NAME:
  getpair
 PURPOSE:
  Read two OSIRIS XD data files and return the difference strip image.
 DESCRIPTION:
 CATEGORY:
  Spectroscopy
 CALLING SEQUENCE:
  getpair,calib,root,i1,i2,diff,hdr,RAW=raw
 INPUTS:
  calib- Anonymous structure containing all pertinent calibration
           information.  This structure is usually loaded beforehand using
           the routine, "ldcalir"
  root - string containing the root of the file name (with leading path
         if desired).
  i1   - Frame id (integer) of first image.
  i2   - Frame id (integer) of second image.
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
  RAW  - Flag, if true will inhibit the column-wise background subtraction.
           The default is to fit the background in each column (along the
           slit) and subtract the fitted background.  Use a linear function
           for the background.
 OUTPUTS:
  diff - i1-i2 after XD spectra extracted to strip image. (see getstrip)
  hdr   - FITS header for image.
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
  Specifically written for OSIRIS cross-dispersed spectral data.
 PROCEDURE:
 MODIFICATION HISTORY:
  95/03/24, Written by Marc W. Buie, Lowell Observatory
  95/09/15, MWB, added usage of calib structure and return of header.