A Guide to Light-Pollution Filters for Visual Astronomy
- 1. Overview
- 2. How Light Pollution Filters Work
- 3. Types of Light-Pollution Filters
- 3.1 Broadband Nebula Filters
- 3.2 Narrowband Filters
- 3.3 Line Filters for Visual Observation
- 4. Filter Bandwidths, Exit Pupils, and Image Brightness
- 5. Recommendations for Light Pollution Filters
- 6. Summary
- 7. Further Reading
"Always avoid the neighborhood of any bright light. Electric lights in particular are an abomination to stargazers." These prophetic words by the astronomy writer Garrett Serviss in his 1896 book Astronomy with an Opera Glass continue to ring true. All modern cities are now enveloped in a dome of scattered light pollution to the extent that entire constellations are rendered invisible by the glow of street and building lights, and many city dwellers are entirely unable to see the Milky Way.
While light pollution is a constant frustration,modern technology can also give back, at least a little, to frustrated stargazers.Light pollution filters, which are sophisticated glass optical filters with dozens of layers of carefully deposited dielectric coatings,can reduce the effects of some types of urban lighting for visual observation of some deep-sky objects, especially those such as nebulae that emit light at discrete wavelengths that are distinct from those of many forms of light pollution. For this reason, they are sometimes called nebula filters, and they work particularly well to improve the contrast of emission nebulae, planetary nebula, and supernova remnants. They are not a cure-all for light pollution, and they don't replace truly dark skies, but when you select the right filters for your observing interests, location, and telescope, they can be an enormous aid in viewing detail in many celestial objects.
This guide to light pollution filters for visual observers will help you understand the pros and cons of each of the three types of light pollution filter so you can make the best choice for your observing interests and equipment.
Much, though not all, of urban light pollution is caused by mercury and sodium street lights (Figure 1). These lighting systems use lamps in which electric current passes through trace amounts of either of these metals, causing their atoms to emit bright light at specific wavelengths. Mercury-vapor lamps emit light at 405nm, 436nm, 546nm, and 578nm in the violet, blue, and yellow region of the visible spectrum. High-pressure sodium-vapor lamps emit light strongest at 570nm, 583nm, 600nm, and 617nm,and low-pressure sodium vapor lamps emit light at 589nm, all mostly in the yellow region of the visible spectrum.This light emanates upward into the urban sky and illuminates dust and vapor in the atmosphere, resulting in a brightened background sky that can overwhelm the view of many deep-sky objects, especially those with low surface brightness.And nature provides another type of light "pollution", a faint light generated in the upper atmosphere by sodium atoms at 589nm. This airglow is only a problem when observing at very dark locations, but it can reduce contrast of some deep-sky objects with very low surface brightness.
The good news is that optical engineers can relatively easily make sophisticated but relatively affordable filters to block discrete wavelengths from sodium and mercury lamps and from airglow. The bad news is that stars also emit light at these wavelengths, and across a broad band of wavelengths of light visible to the eye, which means that a filter that blocks light pollution from mercury and sodium lamps also blocks starlight. So there is no improvement in contrast when visually observing stars, star clusters, and galaxies with such a filter.
Many nebulae, however, are different. They emit light from excited hydrogen and oxygen atoms at discrete wavelengths. Hydrogen emits green light at 486nm (H-beta) and red light at 656nm (H-alpha). Doubly-ionized oxygen atoms (which atomic scientists call 'OIII', or 'oh-three') emit green light at 496nm and 501nm. These wavelengths are well away from the wavelengths sodium and mercury light, so it is possible to engineer optical filters than pass light from nebulae while blocking light from many street lamps (Figure 2). Such filters, in various forms, have been on the market for amateur astronomers since the early 1980s.
It's important to understand that light pollution filters do not increase the brightness of nebulae. In fact, because they have some loss even at "good" wavelengths, they make nebulae dimmer. But they improve the contrast of a nebula against the artificially brightened background sky, making it easier to see detail, especially around the fainter periphery of a nebula.In some cases, the effect is quite remarkable. A nebula that is all but invisible, or barely visible, against a bright background sky can pop into view. Or a nebula that is visible to some degree without a filter becomes much larger and richly detailed. Nebula filters are by no means a replacement for observing under dark skies, but they can help when observing many emission nebulae (such as Orion or the Lagoon), planetary nebulae (like the Dumbbell and Ring), and supernova remnants (like the Veil Nebula). They can even help improve contrast of many nebulae when observing under very dark skies.
When using these filters, it's also critical to keep stray light out of your eyes because such light can reduce the sensitivity of your retina and decrease the size of pupil. They are not a replacement for clear, dark skies.
Light pollution filters are not intended for use on the Moon and planets, and they are absolutely not designed to work as solar filters. Nor can they help with blue-white reflection nebula which reflect broadband starlight such as the nebulosity around the stars of the Pleiades or M78 in the constellation Orion.
And despite the advantages they offer for visual astronomers beleaguered by light pollution, nebula filters can't filter all types of light pollution. They are not effective at improving the contrast of nebula by removing broadband light pollution from LED lights, for example, which are becoming widely deployed in many cities, or from incandescent bulbs.That's because these light sources emit light in a continuous spectrum, including at the discrete wavelengths at which nebulae emit light.
Essentially all light-pollution filters for visual observation are one of three main types: broadband, narrowband, and line filters.They all block the light from sodium and mercury vapor lamps and broadband light pollution to some degree, but they differ by which wavelengths of light they pass from nebulae and how much light pollution is excluded.
All light-pollution filters for visual observation are mounted in threaded metal holders that are inserted into the barrel of an eyepiece or star diagonal. Most manufacturers make them in 1.25" and 2" sizes. Let's have a look at the three types of filters.
Broadband nebula filters generally pass light in a 50-70nm band in the blue-green part of the spectrum, including blue-green light from H-beta (486nm) and OIII (496nm and 501nm), while blocking light from sodium and mercury vapor lamps (Figure 3). Many such filters also pass light in the red portion of the spectrum including H-alpha light at 656nm, although the human eye is not particularly sensitive at this wavelength. Because their passbands are in the blue, green, and red, all of these filters appear somewhat greenish-purple to the eye.
The large bandwidth of broadband nebula filters means they also pass a significant amount of continuous light from stars, so stars still appear reasonably bright and are naturally colored with these filters. Unfortunately, so do sources of broadband light pollution. For this reason, broadband light pollution filters work best in suburban and rural skies that have relatively little light pollution to begin with.They work less well for visual observation under conditions of moderate to severe light pollution.
Some observers note a slight improvement even when viewing diffuse stellar objects like galaxies in darker skies with the broadband filters, but the improvement is generally modest.
There are many incarnations of broadband nebula filters on the market. These filters do not have identical specifications, and they typically differ in the width of the passband in the blue and green part of the spectrum, and in the quality of their coatings and glass substrates. Table 1 gives a summary of commonly available broadband nebula filters.
|Filter Manufacturer||Filter Brand Name||Comments|
|Lumicon||Deep Sky Filter||One of the first on the market; high quality with long track record|
|Celestron||Light Pollution Reduction (UHC-LPR Filter)|
|Baader||UHC-S Filter||Despite the UHC name, this is a broadband filter.|
|Baader||Moon and Skyglow Filter||Uses neodymium glass instead of multi-layer coatings.|
|Astronomik||City Light Suppression (CLS) Filter|
|Optolong||City Light Suppression (CLS) Filter||Also available as clip-in holders for DSLR cameras.|
While broadband filters offer modest improvement in nebulae for visual observers, these filters can be much more useful for imaging. That's because CCD and CMOS cameras are much more sensitive to red light at the H-alpha wavelength than the human eye, so they produce brighter images of objects like stars and nebulae that emit H-alpha light.
But for visual use, which broadband filter works best? Experienced visual observers suggest that there is no one broadband filter that is optimal for every celestial object, for all telescopes and eyepieces, and in all sky conditions. What's more, the visual performance of each filter can be somewhat subjective. All filters will show some improvement for visual observation of nebulae, especially in suburban and darker skies, but most are often overwhelmed by light pollution in urban skies. More expensive broadband filters, on average, tend to have better rejection of out-of-band light, better coatings, and higher quality glass substrates.
To improve visual contrast of nebulae, especially skies that endure moderate to heavy light pollution, amateur astronomers turn to filters with smaller bandwidths that provide greater rejection of unwanted light. These so-called narrowband filters have a bandwidth of about 20-30nm, usually only in the blue-green region of the spectrum. They pass light from H-beta (at 486nm) and OIII (at 496nm and 501nm) where the human eye is most sensitive, but they usually do not pass H-alpha and other red light where it is not.
Figure 6 shows the spectrum of a Lumicon UHC filter, which is typical of narrowband nebula filters for visual use. Because these filters pass only blue and green light, they usually have a blue-green color (see Figure 7), and stars appear blue-green and considerably fainter than without the filter.
Many manufacturers call their narrowband visual light pollution filters "UHC" (for Ultra-High Contrast). This is somewhat confusing because other manufacturers use the term UHC for their broadband filters.
Table 2 shows some of the most common filter offerings of narrowband nebula filters (for visual observing) on the market today. Each differs in the overall bandwidth and material and coating quality. Some narrowband filters have a passband that is almost as large as broadband filters, while some are nearly as narrow as line filters (see the next section). A filter with a narrower bandwidth usually gives a better visual view of most nebulae because it rejects more unwanted light pollution, but the performance does depend on the aperture and magnification of the telescope, the sky, and the subjective experience of the observer. In general, the more light polluted the sky, the narrower the light pollution filter you should use. But visual observing is a subjective experience, and the exact bandwidth of the narrowband filter of the filter is not critical.
|Filter Manufacturer||Filter Brand Name||Comments|
|Lumicon||UHC Filter||A relatively narrow band gives good performance in most situations.|
|Orion||UltraBlock Filter||Similar performance to Lumicon.|
|Optolong||UHC Filter||Narrower than the OptolongCLS, but much wider than Lumicon UHC or Orion UltraBlock; also passes H-alpha.|
|Explore Scientific||UHC Filter||Relatively wide band for this class of filter|
|Astronomik||UHC Filter||Very narrow band in blue green but also passes H-alpha in red. Good for visual and imaging.|
|Astronomik||UHC-E Filter||A wider and less expensive version of the Astronomik UHC filter|
|Tele Vue||Nebustar UHC||Passes H-beta and OIII|
Line filters, as their name suggests, pass only one or two spectral lines emitted by a particular element or ion. They reject even more broadband light pollution than narrowband filters.Line filters can be used in severely light polluted skies to allow an observer to see some nebulae that would be otherwise washed out by bright light-polluted background sky, and they can be used even in darker skies to tease out detail and improve contrast when used under the right circumstances.
The most common narrowband filter for visual observation of nebulae is the OIII filter, so called because it only passes the two blue-green wavelengths emitted by doubly-ionized oxygen at 496nm and 501nm (Figure 8). These filters have a passband just 10-15nm wide, and they do not pass the 486nm spectral line from H-beta. OIII filters tend to work best on planetary nebulae and some supernovae remnants, especially the Veil Nebula, though they do help with some emission nebulae as well. These filters are made by Optolong, Celestron, Baader, Lumicon, Astronomik, and Tele Vue Optics among others.
There are line filters with passbands narrower than 10-15nm, but such filters are mostly intended for imagers, not visual observers.
Because they pass so little light, especially starlight, images of stars in these filters are very dim and colored blue-green. The brightness of the background sky is reduced, but so is the brightness of the nebula itself. For this reason, some vendors and amateur astronomers suggest that OIII and other narrowband filters should only be used with telescopes of 8" aperture or larger to provide a sufficiently bright image. But this is not necessarily true. OIII filters can be used with smaller scopes, especially fast refractors, to improve the view of extended nebulae as long as the exit pupil of the eyepiece used is sufficiently large (more about this in the next section).
Another type of line filter, hydrogen-beta filters, passonly 486nm emittedby hydrogen atoms in a nebula in a band about 10nm wide (Figure 9). These are highly specialized filters, the narrowest and most spectrally selective of all visual nebula filters, and useful for seeing unusual and faint objects like the Horsehead, California, and Cocoon nebulae with large telescopes. While they don't have wide application, these filters are useful if you want to try to see these nebulae visually with a moderate-to-large sized telescope.They are made by many manufacturers including Lumicon, Optolong, Astronomik, Tele Vue Optics, and Thousand Oaks.
Note: H-alpha filters, which pass nebula light at 656nm, are not useful for visual observation of nebulae because the human eye is not sufficiently sensitive at this wavelength. However, these filters are essential tools for night sky astrophotography because CCD and CMOS cameras are sensitive at this wavelength.
Like many nebulae, comets also emit light at discrete wavelengths. As a comet approaches the Sun, frozen compounds in the nucleus are sublimated and energized by the Sun's light. Several atoms, ion, and molecules from a comet emit light in the blue-green region of the spectrum, which is why the coma and ion tails of comets appear blue-green in images and sometimes visually. Some filter vendors, most notably Lumicon, make a specialty filter that passes blue-green light at OIII (496 nm and 501 nm) and light from the Swan Band of diatomic carbon at 511nm and 514nm in the green. The overall passband of the Lumicon comet filter is about 25nm.
A comet filter is a useful tool to help you see more detail in the ion tails of comets and fine detail and structure such as jets in the coma. These filters can also reveal the coma of very faint comets that are otherwise difficult to see. They do not help reveal much detail in the dust tails of comets, however, since these structures shine from reflected white light from the Sun.
Whether you choose a broadband filter, a narrowband filter, or a narrower line filter for visual observation, you must keep in mind the interplay between the brightness of the background sky, the brightness of the nebula, and your eye.
In practical terms, each type of filter works best with a particular range of exit pupil for the eyepiece you use with your telescope and the filter. If the exit pupil of the eyepiece is too small with a particular filter, which is the case for short focal-length eyepieces that give higher magnifications, then the background sky becomes too dark and the surface brightness of the nebula too low to allow for easy visual detection. If the exit pupil is too large (and the magnification too low), then the background sky becomes too bright and there is insufficient contrast with the nebula.
(NOTE: The exit pupil of an eyepiece when used with a particular telescope is simply the focal length of the eyepiece divided by the focal ratio of the telescope. For example, an eyepiece with a 24mm focal length uses with a telescope of focal ratio f/6 gives an exit pupil of 4mm. The maximum useful exit pupil is about 7mm, the maximum dilation of the pupil of the human eye under dark-adapted conditions.)
Lumicon has made recommendations for exit pupils for a number of their filters, and these recommendations carry over to filters from other manufacturers with similar characteristics:
- For broadband filters such as the Lumicon Deep-Sky filter or Optolong CLS filter, the exit pupil should range from 0.5mm to 2mm in bright urban sky and 1mm to 4mm in dark skies
- For narrowband filters such as a Lumicon UHC filter or other UHC filters, the exit pupil should range from 1mm to 4mm in bright urban sky and 2mm to 6mm in dark skies
- For OIII line filters, the exit pupil should range from 2mm to 5mm in bright urban sky and 3mm to 7mm in dark skies
- For H-beta line filters, the exit pupil should range from 3mm to 7mm in bright urban sky and 4mm to 7mm in dark skies
The upshot is that for a narrower bandwidth, a greater exit pupil is required, which means a lower magnification results. In some cases, OIII and H-beta filters can be used with the unaided and completely dark adapted eye to improve the contrast of some large, extended nebulae such at the North America or California nebulae.
Example: An observer has a 127mm f/7 refractor. To observe with a narrowband UHC filter with a 20mm bandwidth that passes H-beta at 486nm and both OIII lines at 496nm and 501nm, the exit pupil should be 1mm to 4mm in dark skies, which means the filter should be used with eyepieces between 7mm and 28mm focal length. In urban sky, the eyepiece should be between 14mm and 42mm focal length.
|Filter Type||Passband (nm)||Wavelegnths||Best Application||Best Exit Pupil (mm)||Comments|
|EN, PN, SNR in darker sky||0.5 to 4||Not enough light-pollution rejection for urban skies; passes H-alpha where eye is not sensitive; good for basic imaging and visual observing of all nebulae in dark skies.|
|EN, PN, SNR in suburban and urban sky||1 to 6||Commonly called a 'UHC' filter; the best all-around nebula filter for visual use, especially for planetary and emission nebula and SN remnants.|
|Line (OIII)||~20-30||OIII (496nm,501nm)||PN and SNR Suburban to urban sky||2 to 7||Best for lower-power views from dark or light-polluted sky; particularly good with planetary nebulae and SN remnants|
|Line (H-beta)||~20-30||H-beta(486nm)||Faint nebulae in dark skies||3 to 7||A specialty filter for Horsehead and California Nebulae|
Most experienced observers agree that a narrowband filter filter, one that passes H-beta and OIII emission in the blue-green while blocking H-alpha and light from sodium and mercury vapor lamps, is the best all-around light pollution or nebula filter for visual observation. These filters, described in Section 3.2, work well in urban and suburban skies, and they even work well in relatively dark skies to reduce the effect of airglow. Because they pass light from hydrogen and oxygen, these filters improve the contrast of most nebulae, including emission nebulae such as the Orion and Lagoon Nebulae, as well as most planetary nebulae and supernova remnants. If you can choose only one filter for visual observation, a narrowband filter is the best choice with most telescopes, especially a filter that has a tighter bandwidth for better rejection.
For observers with larger telescopes, or those with smaller telescopes who want to improve the contrast of large nebulae at low magnification, an OIII line filter is an excellent supplement to a narrowband filter and makes a good second filter. These filters (described in Section 3.3) provide the highest contrast for many planetary nebulae, although they render very dim star images. Their rejection of light pollution means they work well even in urban skies. For challenging objects with low surface brightness, a good OIII filter can mean the difference between seeing a nebula or seeing nothing at all.
Broadband filters (Section 3.1) are generally less effective than narrowband or OIII filters, but they can help with some nebulae in dark skies. And they are quite effective for reducing the effect of light pollution for basic astrophotography with nebula and with stellar objects. These filters are also useful for observation of nebulae at higher magnification (and smaller exit pupils) than narrowband filters or OIII filters.
H-beta filters are the most specialized and least generally useful nebula filter. But these filters offer perhaps the only way to observe large nebulae with strong H-beta emission such as the Horsehead, California, and Cocoon Nebula.
Light pollution or nebula filters are a great help to urban and suburban observers, and to all stargazers who wish to see maximum detail in emission and planetary nebulae and supernova remnants. These filters improve the visual contrast in these celestial objects by selectively filtering discrete wavelengths from mercury and sodium vapor lamps used for street illumination in major cities. They do not help reduce the effects of broadband lighting sources like incandescent and LED lamps. Nor are they a replacement or substitute for truly dark skies.
Of the three main types of light pollution filter for visual use--broadband, narrowband, and line filters--narrowband filters are the most generally useful. These filters have a bandwidth of 20-30nm in the blue-green part of the spectrum, and may or may not pass deep red light from hydrogen alpha. Every observer should have a narrowband filter. If aperture, budget, and sky conditions allow, an OIII line filter is a good second filter with nearly any telescope as long as restrictions on exit pupil are observed. When chosen to match a telescope, eyepiece, and sky conditions, the effects and benefits of narrowband and OIII filters in moderately to greatly light-polluted skies are often quite striking. These nebula filter reveal more detail and faint extensions and tendrils in bright nebulae like the Orion Nebula or the Lagoon Nebula. And even in relatively dark skies, these filters can improve the appearance and contrast of fainter nebulae like the Veil or Helix nebulae.
- Useful Filters For Viewing Deep-Sky Objects, by David W. Knisely at Prairie Astronomy Club website.
- Choosing and Using Astronomical Filters by Martin Griffiths, Springer-Verlag, 2014.