FastCam is an instrument jointly developed by the Instituto de Astrofísica de Canarias (IAC) and the Universidad Politécnica de Cartagena (UPCT), designed to obtain high spatial resolution images in the optical wavelength range from ground-based telescopes (http://www.iac.es/proyecto/fastcam and http://www.iac.es/telescopes/Manuales/manualfastcam.pdf). The instrument is equipped with a very low noise and very fast readout speed EMCCD camera which provides short exposure images to an FPGA-based processor which performs the selection, recenterg and combination of images in real-time (applying Lucky Imaging techniques) to provide diffraction limited resolution images in 1-4 m class telescopes from 500 to 1100 nm. IDOM has contributed to this new state-of-the-art instrument with the design of an optomechanical system conceived to maximize the image scale stability of the system for astrometry. The combination of aluminum plates, carbon fiber (CFRP) rods and stainless steel mounts in the optical bench defines an athermalized and stiff design to meet the requirements of thermal and mechanical stability. This work has been done with the support of the Aerospace Subprogramme of the Spanish Centre for the Development of Industrial Technology (CDTI) and the INTEK programme of the Basque Development Agency (SPRI).