FERMI@Elettra is a Free Electron Laser (FEL) user facility currently under construction at Sincrotrone Trieste in Italy. It will provide a spatially coherent and transform-limited photon beam in the sub-ps regime, covering the VUV/Soft X-ray range (from 100 down to 1.33 nm). Thanks to its high fluence this 4th generation light source will be able to create and probe warm dense matter (WDM) inside the TIMEX end-station. Since the WDM state has a short lifetime (a few ps), measurement of basic physical quantities, such as temperature and density, is a challenge and new approaches are needed. For this reason a new method has been proposed for measuring temperature using a slowly responding pyrometric probe (Principi et al., 2010 [1]). However, the technique does require the spatial photon beam profile to be properly shaped at the sample. This can be done using an active optic (i.e. a deformable plane mirror) placed before the elliptical focusing mirror. Ray-tracing simulations and metrology measurements on a prototype have been performed and the results are presented here. (C) 2010 Elsevier B.V. All rights reserved.