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DIGITAL SENSORS Part 2: Light-Pollution Reduction (LPR) Filters

One of the biggest challenges in doing astrophotography near urbanized areas has always been sky-glow. For a sky-shooter the appropriate LPR filter can help reduce sky-glow. While shooting nightscapes, it's essential to scrutinize three key sensing aspects:

- Is the camera body stock-filtered or modified?
- Does the external LPR filter hinder or complement the spectral response of the sensor?
- Lastly, how severe is the sky-glow the shooter is imaging in?

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Spectral Features in Sky-Glow

Over decades of lighting innovations by lighting companies, today's urban night sky is the most contaminated in history. Here’s a short video showing the approximate timeline for the evolving spectral features in North American sky-glow:



Today, 4 different types of outdoor lamps are used extensively in most North American towns and cities. More than 4 are utilized in Tucson, Arizona, in some parts of Asia, Europe and the Middle-East.

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Clear Nights

Night-sky shooting or observing conditions can be splendid when the sky is absolutely clear and dry. Near the big cities and their suburbs, one generally needs to pay attention to blue spectral features (emissions or bands). That's because shorter (blue) wavelengths scatter more easily than longer (red) wavelengths in excellent conditions. However, examining how light behaves spectrally in our atmosphere, one should be aware that bluer wavelengths get scattered-out closer to their sources while orange and red wavelengths penetrate deeper into a clear atmosphere. This means that sky-glow becomes reddened as it escapes a city.

Although the spectrum of sky-glow can differ from one area to another, from one night to another due to sky conditions, as well as due to the distance from light sources, the spectral features in sky-glow are virtually the same; just their weights can appear different, sometimes vastly different.



Clear-night spectra for the year 2020 are shown above for three diverse locations. The sky-glow spectrum today contains features from a number of light sources and natural sources (airglow) contributing to the same lines or bands over each city. However, the weights (or brightness) of each feature can differ. Most of these features, but not all, are clustered in parts of the spectrum that can be filtered out.

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Which LPR Filter?

As shown in some detail in "Why We Use Light-Pollution Filters", the most important visual nebula emissions are just three lines in the blue-green part of the spectrum. Add one more important photographic nebula line (at least) in the deep red end of the spectrum and you have all the essential nebula emissions.



Above is the clear-night sky-glow for Los Angeles, California, in 2018. The regions marked "Blocked" on the top are the areas of the spectrum where broadband or narrowband LPR filters attempt to eliminate. The "Passed" regions in green text are the areas transmitted by the filters.

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Broadband type Filters

Broadband filters can be considered as general purpose contrast boosting nebula filters that can always be used visually and for imaging. Like the name implies, these filters transmit a fairly wide blue-green window in the spectrum and are also made to transmit red wavelengths above 640 nm allowing the strong red Hα emission line of the nebulae through.

Most broadband filters offered today increase the visual contrast of deep-sky objects slightly, sometimes marginally. These "broad-band" type filters are more rewarding when used with astrophotography.



Above, the up-light (overcast) 2020 spectrum for the small city of Cornwall, Ontario, Canada, acts as the backdrop for the spectral transmission curve of a broadband filter. Although this Svbony filter is relatively new, offered in the last decade, its classic "broadband" profile has a standard transmission outline for broadband filters. The most important nebula emissions are marked as vertical lines in their approximate color.



The spectral transmission curve of the Svbony broadband filter from the previous illustration (in yellow) is inserted here with the transmission curves for a typical stock filtered camera (blue outline) versus an astro-modified filtered body (red outline). The hatched areas are the "effective" red channel responses. Very little is registered with stock cameras (blue hatched area), a great deal more is registered for astro-modified bodies (red hatched area).

Narrowband type Filters

Narrowband filters transmit a narrower blue-green window. They were initially designed to transmit only the three visual nebula lines and are still primarily sold and bought for observing emission and planetary nebulae. However, many companies have also designed Narrowband filters to transmit the deep red Hα line, thus making them very valuable for astro-imaging in bright urban settings.

Line type Filters

Rounding up the classic LPR filter types, the "Line" filters transmit a narrow band about either the O-III or Hβ lines. Although they are only useful on the two types of gaseous nebulae, they remain somewhat popular with die-hard planetary and faint nebula hunters. Using these filters to spot or photograph galaxies, star clusters or reflection nebulae will prove particularly disappointing since their rejection of a large part of the spectrum makes them unsuitable to shoot or observe any object other than a nebula.



The transmission curves for three filters made by Astronomik are plotted above against the 2020 up-light for Cornwall, Ontario, Canada. The filters are: Astronomik's CLS in yellow (a wide Broadband filter); the UHC-E in green (a narrowband and comet filter); and the O-III in orange (a line filter). The Astronomik CLS-CCD filter is similar to the CLS (in yellow) except for an added cutout at 700 nm to eliminate NIR wavelengths.

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NEWER TYPE LPR FILTERS

Let's look at the most modern of LPR filters offered in this century by a slew of filter makers, ostensibly after the arrival and bourgeoning popularity of digital cameras.

Multi-Band pass Filters

Instead of a dual band-pass, like broadband or modern narrowband filters have, "multi-band" filters transmit more than two regions of the spectrum. Their general aim is to block some well known and notorious emission lines in sky-glow. These types of filters rarely list whether they are designed for a stock or modified camera in their description. The main thing to remember is that a "stock" DSLR will have the internal UV/IR-cut filter intact, whereas a modified DSLR, the stock hot-mirror has been removed or replaced.



The multi-band Optolong L-Pro filter, shown above, attempts to block some of the classic and notorious mercury (marked as Hg) and sodium (marked as Na) features still present in the 2020 sky-glow above. The transmission curve for the filter is shown in white against the 2020 up-light for the small city of Cornwall, Ontario, Canada.

Multi-band pass filters, supposedly, are also designed for balanced color. The characteristics of filter transmission allow images to be taken with minimal color shifts. This enables broadband emitting objects in the night sky, such as stars, galaxies and globular clusters, to appear more natural. These filters are ideal with modified astro cameras in mildly light-polluted skies, however, unmodified cameras can benefit by increasing the signal-to-noise ratio, thereby increasing the appearance of the deep red Hα wavelength. Multi-band filters are also very useful for astrophotography in country and dark-sky sites, reducing the light from natural air-glow in our atmosphere. The brightest air-glow emission line occurs at 557.7 nm which is marked as "Airglow" in the image above.

Numerous of these types of filters are offered today. An assortment of at least four different multi-band filters are made by one company alone (IDAS filters). Some filters in this class are:
- The Optolong L-Pro;
- the SkyTech L-Pro Max;
- the Astro Hutech NGS1 Night Glow Suppression;
- and the numerous IDAS LPS series which can be found here.

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Neodymium Filters

Some filters are made with Neodymium (or Neodynium) glass having a light-blue appearance. This type of filter transmits large portions of the spectrum providing selective blocking for a small area in the yellow without significantly reducing the desired nebula emission lines. The selectively blocked area includes the yellow 577/579 nm pair of mercury lines and the 589 nm sodium band (or pair of lines in the case of low-pressure sodium).



The transmission curve for a typical Neodymium type filter is inserted here with the transmission curves for a typical stock filtered camera (in blue) versus an astro-modified filtered body (in red). Please note the high transmission for natural airglow at 557.7 nm, which is atypical for any other LPR filter.

Targets such as galaxies, star clusters and reflection nebulae are some of the more difficult objects to capture from polluted areas. Neodymium filters, also the multi-band filters, are well suited for capturing such broadband sources in the night sky. They can often create better full-color images in both modified and stock CMOS cameras, often with very little post-processing. The Neodymium filters are not recommended for darker skies because most of them easily transmit Earth's natural airglow (at 557.7 nm).

Some filters in this class are named "moon", "natural light" and "nuances". Here is a partial list of Neodymium type filters:
- Baader Moon and Skyglow filter;
- Cokin Nuances Clearsky Light Pollution Filter;
- and NiSi Natural Night Filter.

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CCD Filters

CCD filters originally weren't made for visual use nor were they light-pollution filters for imaging; rather the filtering system followed a standardized photometric scheme (for measuring the light of stars) having a set of well-defined filter pass-bands.

Photometric systems were usually characterized according to the widths of the pass-bands of the filter system employed. Broadband type CCD filters typically had a band-pass wider than 30 nm. LRGB (L=luminance, R=red, G=green and B=blue) and RGB filters for imaging are of the broadband type, with band-passes of about 80 nm and sometimes more. Intermediate-band CCD filters had spectral pass-bands between 10 to 30 nm wide, while narrowband CCD filters had pass bands less than 10 nm wide, being similar to the class of line LPR filters.

With digital imaging becoming ever more popular with amateurs, CCD filters have recently proliferated. These are the most expensive dichroic-type filters available to an astro-imager. For the many CCD cameras now readily available, currently offered are either "broadband" (LRGB and RGB) or "narrowband" type filters.



The diagram above shows two distinct sets of CCD filters. With a light-pollution suppression gap in the orange part of the spectrum, CCD "broadband" type filters (light color) are well suited for country skies. The "narrowband" type CCD filters (solid color) can be utilized in severe sky-glow and even in moonlight.

Narrowband CCD filters transmit a narrow area at the emission lines for which they are named. For example, H-beta filters only transmit a narrow band around the hydrogen-beta emission (near 486 nm) and nothing else. With modern LED sky-glow, some narrowband CCD filters should retain their sky-glow blocking abilities, and may even improve.



In the illustration above, shown against the 2020 up-light spectrum of Cornwall, Ontario, are generally the most common of narrowband CCD filters.

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