;+
; NAME:
;	CHKBKG
; PURPOSE:
;       Quick look at mean zenith brightnesses in all background
;	images for a given date, camera, and filter.
; CATEGORY:
;	
; CALLING SEQUENCE:
;	CHKBKG, YYMMDDCF
; INPUTS:
;	YYMMDDCF: An 8-digit number constructed as follows:
;	      YY: The last two digits of the year;
;             MM: The two digits of the month number;
;             DD: The two digits of the day number (UT);
;              C: The camera number (0/1);
;              F: The filter number (0-4).
; OPTIONAL INPUTS:
;	None.
; KEYWORD PARAMETERS:
;	None.
; OUTPUTS:
;	None.
; OPTIONAL OUTPUTS:
;	None.
; COMMON BLOCKS:
;	None.
; SIDE EFFECTS:
;       A plot is created, showing the mean zenith intensity in
;       each of the background images, its time span, and the number
;	of individual camera images used to form it.
; RESTRICTIONS:
;       The appropriate background file must exist.  The routine
;	only works under Unix right now.
; PROCEDURE:
;	Straightforward.
; EXAMPLE:
;	CHKBKG,93120802		; for 8 Dec 1993, camera 0, filter 2.
; SEE ALSO:
;	TVIEW_BKGRDS.PRO, BFILE_CHECK.PRO
; MODIFICATION HISTORY:
; 	Written by:	D P Steele, ISR, October 1994.
;-
PRO chkbkg,num

IF N_PARAMS() LT 1 THEN BEGIN
    doc_library,'chkbkg'
    RETURN
    ENDIF

fs=4L*256*256+24	; file size in bytes
brmax=250.		; default max zenith brightness
zmax=0.	

; This routine only works under Unix, right now.
name='/jasper/cnsr3_data1/bkgrd/'+STRING(num,FORMAT='(I8)')+'.bkg'
file=FINDFILE(name,COUNT=nfile)
IF nfile EQ 0 THEN BEGIN
    MESSAGE,name+' not found',/INFORMATIONAL
    RETURN
    ENDIF

OPENR,bu,name,/GET_LUN
f=ASSOC(bu,BYTARR(fs))
fb=FSTAT(bu)
n=fb.SIZE/fs		; number of background images found

LOOP:			; label for GOTO below to point at

; Set up plot axes
PLOT,[0,1],/NODATA,XRANGE=[0,24],/XSTY,XTICKS=4,XTICKV=INDGEN(5)*6 $
    ,XMINOR=6,XTITLE='UT Hours' $
    ,YRANGE=[0,brmax],YTITLE='Mean Zenith Background' $
    ,TITLE=name

FOR i=0,n-1 DO BEGIN	; loop through all background images
    fi=f(i)		; load i-th image
    hb=BYTE(fi,0,24)	; get header bytes
    im=FLOAT(fi,24,256,256)	; and image data
    nimgs=FIX(hb,10)	; decode header
    sut=LONG(hb,12)
    eut=LONG(hb,16)
    int=LONG(hb,20)
    zm=mean(im(124:131,124:131))	; mean zenith brightness
    zmax=MAX([zmax,zm])			; update max observed 'zm'
    PRINT,i,strsec(sut),strsec(eut),strsec(int),nimgs,zm $
	 ,FORMAT='(I2,A10,"-",A8," (",A8,")  ",I3,F5.0)'
;   prsc,[i,sut,eut,int,nimgs,zm]
    PLOTS,[sut,eut]/3600.,[zm,zm]	; plot image span and
					; mean zenith brightness
    IF int LT 3700. THEN yo=1 ELSE yo=-6
    XYOUTS,(sut+eut)/7200.,zm+yo,STRTRIM(nimgs,2) $
	  ,ALIGNMENT=0.5,/DATA		; label with # images used
    ENDFOR

IF zmax GT brmax THEN BEGIN
    MESSAGE,'Replotting with better y-axis',/INFORMATIONAL
    brmax=zmax
    GOTO,loop
    ENDIF

FREE_LUN,bu

RETURN
END