pro sp_group,sfname,bfname,ch,ff,ffe,chl,chh,ctonth=ctonth,fout=fout,fnorm=fnorm
;+
;
; Adaptively calculate the boundary channel numbers for a constant 
; signal to noise ratio spectrum
;
;*Input
; sfname, bfname - source and background file names
;
;*optional output:
; ch - mean channel number of the output spectrum bins
; ff, ffe - output spectrum and its erro bars
; chl, chh - the lower and upper channel numbers of the output spectrum bins
;
; ctonth - the signal to noise threshold; def=3
; fnorm - adjusting factor for the background normalization; def= 1.
;       This parameter is useful when background needs to be scaled by a factor
;       (fnorm > 1, higher).
; fout - the output file name (to be used by grppha for example)
;       which can be used in the program grppha for grouping.
;*Note
; the keywords, exposure and backscal in the fits headers of the source and 
; background files must have been defined correctly.
;
;*Example:
; sp_group,'halo2.pi','bgnd_halo2.pi',cton=2,fout='test.grp'
;
; written by wqd, Feb 22, 2001
;
;-
if n_params() eq 0 then begin
print,'CALLING SEQUENCE -- sp_group,sfname,bfname,ch,ff,ffe,chl,chh,ctonth=ctonth,fout=fout'
return
endif

if n_elements(fnorm) eq 0 then fnorm=1.
; input source data
tab=readfits(sfname,ext=1,hdr)
snrow=sxpar(hdr,'naxis2')
sc=tbget(hdr,tab,'counts',lindgen(snrow))
sexpt=sxpar(hdr,'exposure',count=count)
if count ne 1 then stop,'stop: count not = 1'
sarea=sxpar(hdr,'backscal',count=count)
if count ne 1 then stop,'stop: count not = 1'

; input background data
tab=readfits(bfname,ext=1,hdr)
bnrow=sxpar(hdr,'naxis2')
if bnrow ne snrow then stop,'stop: snrow ne bnrow!'
bc=tbget(hdr,tab,'counts',lindgen(bnrow))
bexpt=sxpar(hdr,'exposure',count=count)
if count ne 1 then stop,'stop: count not = 1'
barea=sxpar(hdr,'backscal',count=count)
if count ne 1 then stop,'stop: count not = 1'

;normalization of the background into the source region with the 
; correct exposure
bnorm=fnorm*sexpt*sarea/(bexpt*barea)

;calculate the grouping and output into the file.
get_group,sc,bc*bnorm,ch,ff,ffe,chl,chh,ctonth=ctonth,fout=fout
;-------------------------------
return
end