FUNCTION exposure,files
;
; This function returns a STRING array giving the exposures (i.e.,
; the three digits before the '.') in the STRING array of file
; names passed in 'files'.
; 
out=files
FOR i=0,N_ELEMENTS(files)-1 DO BEGIN
    dotpos=STRPOS(files(i),'.')
    out(i)=STRMID(files(i),dotpos(0)-3,3)
ENDFOR
RETURN,out
END