Finally, we note that the 3D-printed plastic mounts used here are susceptible to a small degree of mechanical creep, leading to a gradual loss of alignment and measurement accuracy. To achieve the 3-millidegree accuracy reported above, careful instrumental alignment was required shortly before use. In everyday use, with minor adjustments only before starting measurements (see guidance notes in the Supporting Information), a 3σ accuracy of 5 -15 millidegrees can be expected. For those requiring the best possible performance from the set-up reported here, we recommend the optical mounts are fabricated in metal using a 3D-printing service, although this obviously comes at a substantial cost premium. The next version of Opol will include an option to use machined metal parts that can be readily milled on a benchtop milling machine.