Presentations at the SPIE International Symposium on Optics & Photonics, the Conference Optics for EUV, X-Ray, and Gamma-Ray Astronomy II (O. Citterio and S.L. O’Dell, Proceedings of SPIE Vol. #5900), August 3 (San Diego, CA US)
Off-plane grazing-incidence Constellation-X grating calibrations using polarized synchrotron radiation and PCGRATE code calculations
J. F. Seely, L. I. Goray, M. Laming, B. Kjornrattanawanich, K. A. Flanagan, R. K. Heilmann, A. P. Rasmussen, C. Chang, and M. L. Schattenburg
Efficiency measurements of a grazing-incidence diffraction grating, planned for the Constellation-X Reflection Grating Spectrometer (RGS), were performed using polarized synchrotron radiation at the NRL Brookhaven beamline X24C. The off-plane TM and TE efficiencies of the 5000 groove/mm MIT test grating, patterned on a silicon wafer, were measured and compared to the efficiencies calculated using the PCGRATE-SX code. The calculated and measured efficiencies are in agreement when using groove profiles derived from AFM measurements. The TM and TE efficiencies differ, offering the possibility of performing unique astrophysical science studies by exploiting the polarization sensitivity of the off-plane gratings. The grating calibrations demonstrate the importance of using polarized synchrotron radiation and code calculations for the understanding of the Constellation-X grating performance, in particular the effects of the groove profile and microroughness on the efficiency. The optimization of grazing incidence gratings, for both the off-plane and in-plane mounts, planned for the RGS and x-ray spectrometers on other missions will require detailed synchrotron measurements and code calculations.
© 2005 SPIE
Wavelength separation of plus and minus orders of soft-x-ray-EUV multilayer-coated gratings at near-normal incidence
L. I. Goray and J. F. Seely
The validity of approximating the efficiency of a multilayer grating operating at close to normal incidence in the soft-X-ray-EUV range with a product of the relative grating efficiency by the reflectance of its multilayer coating has been studied by the rigorous integral method. The widely used approximated approach has until recently been considered accurate enough for analysis of short-wavelength normal-incidence multilayer-coated gratings. Real gratings employed in the soft-X-ray-EUV range are used to demonstrate the inapplicability of this approximation to an analysis of precise positions of efficiency maxima for the external (n > 0) and internal (n < 0) diffraction orders, despite the small ratios of wavelength and groove depth to period. The present authors have performed an analysis of the accuracy inherent in a derived simple expression for spectral separation of the same plus and minus orders with respect to the wavelength, order’s number, incident angle, period, and groove depth. The reason for the observed substantial (a few Angstrom or even nm) wavelength separation between the maxima of positive and negative orders is related to oblique, close-to-normal incidence of radiation on a grating operating in the short-wavelength spectral region and different angles of deviation of respective orders. The modeling carried out with the commercial code PCGrate-S(X) v.6.1 permitted not only prediction of the separation between positive and negative orders for a multilayer Mo/Si 4200-gr/mm grating with AFM-measured trapezoidal groove profile, which is designed for operation in the EIS spectrometer on the Solar-B spacecraft, but obtaining a good agreement with synchrotron radiation measurements, including high orders as well. A conclusion is drawn that high-precision calculations of the efficiency of multilayer normal-incidence soft-X-ray-EUV range gratings have in some cases to be performed, although this may require increasing the computation time by several times compared to the commonly used approximate approach.
© 2005 SPIE