;----------------------------------------------------------------------
; NAME:
;	sub_image2
; PURPOSE:
;       Subtract the chop and nod images in an chop/nod set from the
;       source image.
;
; EXPLANATION:
;       Subtracts chop and nod images from the source image if chop
;       and nod images have been taken. If the images do not contain 
;       either the chop or nod set then only the sets that are present
;       will be subtracted. If the bad_pixel mask is sent
;       as an argument, all of the bad pixels are set to zero.      
;       
; CALLING SEQUENCE:
;       sub_image, 'inputfile','outputfile' [,'badpixelmap']
; INPUTS:
;       imageName - name of the input fits image (full path name
;                   excepted).
;       filename - name of the output file name. The output file name
;                  will be given the prefix 'd' meaning a
;                  'differenced' file.
;
;
; OPTIONAL INPUTS:
;       badPix - name of bad pixel map. Good pixels in the map should
;                have a value of one. Bad pixels in the map should
;                have a value of zero.
;
; OPTIONAL INPUT KEYWORDS:
;
; OPTIONAL OUTPUTS:
;
; NOTES:

; PROCEDURE CALLS:
;       
;
; REVISION HISTORY:
;-------------------------------------------------------------------------


PRO sub_image2, imageName, filename, badpix
 

   image_name = filename

   ;read the data
   data  = READFITS(image_name, header, EXTEN_NO=0)
   data2 = READFITS(image_name, EXTEN_NO=1)
   data3 = READFITS(image_name, EXTEN_NO=2)
   data4 = READFITS(image_name, EXTEN_NO=3)

;perform the differencing if a chop or nod image exists.
   IF data2[0] ne -1 THEN BEGIN
       data = data - data2
       history = 'Second image in a chop or nod set was subtracted: A-B'
       sxaddhist, history, header
   ENDIF
;perform the differencing if chop and nod images exist.
   IF data4[0] ne -1 THEN BEGIN 
       data = data - data3 + data4
       history = 'Full chop and nod images subtracted: (A-B)-(C-D)'
       sxaddhist, history, header
   ENDIF

; if a bad pixel map is supplied, multiply by the bad pixel
; map to make the bad pixels zero, and update the fits header.
   IF N_PARAMS() EQ 3 THEN  BEGIN 
        badMap	= readfits(badPix) 
	data = data * badMap
        history = 'Multiplied by a bad pixel map (BPM).' 
        sxaddhist, history, header
        history = 'BPM = ' + badPix 
        sxaddhist, history, header
    ENDIF

;save the new image with the 'd'suffix added to the image name
   imageName = 'd' + imageName
   writefits, imageName, data, header

end