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The method for taking images at each focus step is also selectable. You may take a single, or n multiple singles, or on some instruments, a subframe basic occultation with n frames (ie NAXIS3=n). In either case, if multiple frames or singles are used, the fwhm or other figure of merit for that step is an average of the analysis values obtained. On LMI the basic occultation is not supported; such exposures may be taken, but are subject to interface image contamination due to lack of an optically masked transfer area.
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