PRO dustem_write_isrf_lv,file,st,help=help

;+
; NAME:
;    dustem_write_isrf_lv
; PURPOSE:
;    Writes file ISRF.DAT used by DustemWrapper
; CATEGORY:
;    Dustem
; CALLING SEQUENCE:
;    dustem_write_isrf_lv,file,st[,/help]
; INPUTS:
;    file : file name 
;    st   : input ISRF structure
; OPTIONAL INPUT PARAMETERS:
;    None
; OUTPUTS:
;    None
; OPTIONAL OUTPUT PARAMETERS:
;    None
; ACCEPTED KEY-WORDS:
;    help      = If set print this help
; COMMON BLOCKS:
;    None
; SIDE EFFECTS:
;    None
; RESTRICTIONS:
;    The dustem fortran code must be installed
;    The dustem idl wrapper must be installed
; PROCEDURE:
;    a Stellar ISRF is added based on the content of !dustem_isrf_star_add
; EXAMPLES
;    
; MODIFICATION HISTORY:
;    Written by J.P. Bernard
;    Contact J.-Ph. Bernard (Jean-Philippe.Bernard@irap.omp.eu) in case of problems.
;-

IF keyword_set(help) THEN BEGIN
  doc_library,'dustem_fit_sed_readme'
  goto,the_end
ENDIF

;=== add stellar component if needed.
;stop
ind=where(!dustem_isrf_star_add.amplitude NE 0.,Nstars)
IF Nstars NE 0 THEN BEGIN
	cmic=3.e14
	pc2m=3.08e16     ;1pc=3.08e16 km
	rsun2m=6.957e8   ;Rsun=6.957e5 km
	wave_angstroem=st.LAMBISRF*1.e4      ;mic to Angstroem 
	stellar_component=fltarr(n_elements(st))
	FOR i=0L,Nstars-1 DO BEGIN
		omega=!pi*((!dustem_isrf_star_add[i].radius*Rsun2m)/(!dustem_isrf_star_add[i].distance*pc2m))^2
		Ilambda=planck(wave_angstroem,!dustem_isrf_star_add[i].temperature)/(4.*!pi) ;ergs/cm2/s/A/sr
		Inu=Ilambda*st.LAMBISRF^2/cmic  ;ergs/cm2/s/Hz/sr
		stellar_component=stellar_component+!dustem_isrf_star_add[i].amplitude*omega*Inu ;
	ENDFOR
	st.isrf=st.isrf+stellar_component
ENDIF

Ncomments=6
c=strarr(Ncomments)

c(0)='# DUSTEM: exciting radiation field featuring'
c(1)='# Mathis ISRF'
c(2)='# Blackbody with     T=   0.0000E+00'
c(3)='# dilution factor wdil=   1.0000E+00'
c(4)='# Nbr of points'
c(5)='# wave (microns), 4*pi*Inu (erg/cm2/s/Hz)'

openw,unit,file,/get_lun

FOR i=0,Ncomments-1 DO BEGIN
  printf,unit,c(i)
ENDFOR

N_waves=n_elements(st)
printf,unit,n_waves

FOR i=0L,N_waves-1 DO BEGIN
  printf,unit,st(i).lambisrf,st(i).isrf
ENDFOR

close,unit
free_lun,unit

the_end:

;stop

END