The IFM is based on a new grazing incidence focusing condition, whose essential features are illustrated below. A reflection grating consists of grooves whose spacings vary systematically along the dispersion plane. Selection of the wavelength transmitted between fixed entrance and exit slits is accomplished by rotation of the grating combined with a simultaneous translation along its surface tangent. Due to the varied spacing, the translation provides a new set of grating parameters where the principal ray strikes the grating surface. This new degree of freedom eliminates defocusing at every wavelength. Second order (coma) aberration is also well corrected by higher order terms in the varied-space function. This yields symmetrical line shapes, and provides a physically compact instrument having a large collection angle.
The optical aberrations of the IFM are typically a factor of 10-100 smaller than a conventional spherical or toroidal grating in a single- element fixed slit mounting. Equivalently, the throughput gain is 10- 100 at that limiting spectral resolution.
The fixed slits permit practical use with immovable sources of light and detection, and hence represent an attractive and cost-effective alternative to the Rowland circle monochromator.
The design philosophy for compact model IFM-SXR-0.5 is maximum throughput at moderate spectral resolution. It employs an axis of rotation fixed in space near the grating pole, and a spherical concave grating surface located approximately midway between the slits. This results in minimal required translation, and a nearly fixed direction for the emerging ray.
The grating mounting plate is lapped optically flat to ensure accurate co-alignment of up to five gratings interchangeable under vacuum. In addition, six discrete slit widths are selectable under vacuum using a feedthrough incorporating a centering adjustment which is factory present for optimal resolution and wavelength calibration. The wavelength scanning is accomplished by means of a double cam residing under vacuum and machined to automatically correlate the combined grating rotation and translation. This is driven by a single externally mounted stepper motor, providing a linear wavelength scale.
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