import matplotlib.pyplot as plt
import matplotlib.image as mpimg
import pyfits
import numpy

def Gaf(x):
    f = pyfits.open(x)
    i = f[0]
    d = i.data
    plt.imshow(d)
    plt.show(d)

def Signal(x,x1,x2,y1,y2,xx1,xx2,yy1,yy2):
    f = pyfits.open(x)
    i = f[0]
    d = i.data
    u = d[x1:x2,y1:y2].sum()-(((x2-x1)*(y2-y1))/((xx2-xx1)*(yy2-yy1)))*(d[xx1:xx2,yy1:yy2].sum())
    print u

Signal("C:\Users\Luca\Desktop\Atlas_pleione_onesec.fit",100,200,100,200,100,200,100,200)