As the performance of space based astrophysics observatories is directly limited by the size of the spacecraft and the telescope it carries, current missions are reaching the limit of the launchers’ capabilities. Before considering to develop larger launchers or to implement formation flying missions or in orbit assembly, the possibility of deploying structures once in orbit is an appealing solution. This paper describes the different technologies currently available to develop deployable structures, with an emphasis on those that can allow achieving long focal lengths. The review of these technologies is followed by a comparison of their performance and a list of trade-off parameters to be considered before selecting the most appropriate solution for a given application. Additionally, a preliminary structural analysis was performed on a typical deployable structure, applied to the case of a mission requiring a 20 m focal length extension. The results show that by using several deployable masts, it is possible to build stiff deployed structures with eigen frequencies over 1 Hz. Finally, a discussion on metrology concepts is provided, as knowledge of the relative position between the telescope and the deployed focal plane instruments is critical.