Add some useful constants

constants.py

@@ -0,0 +1,21 @@
+
+'''
+Useful numbers to have handy
+'''
+
+import astropy.units as u
+
+distance = 840 * u.kpc
+
+
+def ang_to_phys(ang_size, distance=distance):
+    '''
+    Convert from angular to physical scales
+    '''
+    return (ang_size.to(u.rad).value * distance).to(u.pc)
+
+
+co21_freq = 230.538 * u.GHz
+
+co21_mass_conversion = 6.7 * (u.Msun / u.pc ** 2) / (u.K * u.km / u.s)
+beam_eff_30m_druard = 56 / 92.