Like most amateur astrophotographers, I find myself in a constant battle with light pollution. The ever-present glow of modern city lights can be seen for hundreds of miles and even more by sensitive cameras used nowadays.
In a perfect world, we could all send our telescopes to remote mountain top sites, but that’s not an option for most of us. Over the years, I have tried a number of techniques, filters, and processing steps that have allowed me to continue enjoying my passion despite light polluted skies.
In this two-part guide, I will share some of those experiences, with the hope that they will prove useful to others.
In Part 1 we’ll cover the effects of light pollution, broadband imaging (luminance and RGB), and narrowband imaging.
Part 2 will include best practices, CMOS camera settings, sub exposures, integration time, flat Calibration, and useful Pixinsight features (NSG and DBE).
Light pollution is the artificial light at night produced by manmade sources such as industrial lighting, billboards, streetlights, and so on. It’s unfortunate that so many haven’t seen what the unpolluted night sky looks like, and many children will grow up never seeing the Milky Way.
There are several ways of measuring and defining light pollution intensity. On in common use is the Bortle scale. This scale ranges from 1 to 9 with Bortle 1 being an excellent dark sky and Bortle 9 a brightly lit inner-city sky where you’ll see few, if any, stars.
Light pollution plays a very undesirable role in most forms of astrophotography, with deep sky imaging among those most impacted. The image below illustrates how detrimental the effects of light pollution can be. These are unprocessed images without adjustments to the histogram or other more advanced scripts that can greatly mitigate some of the negative effects. That processing will be the subject of the second part of this guide.
Solar System Imaging and Light Pollution
There are several categories of astrophotography, and light pollution effects some more than others.
Lunar imaging: Often the simplest form of astrophotography is lunar imaging. Given the extremely bright target and very short exposures, this type of imaging is almost completely immune to light pollution.
Planetary imaging: Planetary imaging deals with mostly small but bright targets. Here again light pollution has a minimal effect on the outcome. Some of the best planetary images I’ve captured have been from a Bortle 9 location. The short exposures and high signal from the target overwhelm the background light pollution.
Camera Types and Light Pollution
While many astrophotographers use one-shot color cameras (OSC) and DSLR cameras, I have always had better results with monochrome cameras as they are more versatile when it comes to using them filters.
Filters are the number one weapon in our arsenal when it comes to light pollution. Monochrome cameras can exploit the L-RGB imaging technique in which we use a luminance filter to capture the luminance data and then “paint” that with RGB color data. The luminance filter is clear, allowing all the light in to capture data quickly with less noise. It also cuts off the near-IR like the color filters do and keeps the image focused the same as the color filters.
Monochrome cameras are also better suited to work with narrowband filters. These are by far the most effective filters when it comes to tackling light pollution. We will cover these filters a bit later.
One-shot color cameras can be used with narrowband filters, but that combination is quite compromised. The built-in color filters on the pixels (Bayer matrix) will block a lot of the narrowband light. All the green and blue Bayer filters will block the red Hydrogen Alpha and Sulphur II light and the red Bayer filters will block the blue-green Oxygen III light, rendering the system a lot less efficient.
Broadband Deep Sky Astrophotography – Luminance
When imaging faint deep sky objects, light pollution is much more of a problem because the background glow of artificial light can be comparable to that of the target. If the light pollution is excessive, it will overwhelm the signal of the target altogether.
Broadband targets show a continuum of light, that is almost all wavelengths of light rather than discreet wavelength bands. Broadband objects like galaxies and reflection nebulae (like the nebula in M45, the Pleiades) are often more challenging from light polluted skies.
The Luminance filter looks like a clear piece of glass but that not the complete picture. A “clear” filter is one that allows everything through as opposed to a luminance filter that blocks out ultraviolet (UV) and near infrared light (IR), allowing only wavelengths from about 400nm to 700nm to pass through. Both monochrome and color cameras benefit from luminance filters as most telescopes are not designed to deliver sharp images outside of the visible spectrum.
One useful technique is to swap a basic luminance filter with one designed to cut out most of the more prominent wavelengths associated with light pollution, i.e., selective light pollution filters. This is possible because artificial light is often produced at specific wavelengths. Modern LED lights and older incandescent lights are much more difficult to isolate as they are broadband sources, i.e., not concentrated at certain wavelengths. Sources like low-pressure sodium, high-pressure sodium, metal halide, and mercury vapor can be blocked quite effectively, though.
Below is the transmission graph of one example of these filters, the Chroma LoGlow, with the wavelengths it allows through highlighted in green. Note how it selectively blocks the specific wavelengths of the major emission lines of mercury vapor lamps (red lines) and sodium-based lamps (orange).
There are a number of options that selectively block out artificial light wavelengths this way, including the Chroma LoGlow, Hutech IDAS LPS, and Optolong L-Pro. These are still broadband filters, allowing most of the light through so we can record the broadband source. This is different than the more restrictive “Nebula” filters discussed below.
Broadband Deep Sky Astrophotography – RGB
Next, we’ll consider the color information in broadband color imaging (RGB, which stands for Red, Green, Blue). With a monochrome camera, these colors are recorded separately and combined to create a color image in the same way that a color computer monitor works. Here too, there are some filters that are better suited for dealing with light pollution than others.
Typically, RGB filters cover a continuous wavelength range from 400nm to 700nm with about 100nm per color, i.e., 400-500nm for blue, 500-600nm for green, and 600-700nm for red. Some filters, like the Astrodon Generation 2 E-Series and Astronomik Deep-Sky RGB, have a “notch” that blocks out wavelengths from approximately 570nm to 620nm. This is the region where a lot of artificial light is emitted by mercury and sodium industrial and city light sources (also blocked out by filters like the Chroma Loglow).
Below are the transmission graphs of these two RGB filters. Note the missing part of the spectrum (highlighted in grey) between green and red corresponding to the wavelengths of a lot of artificial light sources. These filters will block much of the “orange glow” of city lights.
Narrowband targets are mostly emission nebulae that emit light due to certain gases being ionized, which then emit light at specific wavelengths. These objects include diffuse and planetary nebulae rather than reflection nebulae, stars, and planets.
Fortunately, we can isolate the wavelengths from the target and reject most of the unwanted artificial light, eliminating much of the detrimental effects of light pollution.
The three primary wavelengths most commonly captured using narrowband filters are Hydrogen Alpha (HA), Oxygen III (OIII), and Sulphur II (SII). While narrowband filters are very effective, they are not completely immune to light pollution.
A key factor with narrowband filters is how restrictive they are; the bandwidth, often noted as 3nm, 6nm, and so on, is the range in nanometers of wavelength that each filter will allow to pass. The narrower the bandwidth, the better it can filter out unwanted background light. The downside is that narrower bandpass filters are more expensive and are more sensitive to fast focal ratio telescope. For example, some 3nm filters won’t work well with very fast telescopes below f/4.0.
Below is an image of a target imaged with 3nm and 6nm narrowband filters using the same equipment and exposure time from a Bortle 5-6 location (moderately light polluted). We can see the 3nm is more effective at suppressing background glow and emphasizing the nebulosity.
Another less critical factor to consider is optical density (OD), which is the amount of unwanted light the filter blocks out in the specified bandwidth. An OD of 5 means it only passes 0.001% of the unwanted light; the higher the OD value the better.
Hydrogen Alpha Imaging (Monochrome)
Out of the three common narrowband filters, the best at fending off unwanted artificial light is the Hydrogen Alpha filter. That, and the abundance of hydrogen alpha signal (light), allows significant details to be captured from even heavily light polluted skies. The narrow band most susceptible to light pollution is Oxygen III; special care should be taken in acquiring data and its usually best to aim for 3nm filters for this one.
Under the worst light polluted skies where no other option is viable, the imager can still obtain exquisite deep sky results with Hydrogen Alpha-only imaging, albeit in black and white only.
Overall, I would recommend monochrome cameras as they generally produce better quality data, especially when it comes to imaging from light polluted skies.
Filter choice is a critical factor and will play a decisive role in the final outcome. Learning the differences in each will prove very valuable.
Astrophotography will always be preferable from darker skies. However, we do have a number of resourceful tools that can enable imaging from even the most light polluted locations. In part two of this guide, we will explore how imaging technique and processing will improve our results and further limit the effects of light pollution.
Astrophotography: A Guide to Beating Light Pollution, Part 2 will explain how to get the most signal and the least light pollution in your images.
Rouzbeh Bidshahri has many examples of images taken from light polluted skies with a monochrome camera in his gallery at www.RouzAstro.com