In order to separate the fundamental synchrotron radiation from the high harmonics emitted by an undulator, a low photon energy-pass filter has been designed and built, ensuring a high spectral purity on the vacuum ultraviolet (VUV) SU5 beamline at Super-AGO. It consists bf an absorption cell filled with rare gases and separated from the ultrahigh vacuum of the storage ring and of the beamline by a double differential pumping obtained with thin capillaries. Its conception has been optimized by numerical computation of pumping speed. Admission pressures in the range of 100 Pa in the central part of the filter have been used without any degradation of the upstream or downstream ultrahigh vacuum. The measured attenuation factors above the energy cutoff are above 10(5) and 10(2) (and certainly above 10(3) with ultimate pressure of Ne) for argon and neon absorbing gases, respectively, with no measurable attenuation of fundamental radiation. A sophisticated numerical simulation of the pressure distribution, taking into account the geometry of the whole absorption cell including the first pair of capillaries, has been developed. The corresponding calculated attenuation factors are in very good agreement with the measurements, and thus allow reliable predictions of the expected attenuation factors for any given configuration of the filter.