Understanding the Bortle Scale

You may have noticed that amateur astronomers and astrophotographers will often include their Bortle class when posting pictures or discussing observations. It is a useful way to describe how much artificial light affects the night sky at a particular place and time.

A light pollution map will show that a large city radiates white to red from its center, while rural areas appear green to blue. These colors represent the amount of artificial light in the area, and how bright the night sky will look.

A light pollution map showing the location of my backyard

There are several ways to evaluate how much light pollution affects a location, including the Bortle Scale, SQM readings, and naked-eye limiting magnitude. My personal favorite for describing the overall observing experience is the Bortle Scale.

I have personally battled with light pollution in my backyard when taking deep-sky astrophotography images. My previous home was under Bortle Class 8 skies, the second-brightest class on the scale. As discouraging as that may sound, it does not mean that you cannot enjoy astronomy or astrophotography from the city.

I later moved from a house under Bortle Class 8 skies to a Class 6, which made a big difference in the amount of light pollution I observed from home. Aside from the opportunity to collect astrophotography images with improved signal, I can also visually see many more stars in the night sky from my backyard. 

What Is the Bortle Scale?

The Bortle Scale is a nine-level observational system used to describe the darkness of the night sky. Bortle Class 1 represents the darkest skies found on Earth, while Class 9 represents a brightly illuminated inner-city sky.

Unlike a light meter, the Bortle Scale is based on practical observations. These include the visibility of the Milky Way, zodiacal light, faint deep-sky objects, light domes, clouds, and familiar constellation patterns. A Bortle class is therefore an informed estimate rather than an exact measurement permanently assigned to an address.

The constellation Orion photographed from every Bortle Scale class.

Unfortunately for many backyard astrophotographers, their primary imaging location is much brighter than they would like. For example, my Class 6 backyard sky is described as a “bright suburban sky.” This is an appropriate description of my night sky, as I live in the center of a medium-sized city with a population of 130,000.

For example, the original description of a Bortle Class 1 sky states that an experienced observer may see M33, the Triangulum Galaxy, with the naked eye. This is an exceptionally demanding test that also depends on eyesight, experience, transparency, and the object’s position in the sky.

How to Find Your Bortle Class

The easiest way to find your Bortle class is to start with an online light pollution map and compare the estimate with what you can see outside. A map is a useful starting point, but it does not directly measure the current sky above your backyard.

1. Check an online light pollution map for an initial estimate.
2. Observe on a clear, moonless night after astronomical twilight.
3. Allow your eyes 20–30 minutes to become dark-adapted.
4. Compare the Milky Way, visible constellations, light domes, clouds, and faint objects with the Bortle descriptions below.
5. Use a sky quality meter to get a repeatable measurement at your specific location.
6. Record the date and conditions, as sky brightness can vary from night to night.

Light pollution maps model artificial sky brightness using satellite data. They cannot perfectly account for new construction, nearby streetlights, humidity, smoke, snow cover, transparency, or the direction in which you are observing.

The free light pollution map tool lets you plot a specific point on the map for an estimated zenith sky brightness reading. In the example below, you can see one of our favorite campsites (Rock Point Provincial Park) is a Bortle Scale Class 4. 

The detailed pin on the Light Pollution Map shows the SQM reading and Bortle Scale class for a given location. 

Another easy way to estimate the sky brightness in my backyard and at the dark-sky sites I visit is to use a smartphone app called Clear Outside by FLO. It uses your GPS location to estimate the Bortle class and sky quality. Treat this as a planning tool rather than an exact measurement of the sky that night.

The Clear Outside smartphone app provides an estimated Bortle class

Stellarium, a free planetarium software, lets you adjust the amount of light pollution in the settings. This is useful for matching up the view in the software with the brightness of the night sky from your coordinates. The faint stars begin to disappear as you increase the light pollution number.

Related Post: Recommended astronomy apps for stargazing. 

Adjusting the light pollution settings in Stellarium to match your sky

John E. Bortle introduced the scale in the February 2001 issue of Sky & Telescope. Its purpose was to help amateur astronomers consistently describe the darkness of an observing site using practical celestial observations.

Bortle Scale Quick Reference

Related Article: Astrophotography Results from the City Sky vs Dark Sky

The 9 Classes of the Bortle Scale

Below is a summary of the benchmarks used to classify each of the 9 classes of the Bortle Scale. For a detailed description of each class, you can read the full article from Sky and Telescope.

Class 1: Excellent dark-sky site

• Zodiacal Light, Gegenschein, and Zodiacal Band Visible.
• M33 Galaxy is visible as a naked-eye object with direct vision.
• Scorpius and Sagittarius regions of the Milky Way cast diffuse shadows on the ground.
• Viewing Jupiter or Venus degrades the dark adaptation of your eyes.

A Bortle Class 1 Sky (Texas Star Party, 2023)

Class 2: Typical truly dark site

• The summer Milky Way is highly structured to the unaided eye
• The brightest parts of the Milky Way look like “veined marble” when viewed with binoculars.
• Zodiacal light is bright enough to cast weak shadows.
• Clouds in the sky are visible only as dark holes in a starry background.
• Several Messier globular clusters are visible to the naked eye.

The Milky Way from Cherry Springs State Park (Class 2)

Class 3: Rural sky

• Slight signs of light pollution along the horizon.
• Clouds appear faintly illuminated in the brightest parts of the sky.
• The Milky Way still appears complex.
• Globular clusters M4, M5, M15, and M22 are all distinct naked-eye objects.
• M33 is easy to see with averted vision.
• The zodiacal light is striking in spring and autumn.

The Cygnus region of the Milky Way from our Bortle 3 cabin rental.

Class 4: Rural/suburban transition

• Fairly obvious light-pollution domes are apparent over population centers.
• Zodiacal light is clearly evident but doesn’t even extend halfway to the zenith.
• The Milky Way well above the horizon is impressive but lacks all but the most obvious structure.
• M33 is a difficult averted-vision object.
• Clouds in the direction of light-pollution sources are illuminated, but only slightly.

The closest dark sky site to my home (a 45-minute drive) is an excellent example of Bortle Class 4 skies

Class 5: Suburban sky

• Hints of the zodiacal light are seen on the best spring and autumn nights.
• The Milky Way is very weak or invisible near the horizon, and looks rather washed out overhead.
• Light sources are evident in most if not all directions.
• Clouds are quite noticeably brighter than the sky itself.

 

Class 6: Bright suburban sky

• No trace of the zodiacal light can be seen.
• The Milky Way is apparent only toward the zenith.
• The sky within 35° of the horizon glows grayish-white.
• Clouds anywhere in the sky appear fairly bright.
• M33 is impossible to see without binoculars, and M31 is only modestly apparent to the unaided eye.

The above image shows the difference between my Class 8 backyard and a Class 6

Class 7: Suburban/urban transition

• The entire sky background has a vague, grayish-white hue.
• Strong light sources are evident in all directions.
• The Milky Way is totally invisible or nearly so.
• M44 or M31 may be glimpsed with the unaided eye but are very indistinct.
• Clouds are brilliantly lit.
• Even in moderate-sized telescopes, the brightest Messier objects are pale ghosts of their true selves.

Class 8: City sky

• The sky glows whitish gray or orange.
• M31 and M44 may be barely glimpsed by an experienced observer on good nights.
• Only the bright Messier objects are detectable with a modest-sized telescope.
• Some of the stars making up the familiar constellation patterns are difficult to see.

My previous home was located under a Bortle Class 8 sky. 

Class 9: Inner-city sky

• The entire sky is brightly lit, even at the zenith.
• Stars making up familiar constellation figures are invisible.
• Dim constellations such as Cancer and Pisces are not seen at all.
• Aside from the Pleiades, no Messier objects are visible to the unaided eye.

Bortle Class vs. SQM and NELM

The Bortle Scale, SQM, and NELM all describe aspects of night-sky quality, but they are not interchangeable. Bortle class is an observational estimate. An SQM is an instrument that measures sky brightness in magnitudes per square arcsecond, while NELM represents the faintest star visible to the unaided eye.

How These Sky-Quality Measurements Differ

What Can You See and Photograph at Each Bortle Class?

A dark sky is especially valuable for broadband targets such as galaxies, reflection nebulae, dark nebulae, and natural-color Milky Way images. Emission nebulae are more forgiving from the city because their specific wavelengths of light can be isolated with narrowband or dual-band filters.

How Your Bortle Class Affects Astrophotography

In backyard astrophotography, sky quality can have a huge impact on your images. Deep-sky imaging under a bright city sky generally requires more total integration time than imaging the same broadband target from a rural location.

You’ll need to work harder to improve the signal-to-noise ratio of your final image because the signal (light emitted from the object) is buried underneath a layer of light pollution.

Broadband images (no filter) benefit greatly from dark skies.

Does Bortle Class Affect Exposure Time?

Yes, but there is no dependable rule stating that one Bortle class requires an exact multiple of the exposure time needed at another. Your ideal exposure depends on the camera’s read noise and gain, the telescope’s focal ratio, the filter bandwidth, the target’s brightness, the Moon phase, and the local sky brightness.

Under bright skies, increasing the length of each sub-exposure can quickly cause the background to overexpose. It is often better to capture a larger number of appropriately exposed images and stack them to increase the total integration time. Watch the histogram and avoid clipping either end; choose exposure length based on Bortle class alone.

Do Light Pollution Filters Help?

To capture images with improved contrast and reduced skyglow from light pollution, I often use light pollution filters that isolate specific wavelengths of light. However, no filter completely eliminates light pollution.

Traditional CLS and broadband light-pollution filters were more effective when sodium and mercury-vapor streetlights dominated. Modern white LEDs emit light across much of the visible spectrum, making them harder to suppress without also removing some of the light you want to record. A broadband filter may still provide a modest contrast improvement, but it can also alter natural star and galaxy colors.

In general, broadband filters such as the Optolong L-Pro aim to preserve a natural RGB appearance while reducing selected wavelengths of artificial light. For emission nebulae, a narrowband filter with a monochrome camera—or a dual-band filter with a color camera—can isolate specific wavelengths emitted by glowing gas much more effectively.

For example, a 12nm H-alpha filter like the Astronomik EOS clip-in records a narrow portion of the deep-red light emitted by hydrogen-rich nebulae. Narrowband and dual-band filters are tremendous options for emission nebulae under challenging Class 6–9 skies, but they provide little benefit for broadband targets such as galaxies, reflection nebulae, and star clusters. For those objects, darker skies and more total integration time remain the best solutions.

The Veil Nebula in Narrowband from Class 8 Skies (Ha + OIII)

Limitations of the Bortle Scale

The Bortle Scale is extremely useful, but it is not a perfect scientific measurement. Two observers at the same location may assign slightly different classes, and the same backyard can look different from one night to the next.

Factors that affect your assessment include:

• Visual acuity and observing experience
• Dark adaptation and nearby sources of glare
• Atmospheric transparency, humidity, haze, and wildfire smoke
• Snow cover and reflected artificial light
• Airglow, aurora, and other natural sources of sky brightness
• The altitude and direction of the object being observed
• Moonlight, which should be absent when evaluating a Bortle class

The sky may also be considerably darker in one direction than another. For these reasons, use your Bortle class as a practical description rather than an exact permanent rating.

My Seestar S30 Pro capturing deep-sky images from my bright backyard sky. 

Bortle Scale Frequently Asked Questions

Is a lower Bortle number better?

Yes. Lower Bortle numbers represent darker skies with less artificial skyglow. Bortle Class 1 is the darkest category, while Bortle Class 9 represents a brightly illuminated inner-city sky.

What is a good Bortle class for astrophotography?

Bortle 1–4 skies are excellent for deep-sky astrophotography, especially for broadband targets such as galaxies, reflection nebulae, and the Milky Way. Successful astrophotography is still possible under Bortle 5–9 skies by choosing suitable targets, using appropriate filters, and collecting more total integration time.

Is Bortle 4 considered dark?

Yes. Bortle Class 4 is a rural/suburban transition sky and is a meaningful improvement over typical suburban conditions. The Milky Way is normally visible, although light domes may appear above nearby population centers.

Can you photograph the Milky Way in Bortle 6?

It is possible to photograph the brightest portions of the Milky Way from Bortle 6 skies when they are high in the sky, but contrast and faint dust will be limited. A moonless night, good transparency, careful processing, and travel to darker skies will produce a much stronger result.

Can you do astrophotography from Bortle 8 or 9?

Yes. The Moon, planets, double stars, bright star clusters, and emission nebulae remain rewarding targets. Narrowband or dual-band filters can be especially effective on emission nebulae, while galaxies and reflection nebulae require more integration time and careful gradient removal.

What is the difference between Bortle class and SQM?

Bortle class describes the visual appearance of the night sky using several observational clues. An SQM provides an instrument reading in magnitudes per square arcsecond. They are related, but there is no exact conversion between them.

Does the Moon change your Bortle class?

Your location’s underlying level of artificial light pollution does not change, but moonlight brightens the sky and makes a proper Bortle assessment unreliable. Evaluate your sky on a clear, moonless night after astronomical twilight.

How accurate are light pollution maps?

Light pollution maps are valuable planning tools, but they provide modeled estimates rather than live measurements. Local lights, recent development, haze, smoke, snow, and atmospheric transparency can cause the actual sky to differ from the map.

Can a light pollution filter improve Bortle 8 skies?

It can help with the right target. Narrowband and dual-band filters can dramatically improve contrast on emission nebulae. Broadband filters offer a more modest benefit, especially under modern LED lighting, and no filter can turn an urban sky into a truly dark one.

How can I contribute my own sky-brightness observation?

The Globe at Night citizen-science project allows observers to estimate night-sky brightness, submit observations, and contribute to a worldwide record of light pollution.

If you are interested in learning more about astrophotography, have a look at my top 7 tips for beginners. Do not let light pollution spoil your enjoyment of astronomy. The Moon, planets, double stars, bright clusters, and emission nebulae can all be enjoyed from the city, and modern smart telescopes can stack short exposures to reveal objects that are difficult to see through an eyepiece. 

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Related Posts:

• Astrophotography From the City vs. Dark Sky (Head-to-Head Comparison)
• The Incredible Bortle 1 Dark Skies of the Okie-Tex Star Party
• How Dark is Your Backyard? (And How to Measure Light Pollution)