When it comes to observing some deep space objects, there's always a little trick of the trade that makes a huge difference in what you see. In this case, it's knowing what filter to use when observing nebulae.
If you have routinely dark skies, purchasing a nebula filter might seem like an unnecessary expense. After all, you have a good, dark location and you're able to see the nebula of your choice, so what difference could it possibly make? This is something I told myself time and time again, and I was getting perfectly satisfactory views from my smaller telescopes, and really good ones from large aperture, so why bother to filter? The reason is... you can see so much more.
Even if you live and observe in a rural area, chances are that you are getting at least a certain amount of "sky glow," a form of light pollution that eradicates deep sky viewing from urban locations and shows its ugly head as a glow dome when you live in the country.
There are very specific filters which help improve the view, and they enhance nebulae - but an OIII filter is like a secret weapon for special occasions. Orion's O-III eyepiece filter transmits close to 90% of the light corresponding to the two Oxygen-III spectral lines at 496nm and 501nm while completely blocking virtually all other wavelengths. It rocks when it comes to planetary nebulae, some supernovae remnants and diffuse emission nebulae.
If this doesn't make any sense to you, then let's take a look at the various filters. A light pollution filter is a form of nebula filter. It will make the view much better by adding contrast, but remember... it will also decrease the amount of light you see from stars, star clusters, galaxies, etc. Why? Just like streetlights, security lights and house lights, stars emit light at a specific wavelength and this type of filter is blocking that wavelength, with makes the nebula "pop" against a much blacker background.
The next consideration is broadband or narrowband. The broadband filter will help you to see more detail in certain reflection nebulae, but it is the narrowband filter which produces the most dramatic results. The Orion UltraBlock is a narrowband filter and produces spectacular views, but you want more!
This is where the Orion Oxygen III filter comes into play. It isn't a light pollution filter so much as it is a filter which only allows certain wavelengths of light to pass through. When you use this filter, the visual doubly ionized Oxygen III portion of the spectrum is transmitted. It works by rejecting most other light and this will cause specific nebula to appear brighter and more detailed since they are highlighted against a blacker background of space.
If you have ever wanted to see details in objects such as the Veil Nebula, the Helix Nebula, the Dumbbell Nebula, the Ring Nebula, the Eskimo Nebula and those elusive small planetary nebulae, then this is the filter for you. However, remember this is an aggressive filter and since it is blocking a huge amount of light, you're going to want to use it on a scope which gathers a huge amount of light. It's not a filter for everybody, but it's sure a winner if you hanker for a very special view!
Tammy is a professional astronomy author, President Emeritus of Warren Rupp Observatory and retired Astronomical League Executive Secretary. She's received a vast number of astronomy achievement and observing awards, including the Great Lakes Astronomy Achievement Award, RG Wright Service Award and the first woman astronomer to achieve Comet Hunter's Gold Status.