Light Pollution and Nebula Filters
Light pollution filters and nebula filters are used to enhance the view of deep sky objects. They both work in a similar manner, but are intended for slightly different purposes. In fact, nebula filters really are just a type of light pollution filter. These filters are normally designed to thread into the bottom of an eyepiece and so are 1.25" or 2" in diameter, depending on the eyepiece being used. Some light pollution filters are also designed to be threaded onto the back cell of a Schmidt-Cassegrain telescope, the idea being that you do not have to remove the filter when changing eyepieces. This design tends to less popular because the filter is not always used depending on the object being observed.
How Light Pollution Filters Work
Light pollution filters work by selectively blocking certain wavelengths of light while transmitting others. Many sources of light pollution emit their light at specific wavelengths (usually in the yellow and orange parts of the visible spectrum). This means it is possible to construct a filter to block only those certain colors of light, while passing all the other light. This makes the background (which is lit by artificial skyglow) darker, thereby increasing the contrast of the object being viewed.
Above: The visible spectrum, with typical light pollution sources shown
Above: A light pollution filter transmits all light except that given off by sources of light pollution
Broadband vs. Narrowband
The main difference between light pollution filters and nebula filters is in the wavelengths of light transmitted. General light pollution filters try to transmit as much light as possible by blocking only the main sources of light pollution. This is done because most deep sky objects, such as star clusters and galaxies, emit their light at all wavelengths. Blocking too much light blocks the light coming from the object that is being viewed, which doesn't help much. Light pollution filters are termed broadband filters because the portion of the spectrum they transmit is very wide.
Narrowband filters, on the other hand, allow only a very small portion of the spectrum to be transmitted. This is possible because certain celestial objects (namely nebulae) give off their light as certain specific wavelengths. Therefore, a narrowband filter can isolate just one particular wavelength of interest while blocking all other sources of light. Most nebulae give off the majority of their light at a wavelength of 656.3nm, which is in the red portion of the spectrum. This light is emitted by hydrogen gas, which is the primary constituent of nebulae (and the rest of the universe, actually). However, the human eye, when dark-adapted for viewing at night, cannot see red light. This makes a filter for this wavelength useless.
However, nebulae also give off much light at a wavelength of 500.7nm, in the green part of the spectrum. This is light given off by oxygen gas, also common in nebulae. Conveniently, the human eye at night is most sensitive to 500nm light, making a filter for this wavelength particularly useful for observing nebulae. Most nebula filters have their light transmission curve (see below) centered on this wavelength. This wavelength of light is given off by OIII emission and these filters are usually called OIII filters (although not all narrowband nebula filters are OIII filters).
Above: Wavelengths of light given off by excited atoms within a typical nebula. Most of the light comes from hydrogen (H-alpha, H-beta) and oxygen (OIII), with some coming from sulfur (SII), helium (HeII), and nitrogen (NII).
Above: Light transmission of a narrowband OIII filter. Note that only light from excited oxygen gas is transmitted while all other light is blocked.
Using Broadband Light Pollution Filters
Broadband light pollution filters are designed to be used on any type of deep-sky object. Unlike narrowband filters, broadband filters work well on galaxies and star clusters as well as nebulae. These filters work best when the observer is dark-adapted. It should be remembered that this type of filter does not make the objects being viewed brighter. In fact, it makes them dimmer. But it makes the sky background much dimmer yet, increasing the contrast of the objects being observed. Some objects tend to respond better to light pollution filters, with faint galaxies often benefiting quite a bit from the use of such a filter.
Light pollution filters are not necessary when viewing bright objects such as the moon and planets, since the brightness of these objects allows them to be seen very easily even in very light-polluted skies. Also, bright star clusters may not benefit very much from the use of the light pollution filter since they stand out quite well already.
Broadband filters can also be used for astrophotography, allowing images to be captured from somewhat light-polluted locations.
Using Narrowband Nebula Filters
Narrowband filters work only on emission nebulae; that is, any nebula emitting its own light from excited gas atoms. This means reflection nebulae, which shine by reflected starlight, do not benefit from the use of a nebula filter. This includes objects such as the Pleiades, the northern part of the Trifid Nebula, and NGC1977 just north of the Great Orion Nebula. However, objects like the Orion Nebula itself, the southern half of the Trifid, and most other large nebulae look exceptional with a nebula filter.
More so than with a light pollution filter, it is critical that the observer be dark adapted before using a narrowband nebula filter. This does not mean the filter will not work from a light-polluted area, just that you need to avoid local bright lights which will prevent dark adaptation. It is best to spend 15-20 minutes outside in the dark before trying to use a nebula filter. The wait is well worth it, as the difference between some objects with and without the filter is like night and day.
The table below lists objects which benefit greatly from the use of the common OIII nebula filter and some common nebulae which are not enhanced by an OIII filter.
Good Objects for OIII Filter
Bad Objects for OIII Filter
North America Nebula