Light Pollution Filters
Light pollution filters are special filters that permit or block certain wavelengths of light in an effort to improve contrast of deep sky objects. All filters will dim the view, but their purpose is to dim light pollution and sky glow more than the object of interest by trying to allow the light from the object of interest to pass more freely.
Filters break down into three categories:
- Broadband filters (also commonly called light pollution filters or deep sky filters)
- Narrowband or UHC (ultra high contrast) filters
- Line filters, such as Oxygen-III and Hydrogen-Beta filters
Broadband / Light Pollution Filters
Broadband filters are the most permissive, and seek to block common wavelengths of light pollution that come from pink or orange sodium vapor lights. These filters do not work well if light pollution is predominantly white LED, since most of the light from the white LED light will pass right through the filter.
Narrowband / UHC Filters
Narrowband or "UHC" filters attempt to "whitelist" only certain wavelengths rather than "blacklist" light pollution wavelengths. These filters have considerably higher contrast than broadband filters as a result, but they only work on emission nebulae since they permit only wavelengths commonly emitted by emission nebulae - Oxygen III and Hydrogen Beta.
Line filters are even more strict about what they permit than narrowband filters are. They allow transmission of only specific sepctral lines, such as Oxygen III at 501nm and 496nm wavelengths or Hydrogen Beta at 486nm wavelength. Thus line filters offer the highest possible contrast, but do so at the expense of the most view brightness. Certain nebulae emit light more strongly in the O-III or H-Beta part of the spectrum. For example, the ionized gas that acts as a backdrop behind the Horse Head Nebula emits light most strongly in the H-Beta part of the spectrum, and is very low contrast. A UHC filter is often not high enough contrast to show it, an O-III filter can block it, but an H-Beta filter can be just enough to reveal it.
All filters have two primary characteristics: bandwidth (how broad a range of the visible spectrum they allow), and transmission (what percentage of light they allow within the spectral range they permit. The highest quality filters have the tightest bandwidth and the highest transmission.
Bandwidth is a measure of the width of the spectrum allowed through by the filter, in nanometers. For example, the Astronomik UHC filter has a bandwidth of 24nm, while the Astronomik UHC-E (economy) filter has a bandwidth of 35nm. (2019 Nebula Filters Buyer's Guide).
The wider bandwidth of the UHC-E filter means it will have lower contrast, as it will permit more light beyond the boundaries of its target wavelengths. Conversely, the normal UHC filter does a better job of isolating only the wavelengths it wants to permit, so it has higher contrast as a result.
Transmission is the other important characteristic of a filter. It is desirable to pass as much of the isolated wavelengths of light through the filter as possible to ensure maximum brightness and contrast. Similarly, a filter should minimize the transmission of the blocked wavelengths of light as much as possible. It is common for cheaper filters to maximize transmission of the isolated wavelengths of light by also permitting too much of the blocked light to pass through. Thus it's important to understand not only the light transmission percentage of the desired wavelengths, but also the light transmission percentage of the undesired wavelengths.
Filters and Galaxies
Generally speaking, galaxies do not benefit from filters since galaxies are full spectrum objects and do not emit light in any one particular part of the spectrum. In some cases, the emission nebulae of other galaxies (such as NGC 604) may benefit from filters, but the galaxy itself will not. A broadband / light pollution filter in an area with predominantly sodium vapor lighting may give "cooler" spiral galaxies a slight contrast enhancement, but older "warmer" elliptical galaxies may suffer as a result of the filter blocking the warmer parts of the spectrum. UHC and line filters would be detrimental to galaxy observing.