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Colour me (pun intended) puzzled - given that an OIII filter is designed to let through only a very narrow band of visible light that is definitely *not* red...

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Well, we might need a bit of history here. "In the beginning...." (oops...sorry, went back too far), Lumicon created the OIII filter to see if it could improve on the performance of its UHC design. The early versions were the single passband design (bluish-green transmission of only the OIII lines), and they worked pretty well. However, as production rolled on over a few years, for some reason, a change occurred (probably to make production easier). A broad red "leak" passband appeared in the newer OIIIs, which also allowed me to see some reds in bright objects like M42. For most observer's, the red response of their eyes is almost nil, so the red passband didn't really hurt anything anyway. One night, a friend of mine and I were observing together with our two filters when M42 rose up over the eastern horizon. We turned our scopes towards it, and in my friend's OIII, we saw no hint of red, while in my filter, the reds were faintly but plainly visible (both in 10 inch Newtonians). This puzzled us until we held up our filters to a red light and discovered the difference (ie: his older filter had no red transmission). We contacted Jack Marling at Lumicon and he wanted both filters back for inspection. I recommended that he leave the red passband in, and for a number of years, that is what happened. However, Jack sold the company, it eventually went under, and has now re-emerged as a division of Parks. The current OIII filter in the 2" model appears to now no longer have the red "leak" passband, as the one I bought recently shows no sign of it in my spectroscope. So, those with the *really old* or *really new* filters probably won't see red through the Lumicon OIII filter. Clear skies to you.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info

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