Part 2 - DIGITAL SENSORS & LIGHT-POLLUTION (LPR) FILTERS
The Sky-glow Nuisance
One of the biggest challenges in doing astrophotography near urbanized areas has always been sky-glow.
For the high-speed films at the end of the 20th century, sky-glow became a nuisance that was often very difficult to color-correct. Several films had a high sensibility at some notorious sky-glow lines. Rolls of film and numerous trips to the processing lab were necessary before honing in on a good film emulsion, exposure procedure and a satisfactory processing or printing technique. For film astro-imaging, this was a great downfall.
Although today’s digital sensors are extra sensitive to sky-glow and much more vulnerable to fogging (saturating the sensor) than films were, poor images can simply be deleted with a touch of a button. Good results can quickly be obtained with any digital camera with just a few adjustments of the camera settings. Poor focus, underexposure/overexposure, camera shake or any other such astro-imaging problem can be corrected instantly with a review on a screen and a few turns of the camera dials. Images can also be augmented and perfected afterwards with the appropriate editing software. The ability to quickly obtain pleasing results, with and without external filters, is one of the great advantages of digital astro-imaging.
The appropriate LPR filter can help reduce sky-glow for a sky-shooter. While shooting nightscapes, it's essential to scrutinize three sensing aspects: Is the body a stock-filtered camera or modified? Does the external LPR filter hinder or complement the spectral response of the DSLR? Lastly, how severe is the sky-glow the shooter is imaging in?
Spectral Features in Sky-Glow
Over decades of lighting innovations by lighting companies, today's urban night sky appears to be the most contaminated in history. Four different types of outdoor lamps are currently utilized, extensively and concurrently, in most North American towns and cities. More than four are utilized in some parts of Asia, Europe and the Middle-East.
Here’s a short YouTube video showing the approximate timeline for evolving North American sky-glow: "Sky-glow Emissions Through the Decades".
Night-sky shooting conditions can be splendid when the sky is absolutely clear and dry. Near the big cities or their suburbs, a sky-shooter generally needs to pay attention to the blue spectral features (emission lines or bands) in sky-glow. This is because shorter (blue) wavelengths scatter more easily than longer (red) wavelengths in excellent sky conditions. However, examining how light behaves spectrally in our atmosphere, one should realize that bluer wavelengths get scattered-out closer to their sources while orange and red wavelengths penetrate deeper into the atmosphere. This means that as the beams of light escape farther away from a city, sky-glow becomes reddened.
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 appear a little differently.
Clear-night spectra for three diverse locations for the year 2020 are shown above. The sky-glow spectrum today contains features from a number of light sources as well as natural sources (airglow), contributing to the same lines or bands over each city. However, the weights (or the 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.
Which LPR Filter?
As shown in some detail in the "Why we use light-pollution filters" article, the most important visual nebula emissions are just three lines in the blue-green part of the spectrum near 500 nm. Add one more important nebula line (at least) in the red of the spectrum for photography, and you have all the essential nebula emissions.
Depending on the type of object being observed or photographed, a Light Pollution Reduction (LPR) filter may greatly enhance the object, have a little or no effect at all, or may dim it altogether. Visually or photographically, nebula filters will not make any nebula appear brighter, instead, they will make the surrounding sky darker enhancing the contrast of the nebula.
Here is the clear-night sky-glow for Los Angeles, California, in 2018. The regions marked as "Blocked" on the top are the areas of the spectrum where broadband or narrowband LPR filters attempt to eliminate. The green "Passed" regions are the areas transmitted by the filters.
Broadband type Filters
Broadband filters can be considered as general purpose contrast boosting nebula filters that can always be used visually and for astro-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 Hα emission nebula line through.
Most broadband filters offered today increase the visual contrast of deep-sky objects slightly, sometimes marginally. These filters are more rewarding when utilized for astrophotography. This was, and is, due to a fortunate marriage of filter transmission and digital sensitivity (minus the stock hot-mirror filter), producing a desired window through which all nebulae can be imaged in their splendid colors.
Shown above, the up-light (overcast) 2020 spectrum for the small city of Cornwall, Ontario, Canada, acts as a backdrop for the spectral transmission curve of a broadband clip-in filter. Although this Svbony filter for Canon EOS bodies is a relatively new filter, its classic "broadband" profile has a standard transmission outline for such filters. The most important nebula emissions are marked as vertical lines in their approximate color.
The spectral transmission curve of the Svbony broadband filter clip-in from the previous illustration (in yellow) is inserted here with the transmission curves for a typical stock filtered camera (in blue) versus an astro-modified filtered body (in red). The hatched areas are the effective red channel responses. Very litle 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. Filters in this class were conceived for visual work with severe sky-glow or for spotting very faint nebulae from mildly polluted skies. They were initially designed to transmit only the three visual nebula lines and are still primarily sold and bought in order to observe emission and planetary nebulae. Recently, however, many companies have also designed these filters to transmit the deep red Hα line, thus making them very valuable for astro-imaging in a bright urban setting.
Line type Filters
Rounding up the classic nebula filter types, the line LPR filters, like their name implies, 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 here against the 2020 up-light for Cornwall, Ontario in 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.
NEWER TYPE LPRs
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.
The multi-band name says it all; instead of a dual band-pass, as for the broadband or modern narrowband filters, 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 descriptions. 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 modern sky-glow. 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 (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. Some filters in this class are: - The Optolong L-Pro; - the SkyTech L-Pro Max; - the Astro Hutech NGS1 Night Glow Suppressionl - and the numerous IDAS LPS series which can be found here.
Neodymium Glass Filters
Some filters are made with Neodymium (also 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 just a 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.
CCD filters originally were not made for visual use nor were they light-pollution filters for imaging; rather the filtering system followed a standardized photometric system (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 type LPR filters.
With digital imaging becoming ever more popular, CCD filters have recently proliferated for amateurs. These are the most expensive dichroic-type filters available to an astro-imager. For the many CCD astro-cameras now readily available, offered are generally either "broadband" (LRGB and RGB) or "narrowband" 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.
With modern LED sky-glow, some narrowband CCD filters should retain their sky-glow blocking abilities and may even improve!
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. Shown against the 2020 up-light spectrum of Cornwall, Ontario, are the most common of narrowband CCD filters (image above).