The CALIBRATION Sub-Menu

The CALIBRATION sub-menu contains commands to characterise and calibrate detector distortions such as intensity linearity, spatial distortion, and non-uniformity of intensity response (Hammersley, Svensson, & Thompson, 1994, Hammersley et al, 1995, Moy et al, 1996). Raw images of calibration grid exposures may be treated to define spatial distortion correcting functions, and thus image data may be re-binned from a distorted grid to an ideal regularly spaced grid. Once defined the function coefficients may be saved to a file for later re-use, and within the GUI.

The re-binning operation involves calculating the covered area in the ideal output pixel grid of each of the pixels from the raw distorted image. This involves much calculation and is a relatively slow procedure. To make correction of multiple images much faster it is possible to calculate a look-up table of pixel shifts and the relative contributions of each input pixel to the target and surrounding pixels. Once calculated the correction is 10 times faster than without the look-up table (2.5 seconds for an image of 1242 $\times$ 1152 on an HP-755 125MHz system).

A typical use of the CALIBRATION menu is for the correction of diffraction images from detectors with spatial distortion and non-uniformity of intensity response, for macromolecular and small molecule crystallography. Up to hunderds of images are input, corrected for the distortions, and output in an appropriate file format, for integration using a standard integration program e.g. DENZO (Otwinowski 1993). This correction is performed automatically using macro files (which are described in the Reference Manual).