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What Is The Bortle Scale? Light Pollution Classes Explained

If you are into deep sky astrophotography, you know the struggle of having to juggle with your free time, the absence of a large, bright Moon in the sky, and a clear sky is real.

And now, on top of that, you have to consider man-made light pollution as well.

But how bright is your sky, and what can you do about it? The Bortle scale offers a simple answer that is easy to understand. 

night sky darkness scale
This image illustrates how the sky looks from the different classes of the Bortle Scale.

Why Is Light Pollution Bad For Astronomers

Light pollution is bad for astronomers and astrophotographers as it washes away the details in the night sky.

Many stars become invisible to the naked eye: if you live in the city, you already know that aside from the Moon, Jupiter, Saturn, and few of the brightest stars, the sky is bright and empty.

And things don’t get better if you observe or photograph the sky with a telescope: the higher the sky brightness, the less details you see in nebulas and galaxies.

Bortle Class 6:7 from the city of Brussels, Belgium
A typical light frame captured in one of my usual locations: Andromeda’s core is barely visible in the orange sky glow. Bortle Class 6/7, 40km from the city of Brussels, Belgium.

What Is The Bortle Scale?

Before the Bortle Scale was created, and before it went mainstream, amateur astronomers were used to “eyeballing” the darkness of the sky by assessing the Naked-Eye Limiting Magnitude (NELM).

With NELM, one considers the magnitude of the faintest star visible in the night sky. 

Today, the Bortle Scale is probably the most famous classification scale used by amateur astronomers and astrophotographers to estimate the brightness of the night sky.

The scale is named after John E. Bortle, who published it for the first time in the February 2001 issue of the Sky and Telescope magazine.

Like NELM, this scale is not quantitative, but based on visual observation of the sky. 

The main difference with NELM, is that the Bortle Scale considers the visibility (and quality) of a whole range of astronomical phenomena and entities to define the brightness of the sky, not only the faintest star one could spot.

A short example of the difference between a low and high bortle rating.

List of the Bortle Scale classes

The Bortle scale defines the sky brightness using 9 classes.

Here is a description of the Bortle Scale, together with the color code used in many light pollution maps to display the different classes on the map.

ClassColor CodeNELMDescription
1
Excellent Dark Sky
 7.6 – 8
  • Zodiacal lights visible.
  • M33 is a direct vision naked-eye object.
  • The Milky Way’s core casts an obvious shadow.
  • Airglow visible.
  • Jupiter and Venus spoils night vision adaptation.
  • The surroundings are basically invisible.
2
Dark Sky
 7.1 – 7.5
  • Airglow weekly visible near the horizon.
  • M33 easily visible with naked-eye.
  • Clouds and Surroundings barely visible.
  • Many Messier globular cluster can be seen.
3
Rural Sky
 6.6 – 7.0
  • Some light pollution evident at the horizon.
  • Clouds are illuminated near the horizon.
  • Milky Way still shows structures.
  • M33 easily visible with averted vision.
  • M15, M4, M5, and M22 are distinct naked-eye objects.
  • Surroundings vaguely visible.
4
Rural / Suburban Transition
 6.1 – 6.5
  • Light pollution dome visible in various directions over the horizon.
  • Milky Way above the horizon shows less finer details.
  • M33 is a difficult averted vision object, only visible above 55º.
  • Clouds illuminated but still dark at the Zenith.
  • Surroundings are clearly visible.
5
Suburban Sky
 5.6 – 6.0
  • Milky Way very faint or invisible near the horizon and washed out overhead.
  • Light Source visible in most directions.
  • Clouds are noticeably brighter than the sky.
6
Bright Suburban Sky
 5.1 – 5.5
  • Milky Way only visible at the Zenith.
  • The sky within 35º from the horizon looks grayish-white.
  • Clouds are fairly bright.
  • M33 invisible without at least binoculars.
  • M31 is a modest naked-eye object.
7
Suburban / Urban transition (or Full Moon)
 4.6 – 5.0
  • Entire sky looks grayish-white.
  • Strong light sources are evident in any direction.
  • Milky Way invisible;
  • Clouds are brightly lit.
  • M31 and M44 are almost invisible.
8
City Sky
 4.1 – 4.5
  • Sky glows white or orange.
  • The Pleiades are visible but not much else.
  • Dimmer constellations lack key stars.
9
Inner City Sky
 4.0 at best
  • Only the brightest constellations are visible, and they are missing stars.
  • Sky glow rises up until the Zenith.

Astrophotography, though, is still possible in all classes, even though that may require the use of specialized equipment, such as narrowband filters and monochrome astro cameras.

The Bortle Scale And Astrophotography

As you see, the scale is based on observation, and while it can be used as a guideline for astrophotography too, it is not as straightforward to understand what can be photographed in the different classes.

Even from the city, you can still photograph the brightest deep-sky objects, such as star clusters, the many nebulae in Cygnus, the Great Orion Nebula and the Andromeda Galaxy.

M13, The Hercules Cluster using a Hoya Red Intensifier didymium filter
M13, The Hercules Cluster, from my balcony in Brussels, using a Hoya Red Intensifier didymium filter to reduce the light pollution.

Even the small Ring Nebula, in Lyra, can be photographed with success from the city.

M57 photographed from inner city sky
I photographed M57 from my balcony in Brussel (Inner city sky) with nothing more than a Hoya Red Intensifier Didymium filter. All-in-all, I can’t complain.

And, of course, you can photograph the Moon and the Planets from the city.

photograph the Moon and the Planets from the city
Venus, Jupiter, and a close up of the Lunar surface from Brussels, the capital city of Belgium.

So, don’t give up quite yet, just because you live under a light-polluted sky.

Filters For Light Polluted Skies 

Some filters are able to help reduce the light pollution. These filters can vary wildly in price and performances, but they are not universal.

using an affordable didymium filter on a class 6:7 sky
The effect of using an affordable didymium filter on a class 6/7 sky, where light pollution was mostly due to old Mercury-vapor street lights.

If you are lucky enough to image under a sky scoring a 4 or lower in the Bortle scale, I would not bother with these filters.

In this case, in fact, the loss of signal from the target due to the use of the filter will be higher than the loss of signal due to the light pollution.

A more radical solution for people imaging from light-polluted sky is to invest in a setup for narrowband astrophotography.

How Is Sky Brightness Measured? Sky Quality Meters

A Sky Quality Meter (SQM) is an instrument to measure the luminance of the night sky.

Unihedron Sky Quality Meter
Unihedron Sky Quality Meter.

The values are expressed in “magnitudes per square arcsecond,” and the values can range from 16, corresponding to the brightest sky you can possibly have, to 22, corresponding to the darkest possible sky.

To give you an idea, I usually work in a Bortle class 6/7, and when I measure the SQM at the Zenith, I get values in the high 19 / low 20.

Tightening All Together: Working Under Light Polluted Sky

This is all great theory to know, but how does that translate to practice?

If you are about visual observation, then the message is clear: the darker the sky, the better you will observe the deep sky.

This rule of thumb does apply to astrophotography as well: nothing beats a dark sky, and it is often said the best accessory to fight light pollution is “petrol,” to drive away from the city.

lower bortle class shows more detail in image
The region of Deneb and Sadr in Cygnus is filled with bright emission nebulae. Top: 2 hours worth of data, from Brussels. Bottom: 1h20m worth of data only 40 km away from the city. Olympus E-PL6 with Sigma ART DN 60 f/2.8 and Baader UHC filter.

But great results can still be obtained with standard equipment under light-polluted sky:

  1. Use a filter to help reduce the evil orange sky glow from city lights and street lights. 
  2. Check where the histogram peaks: the peak is the sky brightness, and you do not want to have it all the way to the right. If this happens, lower the ISO first, to gain a better dynamic range). If this is not enough, reduce the exposure time or step down your lens a bit more. Ideally, you want the histogram to peak no more than 50% from the left edge, thus avoiding to clip the highlights to pure white.

A practical note on the use of filters for astrophotography: while didymium filters like the Hoya Red Intensifier can be used in front of short focal lengths.

effect of using a filter removes the orange from the sky
The effect of using the Hoya Red Intensifier Didymium filter in Brussels. You can see the sky is less orange.

Interference filters, such as the Astronomik CLS can’t: because of their working principle, light coming in from the edges is affected differently than that coming in from the middle. You will have horrible halos.

Halos from using an Astronomik CLS filter
Halos from using an Astronomik CLS filter in front of a 12mm wide-angle lens. Let me stress this is NOT a defect of the filter, nor is it an indication of its optical quality. The effect is expected when using an interference filter with a wide-angle lens.

If you have a Canon or Nikon camera, most manufacturers have clip-in filters that sit between the lens and the sensor, rather than in front of the lens. This way, the light is rectified by the lens, and you will not have halos.

SVBONY light pollution filter

The SVBONY UHC clip-in filter for Canon can be used to cut light pollution and enhance the contrast of emission nebulae such as those in Cygnus or Orion.

Plus, one filter will do with all of your lenses: a good way to save money on an expensive hobby.

How Dark Is Your Sky? – How To Check Your Current Bortle Scale

DIY: Look Up For Visible Targets Or Use An SQM Device

This is by far the best method to determine the brightness of the sky at your intended location.

The chart below was made to guide you rating your sky accordingly to the Bortle Scale. 

Bortle Scale chart for your location
This flow chart has been created by Steve Owens as a guide to determine the Bortle Scale for your location. The high-res image of the chart can be downloaded here.

If you want to have a numerical value for the darkness of your sky, though, you can use a SQM device and rate the sky of your preferred locations. 

Light Pollution Map

Light pollution maps allow you to overlay on top of the map a light pollution layer. The layer uses a color-coded scale to display the Bortle scale assigned to the different locations.

screenshot of Light pollution map
Light pollution map built using the World Atlas 2015 data.

In these color-coded scales, Black corresponds to Class 1 (the darkest sky) of the Bortle scale and White to Class 9 ( a typical inner city sky).

John E. Bortle expressed concern about the accuracy of associating his scale with maps. Nevertheless, light pollution maps are widely used, and they can provide a good idea of where to go if you are scouting for a new location.

One very popular website showing maps for light pollution is Light Pollution Map.

Mobile Apps

You can get light pollution maps on your phone, such as Dark Sky Finder and Light Pollution Map (LPM). On Android, you can use Dark Sky Map.

Light Pollution Map for iOS
Light Pollution Map for iOS showing the pollution in the north part of Western Europe, where I live.

For iOS, there is a nice app called Dark SQM Meter that allows you to use your phone as an SQM device to measure the darkness of your sky.

Dark Sky Meter app
This is the sky I see from my balcony: not too bad for being in the city, but definitely not dark. On the left, the measurement I got from the Dark Sky Meter app.

While precision is not on par with that of dedicated instruments, for us amateurs should be good enough to create our own database. You can share your measurement too and see them on a map.

Conclusion

Light pollution is a real nuisance, not only for astronomers and astrophotographers but also for wildlife struggling to cope with brightest nights.

Luckily something is moving in the right direction, but industrialized and urban areas are still emptying the sky from the stars.

While for the visual observer, there is not much to see in such areas, if you want to photograph, there are things you can do, and planetary and deep-sky astrophotography is still possible from the city. Still, nothing beats a truly dark sky. 

But now you know how to find those great skies or, at least, how to cope with an urban sky.

About Andrea Minoia

Andrea Minoia works as a researcher in a Belgian university by day and is a keen amateur astrophotographer by night.

He is most interested in deep sky photography with low budget equipment and in helping beginners along their journey under the stars.