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+# Copyright (c) 2017 Weitian LI <liweitianux@live.com>
+# MIT license
+
+"""
+Chandra ACIS utilities
+"""
+
+import math
+
+
+class ACIS:
+    """
+    Chandra ACIS detector properties and utilities.
+
+    References
+    ----------
+    [1] CIAO - dictionary - ACIS (advanced camera for imaging and spectroscopy)
+        http://cxc.harvard.edu/ciao/dictionary/acis.html
+    [2] CIAO - Dictionary - PI (pulse invariant)
+        http://cxc.harvard.edu/ciao/dictionary/pi.html
+    """
+    # Pixel size
+    pixel2arcsec = 0.492  # [arcsec]
+    # Number of channels
+    nchannel = 1024
+    # Channel energy width
+    echannel = 14.6  # [eV]
+
+    @classmethod
+    def energy2channel(self, energy):
+        """
+        Convert energy [eV] to channel number.
+        """
+        return math.floor(energy/self.echannel + 1)