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For imaging data, removing the distortion explicitly may be necessary in order to derive coordinates for objects in true tangent-plane coordinates. This is important if you want to derive coordinates for mask manufacture since these must refer to the focal plane of the telescope which, to first order, is undistorted.
The distortion is mapped by taking an image through the matrix mask and then using the LEXT procedure ANALYZE_MATRIX as described in Section 4.8. This procedure calculates a set of spline functions which describe the forward and backward transformations. Here, the forward transform means distorting from the mask (i.e. telescope focal plane) to the detector focal plane, and the backward transform means distorting from the detector focal plane back to the mask. By default the forward transform is stored in a file FTRANS, and the backward transform in a file BTRANS.
To apply the distortion corrections use the LEXT command DISTORT:
Since the distortion is wavelength dependent, there will normally be a different pair of transformations for each filter used. To change the file names used by DISTORT, use the commands SET FTRANS ftrans_file and SET BTRANS btrans_file.
The distortions can be applied to lists or images. In the list case the data are treated as coordinates and the distorted coordinates will be output to a new list. In the image case, the whole image is distorted. In most cases, it is faster to derive coordinates from the original image and apply the distortion to the coordinates rather than to derive coordinates from the image after a distortion correction.
It is important to note the distortion correction does not account for the simple scale change between the telescope and detector focal planes. In each case the distortion is done in the same units, so if you are distorting a list of positions in CCD pixel units, the distorted values will also be in pixel units. If you wish to transform between mask and detector units (i.e. to just change the scale) you should use the command TRANSFORM: