Update livetime correction

stixpy/calibration/livetime.py

@@ -62,10 +62,35 @@
 import astropy.units as u
 import numpy as np
 
-__all__ = ["get_livetime_fraction"]
+__all__ = ["pileup_correction_factor", "get_livetime_fraction"]
 
+from stixpy.io.readers import read_subc_params
 
-def get_livetime_fraction(trigger_rate, *, eta=2.63 * u.us, tau=10.1 * u.us):
+
+def pileup_correction_factor():
+    r"""
+    Estimate the fractional area of a single large pixel compared with the rest of the detector group.
+
+    This is used in estimating the probability that when two photons are detected in the same group if
+    the first hits a given large pixel that the second hits a different pixel resulting in anti-coincidence rejection.
+    Otherwise if both hit the same pixel pileup will occur.
+
+    Notes
+    -----
+    This assumes uniform illumination over the detector pair
+    This ignores the possibility of the first hit being a small pixel
+    """
+
+    subc_str = read_subc_params()
+    large_pixel_area = (subc_str['L Pixel Xsize'] * subc_str['L Pixel Ysize'])[0]
+    detector_area = (subc_str['Detect Xsize'] * subc_str['Detect Xsize'])[0]
+    big_pixel_fraction = large_pixel_area/detector_area
+    prob_diff_pix = (2./big_pixel_fraction - 1.)/(2./big_pixel_fraction)
+
+    return prob_diff_pix
+
+
+def get_livetime_fraction(trigger_rate, *, eta=1.10 * u.us, tau=10.1 * u.us):
     """
     Return the live time fraction for the given trigger rate.
 
@@ -83,9 +108,7 @@ def get_livetime_fraction(trigger_rate, *, eta=2.63 * u.us, tau=10.1 * u.us):
     `float`, `float`, `float`:
         The live time fraction
     """
-
-    # TODO implement full calculation code this values was taken from IDL
-    beta = 0.94059104
+    beta = pileup_correction_factor()
 
     photons_in = trigger_rate / (1.0 - trigger_rate * (tau + eta))
     livetime_fraction = 1 / (1.0 + (tau + eta) * photons_in)

stixpy/calibration/tests/test_livetime.py

@@ -1,6 +1,11 @@
 import numpy as np
 
-from stixpy.calibration.livetime import get_livetime_fraction
+from stixpy.calibration.livetime import get_livetime_fraction, pileup_correction_factor
+
+
+def test_pileup_correction_factor():
+    pileup_factor = pileup_correction_factor()
+    assert pileup_factor == 0.9346590909090909
 
 
 def test_get_livetime():