Collisionally excited filaments in HST H$alpha$ and H$beta$ images of HH~1/2

We present new H$alpha$ and H$beta$ images of the HH~1/2 system, and we find that the H$alpha$/H$beta$ ratio has high values in ridges along the leading edges of the HH~1 bow shock and of the brighter condensations of HH~2. These ridges have H$alpha$/H$beta=4to 6$, which is consistent with collisional excitation from the $n=1$ to the $n=3$ and 4 levels of hydrogen in a gas of temperatures $T=1.5to 10times 10^4$~K. This is therefore the first direct proof that the collisional excitation/ionization region of hydrogen right behind Herbig-Haro shock fronts is detected.