Merge pull request #125 from LSSTDESC/u/jrbogart/diffsky_sqrt

ensure np.sqrt doesn't have a negative argument

skycatalogs/objects/diffsky_object.py

@@ -48,16 +48,19 @@ def get_knot_size(self, z):
         """
         # Deceleration paramameter
         q = -0.5
-        # Angular diameter scaling approximation in pc
-        dA = (3e9/q**2)*(z*q+(q-1)*(np.sqrt(2*q*z+1)-1))/(1+z)**2*(1.4-0.53*z)
-        # Using typical knot size 250pc, convert to sigma in arcmin
-        if z < 0.6:
-            return 206264.8*250/dA/2.355
-        else:
+
+        if z >= 0.6:
             # Above z=0.6, fractional contribution to post-convolved size
             # is <20% for smallest Roman PSF size, so can treat as point source
+            # This also ensures sqrt in formula below has a
+            # non-negative argument
             return None
 
+        # Angular diameter scaling approximation in pc
+        dA = (3e9/q**2)*(z*q+(q-1)*(np.sqrt(2*q*z+1)-1))/(1+z)**2*(1.4-0.53*z)
+        # Using typical knot size 250pc, convert to sigma in arcmin
+        return 206264.8*250/dA/2.355
+
     def get_knot_n(self, rng=None):
         """
         Return random value for number of knots based on galaxy sm.