;+
; NAME: EXPOSE
;	
; PURPOSE:
;       To return, from a list of image files passed to it, the files
;       that have been produced from a specified exposure time.
;
; CATEGORY:
;	
; CALLING SEQUENCE:
;	EXPOSE,FILES,TIME
;
; INPUTS:
;       FILES: A STRING ARRAY of file (including the path) names.
;
;       TIME: An INTEGER, specifying the exposure time of the images
;             of interest. If no time is chosen, then by default TIME
;             will be set to 60.
;
; KEYWORD PARAMETERS:
;       None.
;
; OUTPUTS:
;       SIXTY: A STRING ARRAY of image files with exposure times given by the
;              variable TIME.
;
; COMMON BLOCKS:
;	
; SIDE EFFECTS:
;       None.
;
; RESTRICTIONS:
;       This procedure assumes that any three integers before a file extention
;       identifies an exposure time of an image file. Consequently, files that
;       have names with three integers before the extention, can be considered
;       valid files by this program even though they may not be image files at
;       all. Therefore, care must be exercised when using this procedure , if
;       meaniful results are to be produced.
;       
; PROCEDURE:
;       The file names (including path) are passed to the function EXPOSURE. EXPOSURE
;       returns a STRING ARRAY containing the exposure time of the files. These exposure
;       times are then checked against the time specified by TIME. Any files that have
;       exposure times that match TIME are returned in the STRING ARRAY SIXTY. If, no
;       time is specified, then by defalt TIME=60 will be chosen.
;
; MODIFICATION HISTORY:
;       Written May 1994, by T A Oliynyk.	
;-

FUNCTION expose,files,time      
                                                                  
in=exposure(files)      
result=N_PARAMS()    
t='060'    
IF result EQ 2 THEN t=STRING(time,FORMAT='(I3.3)')    
index=WHERE(in EQ t,count)       
IF count NE 0 THEN BEGIN     
        sixty=files(index)      
        RETURN, sixty     
ENDIF ELSE BEGIN     
        seconds=STRMID(t,1,2)    
        MESSAGE, 'No '+seconds+ ' second image files found ',/INFORMATIONAL      
        RETURN, -1     
ENDELSE      
END