How to Make a Median Flat from 6-Dimensional LWS FITS Files                   
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For this data set, the detector was flattened by making a median,             
low-airmass blank sky frame and a high-airmass blank sky frame,               
then subtracting the two sky frames.  This was performed by extracting        
off-source nod-position beams, defined by image frames (*,*,0,0,1,i),         
from several observations.  For example, for the 21 August 2000 data,         
the low-airmass blank sky frames were extracted from the                      
DATA/RAW/20000821/LWS0051.FITS and LWS0053.FITS files.  The                   
high-airmass blank sky frames were extracted from the LWS0049.FITS            
and LWS0050.FITS files.                                                       
                                                                              
The following IDL code was developed and used by the observers to make        
a medianed flat for the LWS.  The code also produces a bad-pixel mask         
for LWS.  The code was not tested by and is not supported by the PDS          
but is provided to help clarify the process of making a median flat.          
                                                                              
                                                                              
pro, lwsflat, flat, bpm                                                       
                                                                              
; Create a flat field and bad-pixel mask for LWS.                             
;                                                                             
; Method: 1. Flat. get off-source nod-position beams of                       
; a few frames at various airmasses, median them together to get              
; a high-airmass stare frame and a low-airmass stare                          
; frame, and then take the difference of those two.                           
;                                                                             
; 2. Mask. just take one of these stare frames and mark                       
; which pixels are very far (5 sigma) from the median.                        
;inputs: none.                                                                
;                                                                             
;outputs: just the flatfield, normalized to 1.0, and                          
; the mask (good pixels 1, bad pixels 0)                                      
                                                                              
;insert the frames here. these are for 21 Aug 2000, lambda=12.5 mu.           
loair1 = readfits('lws0051.fits')  ;12 nodsets. secz=1.25                     
loair2 = readfits('lws0053.fits')  ;12 nodsets. secz=1.25                     
hiair1 = readfits('lws0049.fits')  ;12 nodsets. secz=1.95                     
hiair2 = readfits('lws0050.fits')  ;12 nodsets. secz=1.95                     
                                                                              
                                                                              
;NB: combining algorithm:                                                     
; ( img(*,*,0,0,1,i)-img(*,*,1,0,1,i) ) -                                     
; ( img(*,*,0,0,0,i)-img(*,*,1,0,0,i) )                                       
; (nod A - nod B) - (on A - on B)                                             
                                                                              
hicomb = fltarr(128,128,24)                                                   
for i=0,11 do hicomb(*,*,i)=hiair1(*,*,0,0,1,i)                               
for i=12,23 do hicomb(*,*,i)=hiair2(*,*,0,0,1,i-12)                           
locomb = fltarr(128,128,24)                                                   
for i=0,11 do locomb(*,*,i)=loair2(*,*,0,0,1,i)                               
for i=12,23 do locomb(*,*,i)=loair1(*,*,0,0,1,i-12)                           
                                                                              
hiflat = fltarr(128,128)                                                      
loflat = fltarr(128,128)                                                      
hisd = fltarr(128,128)                                                        
losd = fltarr(128,128)                                                        
                                                                              
for ii=0,127 do for jj=0,127 do $                                             
        hiflat(ii,jj) = median(hicomb(ii,jj,*),/even)                         
for ii=0,127 do for jj=0,127 do $                                             
        loflat(ii,jj) = median(locomb(ii,jj,*),/even)                         
                                                                              
for ii=0,127 do for jj=0,127 do $                                             
        hisd(ii,jj) = stdev(hicomb(ii,jj,*))                                  
for ii=0,127 do for jj=0,127 do $                                             
        losd(ii,jj) = stdev(locomb(ii,jj,*))                                  
                                                                              
flat = (hiflat - loflat)/median(hiflat-loflat)                                
                                                                              
bpm = bytarr(128,128) * 0B + 1B                                               
wherebad = where(abs(loflat-median(loflat)) ge 5*stdev(loflat))               
bpm(wherebad) = 0B                                                            
                                                                              
writefits, 'bpm.21jun00.fits', bpm                                            
writefits, 'flat.21jun00.fits', flat                                          
                                                                              
return                                                                        
end