PRO findcent,img,x,y

;procedure to find the centre of a circular 256*256 image by plotting circles
;on the image displayed at 3 times normal size

r=0. & x= 0. & y=0.
;set large window to view 3X image
WINDOW,0,XSIZE=1000,YSIZE=800
ans=''
REPEAT BEGIN
    PRINT,'Enter the radius, and the x and y coordinates of the circle'
    READ,r,x,y
    TVSCL,REBIN(img,256*3.,256*3.,/SAMPLE)
;plot the circle on the image
    arcs,r*3,0,360,x*3,y*3,/dev
    PRINT,'Do you want to reposition the circle?'
    READ,ans
    ENDREP UNTIL(STRLOWCASE(ans) EQ 'n')

; Return window to normal size
WINDOW,0
RETURN
END