# What objects look best in an O-III filter?

I've heard that an O-III (Oxygen III) filter is great for planetary nebulae.

Is this true for all planetary nebulae, or just some or most?

What other target types are often improved with an O-III filter?

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The dominant OIII emission line is the $^3P-^1D_2$ line, which is at 2.51 eV. If you want to see a pretty picture, you'll want a wide field of emission of this line, which would mean that you need a gas cloud which is exciting the OIII line thermally. The thermal energy required to excite the 2.51 eV line is around 8,000 K, so you need gas clouds with that sort of temperature.