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diff --git a/acispy/acis.py b/acispy/acis.py
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+++ b/acispy/acis.py
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+# Copyright (c) 2017 Weitian LI <liweitianux@live.com>
+# MIT license
+
+"""
+Chandra ACIS utilities
+"""
+
+import math
+import subprocess
+import re
+
+
+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)
+
+    @classmethod
+    def get_type(self, filepath):
+        """
+        Determine the Chandra ACIS type (``I`` or ``S``) according the
+        active ACIS chips.
+
+        Parameters
+        ----------
+        filepath : str
+            Path to the input FITS file
+
+        Returns
+        -------
+        acis_type : str
+            ``I`` if ACIS-I, ``S`` if ACIS-S;
+            otherwise, ``ValueError`` raised.
+        """
+        subprocess.check_call(["punlearn", "dmkeypar"])
+        detnam = subprocess.check_output([
+            "dmkeypar", "infile=%s" % filepath, "keyword=DETNAM", "echo=yes"
+        ]).decode("utf-8").strip()
+        if re.match(r"^ACIS-0123", detnam):
+            return "I"
+        elif re.match(r"^ACIS-[0-6]*7", detnam):
+            return "S"
+        else:
+            raise ValueError("unknown chip combination: %s" % detnam)
+
+    @classmethod
+    def get_chips_str(self, filepath, sep=":"):
+        """
+        Return a string of the chips of interest according to the
+        active ACIS type.
+
+        Parameters
+        ----------
+        filepath : str
+            Path to the input FITS file
+        sep : str, optional
+            Separator to join the chip ranges, e.g., 0:3, 0-3
+
+        Returns
+        -------
+        chips : str
+            ``0:3`` if ACIS-I, ``7`` if ACIS-S;
+            otherwise, ``ValueError`` raised.
+        """
+        acis_type = self.get_type(filepath)
+        if acis_type == "I":
+            return sep.join(["0", "3"])
+        elif acis_type == "S":
+            return "7"
+        else:
+            raise ValueError("unknown ACIS type")