** Next:** B Overlapping Orders

** Previous:** A.2 Focusing the Spectrograph

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