MULTILAYER-COATED GRATING SPECTROMETER OPERATING IN THE EXTREME-ULTRAVIOLET REGION AND BASED ON THE SEYA-NAMIOKA MOUNT

The performance of a spectrometer that uses a blazed multilayer-coated grating and that is based on the Seya-Namioka mount is analyzed. In the extreme-UV wavelength region, the efficiency of the grating is relatively high in a narrow bandpass that is defined by the reflectance of the multilayer coating. The bandpass can be shifted in wavelength by rotation of the grating, as is the case for the Seya-Namioka mount, for which the angle of incidence on the grating is near 35-degrees. It is shown that when the source is slightly off the Rowland circle, as occurs when the grating is rotated and the source-to-grating distance is fixed, one can maintain good focusing can be maintained by moving the detector over a small range of distance and angle. The multilayer coating can be designed so that the grating remains on-blaze in wavelength regions of interest and in particular orders as the grating is rotated. A spectrometer composed of a multilayer grating and a movable detector can have relatively high throughput and resolving power in the extreme-UV region. When the grating is rotated, the wavelength coverage is wider than is the case for incidence angles near normal.