acal

 ACAL N1 N2 A1 A2 METHOD DA DT DX -- Applies arc calibration to a set of 
                                     spectra

 Parameters:
            N1, N2 -- The range of spectra to be wavelength calibrated
            A1, A2 -- The range of arc spectra to use for calibration
            METHOD -- N -- Nearest arc in time is used
                      I -- Interpolate (linearly) between two nearest arcs,
                           take nearest if arcs only on one side (in time)
                      E -- Interpolate as above, but extrapolate if arc is
                           only on one side

            DA    -- Maximum displacement in minutes of arc on sky that is
                     allowable between an arc and an object. This prevents
                     an arc taken for one object being used to calibrate
                     another (unless DA is made huge). If a position
                     cannot be found in the arc's header, it will taken to
                     be OK.

            DT    -- Maximum time offset between exposure and arc in
                     minutes. Allows one to eliminate possibility of
                     one night's arc being applied to next.

            DX    -- Maximum difference in extraction position in pixels.
                     If the real header item 'Extract position' is set,
                     this can be used to make sure that an arc has been
                     extracted at the same position as the object.

 The run numbers of the arc or arcs used to derive the wavelength scale are
 written in a character header item called "Arc(s) used".
 
 NB The arc and target MUST have the same number of pixels. If it never
    seems to find any arcs, you may want to check this amongst other
    things. If your spectra have the header parameter 'Fibre' defined 
    representing a fibre number (integer) then both arc and object must match
    exactly for an arc to be used.

This command belongs to the class: calibration