FUNCTION dustem_make_empty_fortran_st,Ngrains

;This makes an empty fortran structure.
;May have to be adapted for polarized models ...

;st=dustem_make_empty_fortran_st(3)

;IDL> help,st,/str
;** Structure <b4023ea8>, 2 tags, length=57600, data length=54400, refs=1:
;   SED             STRUCT    -> <Anonymous> Array[800]
;   EXT             STRUCT    -> <Anonymous> Array[800]

;IDL> help,st.ext     
;<Expression>    STRUCT    = -> <Anonymous> Array[800]
;IDL> help,st.sed
;<Expression>    STRUCT    = -> <Anonymous> Array[800]

;IDL> help,st.sed,/str
;** Structure <c4007d08>, 5 tags, length=40, data length=36, refs=2:
;   WAV             FLOAT         0.0400000
;   EM_GRAIN_1      DOUBLE           0.0000000
;   EM_GRAIN_2      DOUBLE           0.0000000
;   EM_GRAIN_3      DOUBLE           0.0000000
;   EM_TOT          DOUBLE           0.0000000

;IDL> help,st.ext,/str
;** Structure <b4020c68>, 4 tags, length=32, data length=32, refs=2:
;   WAV             FLOAT         0.0400000
;   ABS_GRAIN       FLOAT     Array[3]
;   SCA_GRAIN       FLOAT     Array[3]
;   EXT_TOT         FLOAT           0.00000

lambir=dustem_get_wavelengths()
Nwavs=n_elements(lambir)

;do the sed structure
instruc='one_st={wav:0.,'
FOR i=0L,Ngrains-1 DO BEGIN
  instruc=instruc+'em_grain_'+strtrim(i+1,2)+':0.,'
ENDFOR
instruc=instruc+'em_tot:0.}'
toto=execute(instruc)
sed_st=replicate(one_st,Nwavs)
sed_st.wav=lambir

;do the ext structure
abs_g=fltarr(Ngrains)
ext_g=fltarr(Ngrains)
one_st={wav:0.,abs_grain:abs_g,sca_grain:abs_g,ext_tot:0.}
ext_st=replicate(one_st,Nwavs)
ext_st.wav=lambir

st={sed:sed_st,ext:ext_st}

RETURN,st
END