Spatial distortions, intensity non-linearity, flat-field non-uniformity of response, and for image plates the decay of the latent image with time, can be calibrated and corrected. The process of measuring a spatial calibration grid, defining an approximate interpolating function, and of subsequent correction to images is divided into a number of separate operations. This is to allow the maximum of flexibility to cover different detector systems and different approaches to the calibration process e.g. different interpolating functions. The program tries to guide the user through a logical sequence of commands for the spatial distortion calibration process, but this may not always correspond to a user's needs. (Use the "?" to obtain a full list of available commands.)
The calibration and correction process may be divided into two processes:
In the case of spatial distortion this means: a. producing a calibration interpolating spline function, b. Using the spline function to correct a distorted image. For the uniformity of response the calibration stage means producing a normalised ``flat-field'' image (or its inverse), and the correction stage means pixel by pixel division (or multiplication) of the distorted data by the calibrated flat-field image.
The two operations may be performed completely separately; hopefully a detector system may be calibrated once and the measured distortion corrected on many subsequent data images. The correction process may eventually be performed on an on-line processor as the data are being acquired. (The correction process is much simpler than the calibration process.)