IMG.corimg
  imgtype
    set to PTR_NEW
  nsx_clear_image
    set to PTR_NEW
  nsx_read_image
    set to pointer to [nc x nr] 
  nsx_correct_image
     IA.corimg calculated from IA.clnimg  
    [IB.corimg calculated from IB.clnimg]
  nsx_main
    write IA.corimg as IA.root + "-corrected.fits"
   [write IB.corimg as IB.root + "-corrected.fits"]
   [IAB.corimg = IA.corimg - IB.corimg]
   [write IAB.corimg as IB.root + "-corrected.fits"]
 nsx_create_background_image
   IA.bckimg calculated from IA.corimg  
  [IAB.bckimg calculated from IAB.corimg]
nsx_extract_spectrum
  image = IA.corimg - IA.bkcimg
 [image = IAB.corimg - IAB.bkcimg if IB.X]
  SPX.spobj extracted from image

------
Mar 10, 2020 - translated nsx enough to extract spectra.
  c code in /usr/local/nsx
------
Mar 12, 2020
  Writing nsxng (nsx, no global variables).
------
Nov 28, 2020
  Copy from eris_nires19/anal/common to eris_nires19/anal/common
  Modified:
    nsxng_fractional_pixel_rb.pro
    nsxng_arcsec_profile_mash.pro
    nsx_fractional_pixel_rb.pro
    nsxng_avpinv.pro
    nsxng_constants.pro
    nsxng_difatmref.pro
    nsxng_wsctype.pro
    nsxng_load_wsc.pro
    nsxng_centroid2.pro
    nsxng_clear_spx.pro