Allow template correlation for spectrum with no uncertainty

specutils/analysis/correlation.py

@@ -253,8 +253,12 @@ def _normalize(observed_spectrum, template_spectrum):
         A float which will normalize the template spectrum's flux so that it
         can be compared to the observed spectrum.
     """
-    unc = observed_spectrum.uncertainty.represent_as(StdDevUncertainty)
-    num = np.nansum((observed_spectrum.flux*template_spectrum.flux)/(unc.array**2))
-    denom = np.nansum((template_spectrum.flux/unc.array)**2)
+    if hasattr(observed_spectrum, "uncertainty") and observed_spectrum.uncertainty is not None:
+        unc = observed_spectrum.uncertainty.represent_as(StdDevUncertainty)
+        num = np.nansum((observed_spectrum.flux*template_spectrum.flux)/(unc.array**2))
+        denom = np.nansum((template_spectrum.flux/unc.array)**2)
+    else:
+        num = np.nansum(observed_spectrum.flux*template_spectrum.flux)
+        denom = np.nansum(template_spectrum.flux**2)
 
     return num/denom

specutils/tests/test_correlation.py

@@ -50,6 +50,17 @@ def test_autocorrelation():
     maximum = np.argmax(corr)
     assert maximum == midpoint
 
+    # Test that this works without uncertainty
+    spec1 = Spectrum1D(spectral_axis=spec_axis,
+                       flux=flux1,
+                       velocity_convention='optical',
+                       rest_value=5020.*u.AA)
+
+    corr, lag = correlation.template_correlate(spec1, spec2)
+
+    assert corr.unit == u.dimensionless_unscaled
+    assert lag.unit == u.km / u.s
+
 
 def test_correlation():
     """