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Mounting the aperture masks is not a straightforward operation and it is possible for a mask to be installed so that the centre of the mask is shifted with respect to the centre defined by the matrix mask. If the masks are not made with the milling machine at the telescope, it is probable that the centre of the mask will be a considerable distance away from the correct position.
It is therefore desirable (essential in the case of masks not made by the on-site facility) to calibrate the positions of the masks. This can be done by taking a direct exposure through the appropriate mask with white light from the calibration system, using the same filter as for the master distortion exposure that you wish to relate everything to.
The LEXT procedure ANALYZE_MASK will calculate the relationship between the mask positions in the exposure and the predicted position from the catalogue file. This information will be stored in a new .SET file which can be used subsequently for all target acquisition operations with this mask.