Version 2018-05-31 :
The nsx program now adjusts an overall (row) shift in the echelle orders on the detector.
(Note that a shift of about 23 rows was seen between 2018-01-20 and 2018-03-04.)

Version 2018-08-31 :

-- I have created maps of abnormal high(hot) and low pixels using darks and flat field
   exposures.  See the files 'nsx/cal/HotPix.fits' and 'nsx/cal/LowPix.fits' to see where
   these pixels are located.  These pixels are replaced by interpolations before the spectra
   are extracted.  You can prevent this by using the '-NoHotClean'  and '-NoClean' options. 

-- I have added a larger tolerance (+/- 200 columns) for a skyline shift.  The previous
   version assumed only a maximum of a few column shift.  

-- I have also added flat field corrections which are automatic unless the '-NoFlat' 
   option is used.  This uses a flat from March 2018 (the program does not yet accept 
   user flat fields).