Comparisons of δτ_SN between MDI (blue) and HMI (red) without center-to-limb correction as a function of latitude for different ranges of travel distances. A P-angle correction of 0.21° is applied to the MDI observations. The range of travel distances and their corresponding radii of the lower turning point from the ray approximation are indicated in each panel; the shallowest is shown at the top and the deepest at the bottom. The data values are binned every 7.2° in latitude. The error bars give the standard error of the mean in each binning interval.