In some cases a sample has cylindrical symmetry,, such that in reciprocal space the data are circularly
averaged about the long axis of the fiber, or the normal to the substrate of a thin film, also known as the "meridian." The meridian often coincides with a
particular crystallographic direction. This results in a distortion of the diffraction pattern observed on a flat
area detector (or film) because portions of the Ewald sphere are inaccessible. The Fraser Correction
(R. D. B. Fraser, T. P. Macrae, A. Miller, R. J. Rowlands, J. Appl. Cryst. 9, 81 (1976)) maps the observed data onto
a Cartesian grid. To apply this correction, it is essential to start with a good set of calibration parameters
(center, Q-range, etc., all properly set). Datasqueeze will only apply the Fraser correction if data
are calibrated in the small-angle mode. Clicking on "Calculate Fraser (Fiber) Correction" brings up a new window and
three features on the image: a line passing through the origin and two slightly curved lines perpendicular to the central line.
There are two parameters that must be set. The equator direction specifies the orientation of the fiber equator
(perpendicular to the fiber axis/surface normal, or meridian). The angle Beta specifies the amount by which the fiber is
tilted away from a direction normal to the beam. Each of these can be set by typing into the
appropriate boxes in the new window or graphically. To set them graphically, drag the blue dots so that
the two lines forming the sides of the "H" pass through layer lines, as shown to the right. The
orientation of the H establishes the direction of the equator (which could be anything); small differences in the
distances of the two lines establishes the value of Beta (which is usually 20o or less). Clicking
Apply results in a recalculated image, in which the data have been mapped onto a Cartesian grid. (See the pdf manual for typical before-and-after images).
The qx-axis corresponds to
the equatorial direction, the qy axis corresponds to the meridional axis, and there is a dark region along the meridian where the
Ewald sphere is inaccessible. The calculated pattern is symmetric under reflection about a vertical axis (qx and -qx are exactly equivalent)
but the intensities are not in general symmetric under reflection about a horizontal axis. When making plots of
Fraser-corrected data, the qx-qy options may be more useful than q-chi or 2theta-chi type plots.
After the Fraser parameters have been set, the user can switch between uncorrected and Fraser-corrected
images using the SAXS Mode and Fiber Mode buttons on the Calibrate panel.
In Batch mode see SETFIBERBETA, SETFIBEREQUATOR, APPLYFRASERCORRECTION, and ENABLEFIBERMODE options. |