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The correct position of the collimator may also be determined (not as accurately) from a plot of line FWHM vs collimator position. Take a series of regularly-spaced exposures, copy them to SCRATCH as files FOCUSi.DST, type FIGARO, then FOCUS_FWHM and respond to the prompts.
The parameters which may be changed by the user are: the number of images; the wavelength direction (X or Y); the first and last pixels in the orthogonal direction to be used; the start collimator position; the increment in collimator position; whether the maximum pixel-value is to be determined for each of the images (so that the user can check for saturation); whether the region of the spectrum to be used is cursor-selected or defaults to the whole spectrum; and the number of brightest lines to be used to determine the mean FWHM.
A version of FIGARO's EMLT program is used to determine the mean FWHM for the brightest lines on each extracted spectrum. In-focus position is estimated by determining the minimum in a parabola fitted to the 3 points flanking the lowest-measured FWHM in the relation FWHM vs focus position. The program plots this relation on a device of the user's choosing e.g. PERICOM_MG, IKON_1, CANON_P, VAXstation TEK_4010 window.