• J.J. Welch, A. Brachmann, F.-J. Decker, Y.T. Ding, P. Emma, A.S. Fisher, J.C. Frisch, Z. Huang, R.H. Iverson, H. Loos, H.-D. Nuhn, P. Stefan, J.L. Turner, J. Wu, D. Xiang
  SLAC, Menlo Park, California
• R.M. Bionta
  LLNL, Livermore, California
• D.F. Ratner
  Stanford University, Stanford, California
• H. Sinn
  European X-ray Free Electron Laser Project Team, c/o DESY, Hamburg

Precision in-situ measurements of relative undulator segment K parameters were made at the LCLS and are reported here. We describe the methods used, systematics errors, and signal levels. A method for determining the central ray from each undulator segment was developed to control the effect of angle-energy correlation of the spontaneous radiation on the photon energy spectrum. A variety of photon-energy sensitive detectors were employed, including: Ni foil, the yttrium component in a YAG screen, and a narrow band monochromator followed by either a photodiode or a YAG screen. Different harmonics of the spontaneous radiation were also used.

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