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Using the command ARC or RUN obtain 3 or 5 pairs of Hartmann left and right arc-lamp exposures at different values of the collimator position. Copy them onto a scratch area on one of the Vaxcluster scratch disks as files HART_Li.DST and HART_Ri.DST where i takes values e.g. 1 - 3 or 1 - 5. Type FIGARO, then FOCUS_HART and respond to the prompts.
The parameters which may be changed by the user are: the number of left-right pairs 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; and whether the maximum pixel-value is to be determined for each of the images (so that the user can check for saturation).
The cross-correlation between spectra extracted from left and right images is performed by the FIGARO routine SCROSS. In-focus position is determined in two ways: by linear interpolation between two values flanking zero line-shift, and by fitting a least-squares straight line to the relation between focus position and line shift. 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.