At present the spatial distortion is measured from a known grid of holes on a calibration mask [8,30,34]. The calibration mask should cover the whole of the detector area and should ideally be placed flat against the detector and exposed to X-rays (or other radiation) at some distance from the detector. (The larger the distance the less the problem of parallax.) Alternatively, a point source at the sample position can be used to produce exactly the same parallax as the sample.
The calibration data should be input in the normal manner (See
Section 15.51, Page ![[*]](./crossref.png)
). Before entering the
calibration sub-menu the ROI may need to be defined (using
the REGION command, the PIXEL REGION command,
or the IMAGE graphics sub-menu). The
calibration measurement and function definition is limited to the ROI.
Experience shows that it is important to inspect the calibration
grid image and to define an ROI which includes all valid grid peaks,
but does not include partially recorded grid peaks or other spurious
features33.
It may also be useful to use the Z-SCALE command to
select an intensity range to be displayed.
The normal order of commands for distortion calibration is (commands
with a dagger (
) are recommended but not strictly necessary):
: View (verify) calculated distortion
values as 2-D images
: Save distortion function
coefficients in an ASCII file for subsequent correction of data.
Additionally the command RE-CALCULATE DISTORTION may be used to change the assumptions used to define the distortion, and the command VIEW PEAKS may be useful to view by row and by column the measured peak centres and the calculated distortion. SAVE PEAKS may be used to output all the measured peaks to an ASCII file. This can then be used as input for a spread-sheet or graph plotting program for further processing or display.