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If you doubt your astrometry, or the location of the mask is uncertain, you can use another LEXT procedure REFINE_TWEAK to correlate the target objects found in a direct image of reasonable depth (> 60s depending on the detector) with an image of the mask illuminated with white light or sky. This necessitates taking two extra exposures (a) a white light (or sky) mask image before the first of the direct images of the mask against the sky and (b) an image of the sky with no mask after the initial TWEAK. The residual TWEAK found using REFINE_TWEAK can be applied during the subsequent spectroscopic exposure. Note that you must first have done a positional calibration of the mask using ANALYZE_MASK (Section 4.9) to determine the fine details of the mask mounting before you run REFINE_TWEAK.