I've never heard of a pure 604nm, but I'm always learning something new, and REO is constant in that they are unique. Relatively speaking to Uni, MG, Hughes, etc.
I have a 612nm tube that does the 604nm line as well as ~609 and 594. The only way I can think of to get it pure is like you said, with an external prism similar to the REO-LSTP's. I tried to setup an external cavity tunable rig with my REO 4 line, but couldn't kill the original resonation between the intended mirrors. So "tuning" led to either a bright 612nm output, and several 594nm spots, or a bright orange spot, with several other orange spots. I could also tune to the middle with yellow on one side, and orange on the other.
In the end, did not work. However it was using a prism, and a yellow -> green broadband HR. So perhaps better results could be had with a yellow -> red optic. Will try it tonight.


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HeNes to Krypton, all gas will be considered 
