A demonstration of the stack cross-correlation. Cas A spectra are stacked for Cα RRLs across a range of redshifts. The plot on the left shows the optical depth integrated at the central velocities at each redshift. Middle plot: integrated optical depth resulting from stacking the template spectrum. This was done for a redshift of z = 0.000158. We cross-correlate the left plot with the middle plot to obtain the stack cross-correlation on the right.