set t_exp to pointing.exp_time for shutter effect

observation_sim/ObservationSim.py

@@ -45,7 +45,7 @@ class Observation(object):
             chip.shutter_img = np.ones_like(chip.img.array)
         else:
             chip.shutter_img = effects.ShutterEffectArr(
-                chip.img, t_shutter=1.3, dist_bearing=735, dt=1E-3)
+                chip.img, t_exp=pointing.exp_time, t_shutter=1.3, dist_bearing=735, dt=1E-3)
         chip.prnu_img = effects.PRNU_Img(xsize=chip.npix_x, ysize=chip.npix_y, sigma=0.01,
                                          seed=int(self.config["random_seeds"]["seed_prnu"]+chip.chipID))