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diff --git a/fg21sim/extragalactic/pointsources/psparams.py b/fg21sim/extragalactic/pointsources/psparams.py
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+# Copyright (c) 2016 Zhixian MA <zxma_sjtu@qq.com>
+# MIT license
+
+"""
+Basic parameters to be used in point sources simulation.
+
+References
+----------
+[1] Angular diameter distance,
+     https://en.wikipedia.org/wiki/Angular_diameter_distance
+[2] FlatLambdaCDM,
+     {http://docs.astropy.org/en/stable/api/astropy.cosmology.
+     FlatLambdaCDM.html#astropy.cosmology.FlatLambdaCDM}
+"""
+
+from astropy.cosmology import FlatLambdaCDM
+
+
+class PixelParams():
+    """
+    A class to transform cosmology distance to angles or pixels.
+    Parameters
+    ------------
+    H0: float
+        The hubble constant at z = 0, whose unit is km/s/Mpc
+    Om0: float
+        The total matter density.
+    ang_res: float
+        Angular resolution, i.e. degree per pixel. (May be useless)
+    ang_total: list
+        Total angles of the simulated sky region,whose unit is degree (\deg)
+    z : float
+        Redshift
+    scale: float
+        The real object scale.
+    Example
+    ------------
+    >>> PixelParams = PixelParams(img_size=(1024,1024),ang_total=(5,5))
+    """
+
+    # Hubble constant at z = 0
+    H0 = 71.0
+    # Omega0, total matter density
+    Om0 = 0.27
+    # Redshift
+    z = 0.0
+    # Cosmology calculator
+    cosmo = 0.0
+    # angular diameter distance, [Mpc]
+    dA = 0.0
+    # angular resolution
+    ang_res = 0.0
+
+    def __init__(self, z=0.0):
+        self.z = z
+        self.cosmo = FlatLambdaCDM(H0=self.H0, Om0=self.Om0)
+
+        # angular diameter distance, [Mpc]
+        self.dA = self.cosmo.angular_diameter_distance(self.z).value
+
+    def get_angle(self, scale=1.0):
+        """
+        Input real object scale, and output the respect observed
+        angle, and pixels.
+        """
+        ang = scale / self.dA  # [rac]
+
+        return ang
+
+    def get_scale(self, ang=1.0):
+        """
+        Input real observed scale, and output the respect
+        real object scale, and pixels.
+        """
+        scale = ang * self.dA  # [Mpc]
+
+        return scale