1 files changed, 78 insertions, 0 deletions
diff --git a/acispy/cosmo.py b/acispy/cosmo.py
new file mode 100644
index 0000000..c3d6096
--- /dev/null
+++ b/acispy/cosmo.py
@@ -0,0 +1,78 @@
+# Copyright (c) 2017 Weitian LI <liweitianux@live.com>
+# MIT license
+
+"""
+Cosmology calculator for Chandra ACIS.
+"""
+
+import math
+
+from astropy.cosmology import FlatLambdaCDM
+import astropy.units as au
+
+from .acis import ACIS
+
+
+class Calculator:
+    """
+    Calculate various quantities under a specific cosmology, as well
+    as some values with respect to Chandra ACIS detector properties.
+    """
+    def __init__(self, H0=71.0, Om0=0.27, Ob0=0.046):
+        self.H0 = H0  # [km/s/Mpc]
+        self.Om0 = Om0
+        self.Ode0 = 1.0 - Om0
+        self.Ob0 = Ob0
+        self._cosmo = FlatLambdaCDM(H0=H0, Om0=Om0, Ob0=Ob0)
+
+    def luminosity_distance(self, z, unit="Mpc"):
+        dist = self._cosmo.luminosity_distance(z)
+        return dist.to(au.Unit(unit)).value
+
+    def angular_diameter_distance(self, z, unit="Mpc"):
+        dist = self._cosmo.angular_diameter_distance(z)
+        return dist.to(au.Unit(unit)).value
+
+    def kpc_per_arcsec(self, z):
+        """
+        Calculate the transversal length (unit: kpc) corresponding to
+        1 arcsec at the *angular diameter distance* of z.
+        """
+        dist_kpc = self.angular_diameter_distance(z, unit="kpc")
+        return dist_kpc * au.arcsec.to(au.rad)
+
+    def kpc_per_pix(self, z):
+        """
+        Calculate the transversal length (unit: kpc) corresponding to
+        1 ACIS pixel (i.e., 0.492 arcsec) at the *angular diameter distance*
+        of z.
+        """
+        pix = ACIS.pixel2arcsec * au.arcsec
+        dist_kpc = self.angular_diameter_distance(z, unit="kpc")
+        return dist_kpc * pix.to(au.rad).value
+
+    def cm_per_pix(self, z):
+        """
+        Calculate the transversal length (unit: cm) corresponding to
+        1 ACIS pixel (i.e., 0.492 arcsec) at the *angular diameter distance*
+        of z.
+        """
+        return self.kpc_per_pix * au.kpc.to(au.cm)
+
+    def norm_apec(self, z):
+        """
+        The normalization factor of the XSPEC APEC model assuming
+        EM = 1 (i.e., n_e = n_H = 1 cm^-3, and V = 1 cm^3)
+
+        norm = 1e-14 / (4*pi* (D_A * (1+z))^2) * int(n_e * n_H) dV
+
+        This value will be used to calculate the cooling function values.
+
+        References
+        ----------
+        * XSPEC: APEC model
+          https://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/manual/XSmodelApec.html
+        """
+        da = self.angular_diameter_distance(z, unit="cm")
+        norm = 1e-14 / (4*math.pi * (da * (1+z))**2)
+        return norm