My approach here  is essentially to provide a chronological catalogue of cosmic X-ray satellite missions - especially if the evolution of our understanding of the X-ray sky is summarised alongside - narrating how an initial 'fuzzy' glimpse through a new window into the universe has become ever sharper and more complete. 

I also thought it would be important to explain how the satellite instruments would not have been possible withouth the technical and observational outputs of the UK/US/other nations rocket programmes as well as the cadre of young capable researchers who emerged as a result. Thanks to the  recent  Webinar video  provided  by  Prof.Dr. G.Branduardi-Raymont my colleague and friend from UCL-MSSLUK to give  the readers   a vivid explanation preseneted by the pioneers of the work done in this era. I shared  its link in its  appropriate  part of the book.

It would be very good to show how X-ray observations have both complemented observations performed in other  wavelenghts  and  resolved certain issues. However  this would have required a very big effort and this book would be much more complete and beautiful in this sense. I have tried to do it in some parts of the book by giving references for some certain areas of  X-ray studies  but I cannot say that I have completed this important mission here.

I mostly mentioned about the galactic and extra-galactic X-ray astronomy and its  satellites  with a little touch  for  Solar X-ray observations.  I am aware of the significant transfer of technology insight and expertise from the Solar X-ray rocketry and satellite work therefore I opened a small window for the X-rays from the Sun and the Solar System towards the end of the book by presenting some of the Solar X-ray satellites chronologically and  thus partial observational results .  I must point out that there are plenty of articles in the literature besides the Solar Astronomy and  Astrophysics  is not directly related with my area of expertise. I would like to thank  NASA for opening all the precious pictures-photos  obtained from observations  through the various satellites to the public. Without them this book would be a very  deficient one.

It is a pleasure for me to mention that being a part of  MSSL-UCL was my bonus over my life time. I enjoyed very much every second of my presence there  during  my  PhD studies  in the beginning of 1980s . I grew up there and picked up all the useful aspects like doing research in an area which is completely foreign to me in the very beginning of my  X-ray astronomy and astrophysics career.
                                                                                                                                                                                            Prof.Dr. E.Nihal Ercan
Boğaziçi University Physics Department