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The optical system produces small radial distortions amounting to approximately 2% of the radius at the field edge. 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 map is obtained using the matrix mask (see Section 1.2) which contains small holes spaced on a regular square grid. The mask should be observed without a grism with white light from the calibration system (plus neutral density filters to avoid saturation of the CCD). It is important that this should be done with a broad-band filter so that the image of each hole is not blurred by chromatic distortion effects.
Exposures should be taken through each broad-band filter (or custom filter) that you intend to use so that the chromatic component can be accounted for although this may be small enough for you to ignore.
The exposures should be analyzed using the LEXT procedure ANALYZE_MATRIX which does the following:
If the matrix image is too far from the nominal position, the automatic centroiding may fail. The simplest remedy is to manually set the parameters X_CENTRE and Y_CENTRE to approximately the correct values. Then re-run ANALYZE_MATRIX.
The new global parameters, and BTRANS and FTRANS files are required to generate mask data from direct LDSS-2 images using the LEXT procedure COORDS_TO_MASK.