Off-plane grazing incidence Constellation-X grating calibrations using polarized synchrotron radiation and PCGRATE code calculations

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.