Gadget-wise, today, astro-imagers and amateur astronomers have never had it so good. At their disposal are many wonderful instruments and tools aiding and expanding their astronomical pursuits. At the same time, the biggest setback confronting all astronomers today is light pollution. Many newcomers to the world of astronomy are located smack in the middle of massive amounts of urban and suburban sky-glow. One of the first items snatched up by an eager Milky Way photographer or observer is a Light Pollution filter for this understandable reason.
Light Pollution Filters – Reject vs Reduce vs Nebula
Also known as “Light Pollution Rejection” or “Light Pollution Reduction” filters, Light Pollution filters are also called “Nebula” filters by many amateur astronomers. In the landscape photography world, they are simply referred to as Light Pollution filters. The terms LPR, Nebula, and Light Pollution filter are often interchangeable.
These precision filters are considerably more expensive and more delicate than ordinary photographic filters. This is because they can be designed to transmit broad portions of the spectrum, very narrow bands, or just a few wavelengths. They are also designed to reflect (reduce or reject) the wavelengths of light where sky-glow occurs.
The filters are made by depositing ultra-thin layers of transparent and reflective dielectric coatings under stringent control on optical grade glass. That is why they are the costliest filters in the optics world. And they are among the more expensive accessories any night photographer or amateur astronomer will want.
However, before investing in a good precision filter, a keen astro-imager or observer should know precisely why these filters were created in the first place and for which celestial objects.
THE FIRST NEBULA FILTER
In the mid-20th century, a mixture of incandescent and mercury lighting existed side-by-side in cities for about a decade. Major lighting companies pushed for full-blown conversions to mercury-vapor for general outdoor lighting in the early 1960s. The result was that city-wide light levels on streets soon jumped three-fold. In some cases, nearly ten times the former incandescent levels. Light pollution, as we identify it today, was born at this juncture.
Daystar Nebular Filter
As a direct result of an ever-increasing number of outdoor lamps and escalating mercury levels in towns and cities, sky-glow had finally grown to the extent that amateurs looked for a technical solution. The first nebula filter to hit the markets catering to amateur astronomers was the Daystar Nebular Filter. Made by a long-standing solar filter maker and still operating today, the DayStar Corporation, the filter was introduced in October of 1978. The initial cost of the Daystar Nebular filter was $175 US dollars. Within a year, half-a-dozen companies were selling LPR filters claiming to block some sky-glow. Quickly, their cost dropped to about $50 US dollars (1979) for a standardized 1¼ inch broadband filter for eyepieces.
The first nebula filters were offered near the end of the mercury-vapor era for most cosmopolitan cities (1960 to 1983-ish period). These filters were manufactured to block mercury emissions in the sky very well. A new form of outdoor lighting appeared in the 1980s ― lamps with high-pressure sodium (HPS). As sky-glow increased faster than population growth, filter makers had to speedily perfect filter suppression-transmission characteristics.
LPR filters are designed to allow just three visual nebula emissions to pass through with very high efficiency. Broadband filters are also made to transmit at least one deep red nebula emission line, essential only for astro-imaging, again with a high transmission. Trying to spot galaxies and dim star-clusters will prove disappointing ―truer today than in the early years. On the other hand, since all digital sensors (including films of the past) use a completely different imaging method than our eyes, the broadband filters can indeed improve the photographic contrast of most objects in the sky.
With night-sky astrophotography becoming increasingly popular, broadband type LPR filters came into their own. These filters proliferated after the start of the new millennium as filter makers introduced the City Light Suppression (CLS), Nuance, and Enhance filters. All of which are tailored to increase the imaging contrast of celestial objects in mildly contaminated night skies. Although practically useless in an urban environment, these types of filters are also useful for astrophotography in the countryside and dark-sky sites. They reduce the light from the barely perceptible air-glow while increasing the contrast of Milky-Way delights in a pleasing manner.
Precisely why the three visual emissions from space were selected, we need to look at three effects. To see which parts of the spectrum celestial bodies radiate light, the spectral response of our eyes, and the spectral composition of light pollution.
For a more detailed look at digital imaging using CMOS sensors (DSLR and point-and-shoot cameras) or with CCD sensors (high-end astro-imaging), look for our upcoming article “How digital sensors work with Light-Pollution-Filters.”
All stellar sources in the sky have continuous spectra with varying numbers and intensities of superimposed absorption lines. We see this in the spectrum of our sun. Most stars display what are called absorption line spectra (plural for spectrum). These include stars in our galaxy, open star clusters, globular clusters, and nearby inactive galaxies. Very few stars have continuous spectra with emission lines, and only rare stars have emission lines solely. That conveniently gives us the answer as to what the spectra of the vast majority of objects in the sky look like, but not for the nebulae.
Nebulae can be classed into three different types: Reflection, Emission, and Planetary.
Light from reflection nebulae is scattered starlight. Remnants of star formation, reflection nebulae get their light from embedded or nearby stars. As a result, they show continuous spectra like the stars supplying their light with their absorption lines. These are often a blue color owing to the very young stars formed within them or nearby. A prime example of this type of bluish nebula is the nebula surrounding the celebrated Pleiades star-cluster in Taurus.
More common than reflection nebulae (and more interesting for many) are clouds of rarefied gas. They absorb energy from hot embedded stars. These then re-emit light almost entirely at wavelengths characteristic of the atoms or ions (atoms stripped of their electrons typically) making up the nebula. Consequently, all emission and planetary nebulae exhibit mainly emission-line spectra. Both types of nebulae are essentially related in that they are made from nearly the same material.
Emission nebulae are usually vast regions of rarefied gas associated with star formation. They are called H-II regions for ionized hydrogen. Hot, young blue stars near these regions emit copious quantities of ultraviolet light, which ionizes the gas. This gas is primarily hydrogen. The lines generated are the Balmer-series of hydrogen in the visible and photographic part of the spectrum. (They are named after their discoverer and noted by the Greek alphabet α, β, γ, δ, etc.) Other elements, primarily from helium, oxygen, and nitrogen, produce several emission lines in varying strengths.
The strongest line commonly in emission nebulae is hydrogen-alpha (Hα), at 656.3 nanometers (nm), in the spectrum’s deep red part. Unfortunately, this emission line is invisible (we will soon see why) but can easily be photographed if a camera is unfiltered externally and internally. The hydrogen-beta (Hβ) emission line, at 486.1 nm, is the visual line. It is at best about a third of the hydrogen-alpha line’s strength. This bluish 486 nm emission is the main reason we can glimpse these great expanses of gases at the telescope or with binoculars. The Great Orion Nebula (Messier 42), the Lagoon (Messier 8), or Eta Carina nebula (in the southern sky) are great examples of emission type nebulae.
Associated with star death, rather than star birth, planetary nebulae are arguably some of the sky’s loveliest objects. They differ from H-II regions in that they generally have higher gas densities. Collisions between atoms and ions occur more frequently, yet the gases are still rarefied by earthly standards.
Two lines of emissions from doubly ionized oxygen (O-III), in the blue-green part of the spectrum. They are at 495.9 and 500.7 nm and are the visually significant emissions produced by planetary nebulae. The Balmer-series of hydrogen is present, but these emissions are typically not stronger than for doubly ionized oxygen.
CONES AND RODS
Now let’s have a close look at how the human eye senses light on dark nights and how fortunate we are to have the ability to glimpse all nebulae.
The Human Eye
Our eyes are excellent detectors of a remarkable range of contrast and light levels. The retina of the human eye contains three types of photosensitive cells. Two types of cells, photopsin (cones) and rhodopsin (rods), provide vision through the complete range of natural light on Earth. A third type (the melanopsin containing retinal cells) is involved in regulating and synchronizing circadian rhythms and, to some extent, pupil size.
Vision is accomplished by the cones and rods, which have different functions. Cones are color detectors in daylight or with stronger night-time light. The process is called photopic vision. Rods are efficient low-light detectors and are responsible for most of our peripheral vision. They cannot discern color but sense extremely faint light in shades of grey. This process is called scotopic vision.
The Distribution of Photoreceptors
The human eye contains approximately 7 million cones and 100 million rods. Cones are concentrated near the center of vision in the fovea and are responsible for discriminating fine detail and color. Only cones are found in the location of the eye’s optical axis, subtending an angle of one or two degrees. Further from the optical axis, rods become apparent, quickly becoming more numerous than cones. Their numbers reach a maximum near 20° from the center of vision, after which they begin to decrease again. This occurrence gives us averted vision, widely exploited by observers at the telescope.
In dim light, rods become much more sensitive than cones leading to superior increases in relative sensibilities. During dark adaptation, rods accumulate chemical substances not found in cones. They can then detect shades of light in a considerable range of levels below the color vision threshold.
For extended celestial objects like the nebulae, even for very bright ones such as the Great Orion Nebula, our eyes work at or below the color vision threshold. For faint objects at the telescope, rods are the primary detectors of the eye. On the other hand, because of their higher surface brightness, most stars seen by direct vision can be sensed by cones and often in regal colors. Fainter stars perceived by averted vision are mostly colorless.
The spectral sensitivity curves of our scotopic and photopic vision are dissimilar. Both rods and cones have more-or-less bell-shaped sensitivities covering sections of the visible spectrum. The overall peaks of the two receptors are typically about 50 nanometers apart (or 500 Ångstroms). Photopic vision has a broad peak in the green near 555 nm, while scotopic vision peaks near 507 nm in the blue-green.
Up-front in most planetary nebulae are the doubly ionized oxygen (O-III) emissions at 495.9 and 500.7 nm. Luckily for a planetary nebula hunter, they fall fairly close to our rods’ peak sensitivity (scotopic vision). These O-III lines are nebula wavelengths that are also possible to sense in color, but only if bright enough or when using large telescope apertures.
For bright emission type nebulae, the red hydrogen-alpha (Hα) line, at 656 nm, is just too distant from the overall peak of photopic vision. It is about 101 nm away for cones and is not sensed by our rods at all. The visual nebula line from the Balmer-series is the hydrogen-beta (Hβ) line, at 486.1 nm, but due to its strength in the nebulae, effectively this emission can only be sensed by our rods.
The Most Significant Lines
Therefore, the most significant visual nebula lines are the two O-III forbidden lines (at 500.7 and 495.9 nm) and the Hβ emission (at 486.1 nm), all in the blue-green part of the spectrum. For planetary type nebulae, the O-III lines are visually the more significant emissions. For emission type nebulae, the Hβ line is more significant, but that is it. These are the three visual emissions that can easily be sensed!
Light Pollution Filters and Nebulae
All broadband Light Pollution Reduction filters for astrophotography transmit these very same emission lines. Additionally, these filters are designed to pass the invisible deep red Hα emission, at 656.3 nm plus the much weaker emission lines in the immediate area from sulfur (S-II) and nitrogen (N-II). The passbands in the blue-green plus deep-red parts of the spectrum allow light to be registered by all three sensing channels on RGB sensors inside DSLR cameras.
Using a broadband filter in front of the lens, a shooter can quickly get stunning images on-camera by merely adjusting the color temperature or shifting for the white-balance correction.
CCD cameras are different. They have monochrome (B&W) sensors requiring the user to select the spectral band to the image. We will have more on CCD cameras and their dedicated complement of filters on the next installment of “How digital sensors work with Light-Pollution-Filters”
In summary, just one Balmer line, the Hβ line, plus the two O-III lines can be genuinely sensed by our eyes. We can image the entire Balmer series from celestial subjects and other elemental emissions in glorious colors with the appropriate filter and digital sensor.
Visual Light Pollution Filters
For additional information about which filters to use visually, read the Prairie Astronomy Club’s article about “Useful Filters For Viewing Deep-Sky Objects.”
Since the incandescent days of the early 20th century, human-made emissions have been evolving in sky-glow. Although far from perfect, the first Light Pollution Reduction filters worked very well against mercury-lighting (1978 to ~1983). The major spectral emissions from mercury lamps were not many. The widely spaced lines were adequately far from the most desired emissions of the nebulae.
The Lights They Are a-Changin
With the arrival of HPS and meta-halide lighting, sky-glow acquired undesired emissions over the entire spectrum, including within the all-important blue-green filter transmission window. The red end of the spectrum near the Hα line, however, has remained rather clear. European, Asian, and some Middle Eastern cities eventually got far worse than North American cities. They have numerous types of light-sources deployed outdoors, sources with radiating elements not found over North American cities.
LPR filter makers continued to perfect their products for very high transmission – more than 90% and sometimes up to 97% for the three significant blue-green nebula lines plus the red Hα light.
LED Lighting is Not Helping
Sadly, just a decade into the new millennium, outdoor lighting changed again – for efficiency reasons. As LED conversions in cities unfolded, sky-glow quickly acquired a continuum (bands actually) through parts of the visible spectrum. Additionally, even with 100% LED street-lighting in some towns, spectral features from all other illumination methods are still apparent in sky-glow spectrograms! I have recent sky-glow spectra of numerous cities presented at yourlightpollution.info.
The Narrower, The Better
How the different types of filters function in sky-glow with LED lighting remains to be seen; intuitively, the narrower the transmission windows of filters, the better they should perform.
Happily, for today’s nebula hunter and astro-imager, amateur astronomers have a proliferation of filter makers and companies offering dozens of various nebula filters. And some at reasonable prices too.