diff --git a/spectrum.py b/spectrum.py
index 73561d1..2e222ea 100755
--- a/spectrum.py
+++ b/spectrum.py
@@ -11,7 +11,7 @@ import pathlib
 from scipy.ndimage import rotate
 from astropy.io import fits
 
-cache , filename , output , calibration , intensity_calibration , verbose , no_cache = '' , None , None , None , None , False, False
+cache , filename , output , calibration , intensity_calibration , verbose , no_cache = None , None , None , None , None , False, False
 if len( sys.argv ) < 2:
     raise Exception( 'spectrum.py: type \'spectrum.py -h\' for more information' )
 
@@ -110,6 +110,26 @@ if verbose:
            \n===========================================' )
 # TODO: check in advance file to check if exists or writeable
 
+files = {}
+if calibration != None:
+    files[ 'wavelength' ] = pathlib.Path( calibration )
+if intensity   != None:
+    files[ 'intensity'  ] = pathlib.Path( intensity   )
+if cache       != None:
+    files[ 'cache'      ] = pathlib.Path( cache       )
+
+files[ 'filename' ] = pathlib.Path( filename )
+
+for name , path in files.items():
+    if name in [
+        'wavelength',
+        'intensity' ,
+        'spectrum'  ,
+    ] and not path.is_file():
+        raise Exception(
+            'spectrum.py: could not open the ' + name + ' file'
+        )
+
 hdul = fits.open( filename )
 data = hdul[0].data
 head = hdul[0].header
@@ -117,12 +137,10 @@ hdul.close()
 if verbose:
     print( 'data loaded' )
 
-cache_file = pathlib.Path( cache )
-
-if cache_file.is_file() and not no_cache:
+if 'cache' in files and files[ 'cache' ].is_file() and not no_cache:
     if verbose:
         print( 'using cache' )
-    with shelve.open( str( cache_file ) ) as cache:
+    with shelve.open( str( files[ 'cache' ] ) ) as cache:
         for key in [ 'data' , 'border' , 'calibrations' ]:
             if key not in cache:
                 raise Exception( 'spectrum.py: missing data in cache file' )
@@ -473,10 +491,10 @@ else:
     if verbose:
         print( 'calibration isolation finished' )
 
-    if not cache_file.exists() and not no_cache:
+    if 'cache' in files and not files[ 'cache' ].exists() and not no_cache:
         if verbose:
             print( 'writing result in cache' )
-        with shelve.open( str( cache_file ) ) as cache:
+        with shelve.open( str( files[ 'cache' ] ) ) as cache:
             cache[ 'data' ]         = data
             cache[ 'border' ]       = border
             cache[ 'spectrum' ]     = spectrum
@@ -562,6 +580,17 @@ bias = {
 
 mean_bias = np.mean( [ bias[ 'top' ] , bias[ 'down' ] ] , axis = 0 )
 
+plt.imshow(
+    data[
+        calibrations[ 'down' ][ 'y' ][ 'max' ]      :
+        calibrations[ 'down' ][ 'y' ][ 'max' ] + 200,
+        calibrations[ 'down' ][ 'x' ][ 'min' ]      :
+        calibrations[ 'down' ][ 'x' ][ 'max' ]
+    ]              ,
+    aspect = 'auto',
+)
+plt.show()
+
 if verbose:
     print( 'bias substraction finished' )