Design and fabrication of an active polynomial grating for soft-X-ray monochromators and spectrometers

An active polynomial grating has been designed for use in synchrotron radia tion soft-X-ray monochromators and spectrometers. The grating can be dynami cally adjusted to obtain the third-order-polynomial surface needed to elimi nate the defocus and coma aberrations at any photon energy. Ray-tracing res ults confirm that a monochromator or spectrometer based on this active grat ing has nearly no aberration limit to the overall spectral resolution in th e entire soft-X-ray region. The grating substrate is made of a precisely mi lled 17-4 PH stainless steel parallel plate, which is joined to a flexure-h inge bender shaped by wire electrical discharge machining. The substrate is grounded into a concave cylindrical shape with a nominal radius and then p olished to achieve a roughness of 0.45 mn and a slope error of 1.2 mu rad r ms. The long trace profiler measurements show that the active grating can r each the desired third-order polynomial with a high degree of figure accura cy. (C) 2001 Elsevier Science B.V. All rights reserved.