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A.3 Aligning the Spectrum on the Chip

The final step involves adjusting the detector so that a comparison line will fall along a row or a column to a few tenths of a pixel. In IRAF the subtraction of the sky spectrum is assumed to be along a row or column, and not perpendicular to the object trace. (The object trace basically defines the practical ``dispersion" direction, which may not be simply othogonal to the slit due to differential refraction, or if the grating is slightly misaligned in its cell.)

Mar 06 1996