S.~Vilcins, D.~Lipka, "Mechanical Design of Cryogenic Vacuum Feedthroughs for X-FEL Button BPMs", in Proc. 3rd International Beam Instrumentation Conference, Monterey, CA, USA, paper TUPF11, pp. 332–336, ISBN: 978-3-95450-141-0, \url{http://jacow.org/IBIC2014/papers/tupf11.pdf}, 2014.