;----------------------------------------------------------------------
; NAME:
;	update_aimass
; PURPOSE:
;       corrects the airmass in the image headers and puts a history
;       in the header that the airmass was corrected.
;
; EXPLANATION:
;       Takes a list of images and updates the airmass to the correct
;       values.
;       
; CALLING SEQUENCE:
;       update_airmass, 'imlist' [, '00:00:00.0', '00:00:00.0']
; INPUTS:
;       imlist - a filename for a file containing a list of images
;                that need to be pattern corrected.
;
; OPTIONAL INPUTS:
;       ra2 - the RA of the object precessed to the appropriate epoch
;       dec2 - the Dec of the object precessed to the appropriate epoch
; OPTIONAL INPUT KEYWORDS:
;
; OUTPUTS:
;
; OPTIONAL OUTPUTS:
;
; NOTES:
;      This procedure requires that the path name for the filelist
;      template is set to the appropriate path name. Pleas alter the
;      path name for this file accordingly.
;      

; PROCEDURE CALLS:
;      airmass2
;      decparse
;      raparse
; REVISION HISTORY:
;-------------------------------------------------------------------------

pro update_airmass, imlist, ra2, dec2

;retrieve the list of files
files_file=make_array(1, /STRING)
restore, '/d1/data1/mirsi/analysis/filelist_template.idl'
files_file=read_ascii(imlist, template=filelist_template)
filelist=files_file.field1
nfiles=n_elements(filelist)

;loop over the list of files
for i=0, nfiles-1 DO BEGIN $
 data = READFITS(filelist(i), header)

 ra_s = sxpar(header, 'RA')
 dec_s = sxpar(header, 'DEC')
 lat_s = sxpar(header, 'TEL-LAT')
 long_s = sxpar(header, 'TEL-LONG')
 ut_s = sxpar(header, 'TIME-STR')
 date = sxpar(header, 'DATE-OBS')

;if user supplies coordinates for the object the
; new coordinates are updated here.
 if n_params() EQ 3 THEN BEGIN
     ra_s = ra2
     dec_s = dec2
 ENDIF

;convert the dec, ra, lat, long, ut to the appropriate units
 dec = decparse(dec_s)
 ra  = raparse(ra_s)
 lat = decparse(lat_s)
 long = decparse(long_s)
 ut = raparse(ut_s)

;compute the airmass
 air = airmass2( ra, dec, lat, long, ut, date)

;rewrite the data and the updated header information.
 sxaddpar, header, 'AIRMASS', air
 sxaddhist, 'Airmass corrected on ' + systime(), header 
 writefits, filelist(i), data, header
ENDFOR

end