Exact optics: a unification of optical telescope design

A strong emission line at 2.8935 μm discovered by Rubin et al. in an ISO SWS02 spectrum of the Orion Nebula is identified as the     multiplet of O  i . Line formation is due to de-excitation cascades following UV-pumping of high ³S^o and ³D^o terms and occurs in the O  i zone immediately behind the hydrogen ionization front. This cascade mechanism also accounts for permitted O  i triplet lines in the optical spectrum of the Nebula, as shown by Grandi. An escape probability treatment of the O  i cascades accounts for the strength of the λ 2.89-μm line and suggests interesting diagnostic possibilities for the optical lines.