This thesis describes the pre-flight preparation and analysis of results from two satellites containing experiments for cosmic X-ray Astronomy. The Copernicus satellite could be pointed to within thirty arc seconds with high spacial resolution, and was consequently used to map three extended extragalactic X-ray sources in clusters of galaxies. The previous observations of all three clusters are reviewed, together with the possible X-ray emission mechanisms capable of producing such extended sources. The Copernicus observations showed that all three extended X-ray regions in the Coma, Perseus and Virgo clusters were non uniform, and indeed very complex, being probably each centred on an active galaxy. Pre-flight preparation for two experiments on Ariel 5; a Spectrometer/Polarimeter, and a Sky Survey scanning experiment, are described in detail. Preliminary results on Cygnus X-3 from both experiments are presented in Appendix III, which indicate the source was in a high intensity low temperature state at the time of observation by Ariel-5.