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I observed color in a nebula for the first time this past weekend when looking at the Ring Nebula through my Orion XT8. I noticed a bit of red on the outer edges. I placed my Orion Ultrablock Narrowband filter on to attempt to observe more detail when I realized that it washes the field of view out in green, so I could no longer see the red.

Is this normal? Are there any other narrowband filters that preserve color?

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up vote 7 down vote accepted

By definition, a narrow band filter is exactly that, a narrow band. That means that it only lets light of a very small wavelength range through the filter. Since wavelength corresponds to color, all narrow band filters are only going to let through a very narrow color range and will "tint" the image accordingly.

In your case, I suspect the filter you used is centered on the [OIII] lines which are in the green part of the spectrum and so the filed appeared green as that is the only color that would get through. If you were using a narrowband H-alpha filter, it would have all appeared red as that is in the far red portion of the spectrum at 6563 Aangsroms.

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According to Orion's website (…), it passes H-beta and ionized oxygen wavelengths through and blocks everything else. Does that correspond to the green that I'm seeing? – James Skidmore Jun 2 '11 at 15:07
Yep, Ionized Oxygen is the [OIII] line at 500.7 nm and H-beta is at 486 nm right in the blue-green part of the spectrum. Blue is at 475 and green at 510 so a filter that lets in 485-505 is going to definitely have a greenish cast to it. – dagorym Jun 2 '11 at 15:15
Excellent explanation. – James Skidmore Jun 2 '11 at 15:17

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