Astrophotography is a rewarding, but challenging hobby. The equipment and software learning curve tends to be high. But learning equipment and software is not the only challenge. Light pollution also can severely impact image quality.
One hundred years ago, almost everyone on earth could go outside at night and see the Milky Way Galaxy in all its glory. With population growth, encroachment by bright city lights has followed, particularly in urban areas. Residents must often drive or live tens of miles from cities to glimpse even a portion of the Milky Way. And because dim astronomical objects often require long exposures to capture faint details, city sky glow can often whitewash these long exposures, effectively completely obscuring the target object. There are several ways to measure the degree of sky glow, ranging from extreme light pollution in densely populated urban areas, all the way to pristine skies free from any measurable light intrusion. Each of these scales can help give an idea of what a resident can see with the naked eye as well as how challenging deep sky imaging will be from their location.
How do you measure light pollution?
One of the most used measures of light pollution for a particular location is the Bortle Scale. The Bortle Scale is a numerical scale ranging from 1 to 9 that measures the brightness and quality of the night sky, based on the degree of light pollution, and describes the general appearance of the sky, how much sky glow will be seen, and the brightness of stars and other celestial objects that will be visible without aid. It was developed by John Bortle, an amateur astronomer, and first published in the February 2001 issue of Sky & Telescope magazine.
Here is a more detailed description of each class:
Bortle Scale
| Bortle Class | Description |
| Class 1: Excellent dark-sky site | The zodiacal light, Gegenschein, and faint aurora are visible.*The Milky Way casts noticeable shadowsThe sky is completely dark, with no artificial light visible. |
| Class 2: Typical truly dark site | The zodiacal light, Gegenschein, and faint aurora are visible.The Milky Way is very prominent and well-defined.The sky is very dark, with only a few distant artificial lights visible on the horizon. |
| Class 3: Rural sky | The Milky Way is still quite prominent, but with some loss of detail.Some light pollution is visible on the horizon.Most celestial objects are visible, although some fainter ones may be difficult to see. |
| Class 4: Suburban sky | The Milky Way is only visible in its brightest parts.Light pollution is clearly visible, with a yellowish glow visible in some directions.Only the brightest celestial objects are visible. |
| Class 5: Bright suburban sky | The Milky Way is invisible or barely visible.Light pollution is a significant problem, with a yellow or orange glow dominating the sky.Only the brightest stars, planets, and the Moon are visible. |
| Class 6: Suburban/urban transition | The sky is bright and hazy, with little or no visible stars.Light pollution is severe, with a yellow or orange glow from streetlights and buildings.Only the brightest celestial objects are visible, and even they are difficult to see. |
| Class 7: Urban sky | The sky is very bright, with only a few of the brightest stars visibleLight pollution is extreme, with a bright orange or yellow glow dominating the sky.Only the Moon, planets, and a few bright stars are visible. |
| Class 8: Inner-city sky | The sky is bright and almost completely devoid of stars.Light pollution is overwhelming, with a bright orange or yellow glow from streetlights and buildings.Only the Moon, planets, and a few very bright stars are visible. |
| Class 9: Bright inner-city sky | The sky is completely bright and washed out, with no stars visible.Light pollution is extreme, with a bright orange or yellow glow dominating the sky.Only the Moon and a few planets are visible. |
*Gegenschein is a nebulous patch of light appearing opposite the sun thought to be sunlight reflected off of dust
* Zodiacal light is a faint conical glow of diffuse light appearing just after twilight and before dawn, visible in a very dark sky
The higher the class, the more difficult it will be to capture fine details. Fortunately, there are ways to minimize the effect of light pollution, making it possible to get very nice images from even the brightest night skies. Other than setting up your equipment in a dark sky location, filters are the most useful way to combat light pollution.
Broadband vs Narrowband
Filters can be divided into narrowband and broadband categories. Broadband filters are designed to allow most light wavelengths to pass through but filter out the most common wavelengths emitted by city streetlights. Red, green and blue filters for monochrome cameras can also be considered broadband, since they allow to pass a fairly large range of wavelengths of that particular color. Narrowband filters, as the name suggests, limit a very narrow range of wavelengths to pass. These are the typical wavelengths emitted by specific celestial objects, such as nebula. All other wavelengths are filtered out. This completely negates the effects of light pollution at the expense of longer exposure times and losing the other less predominant colors of the object. The narrower the wavelength range, the more expensive the filter. There are many different types and sizes of both narrowband and broadband filters. Some screw into standard astronomical threads in various sizes. Others clip in, or sit on top of DSLR or mirrorless sensors. Also on the market are filter holders that can carry several different filters that can be rotated into the photographic field without moving or removing the camera. Check out my more detailed article about different filters. Make sure you buy one that fits your gear!
For nebulae, I use the Optolong L-Enhance filter, which significantly blocks light pollution while allowing up to 90% transmission of H-Alpha, H-Beta and Oxygen III emission lines.
For broadband city light suppression, I like the Astronomik CLS (city light suppression) filter that is designed to clip into my Canon EOS Ra camera. Amazon has alternative CLS filters.
