THz band-pass filters were fabricated by femtosecond-laser ablation of 25- μ m-thick micro-foils of stainless steel and Kapton film, which were subsequently metal coated with a ∼70 nm film, closely matching the skin depth at the used THz spectral window. Their spectral performance was tested in transmission and reflection modes at the Australian Synchrotron’s THz beamline. A 25- μ m-thick Kapton film performed as a Fabry–Pérot etalon with a free spectral range ( F S R ) of 119 cm − 1 , high finesse  F c ≈ 17 , and was tuneable over ∼ 10 μ m (at ∼5 THz band) with  β = 30 ∘  tilt. The structure of the THz beam focal region as extracted by the first mirror (slit) showed a complex dependence of polarisation, wavelength and position across the beam. This is important for polarisation-sensitive measurements (in both transmission and reflection) and requires normalisation at each orientation of linear polarisation.