the surface the wider the FWHM (see Figure ). This behaviour can be explained by looking at the z 0 dependence of A(β m , η n ). For a spherical absorber this (see equation 10). This expression will always be positive for low z 0 but for z 0 ≈ h mat will be positive for even n be positive and for odd n be negative. As the mode shape in r ∝ J 0 (β m r) this means that for absorbers close to the surface the wider lower order modes in m for n = 0 are counteracted by negative contributions from n > 0.