Sky Quality

Bortle Scale

The Bortle Scale (also called the Bortle Scale or Bortle Class) is a practical classification that quantifies the brightness of the night sky. It was introduced in 2001 by American amateur astronomer John E. Bortle. Its purpose is to provide a simple, visually based way to describe the light pollution of a location, allowing astronomers (both professional and amateur) to assess and compare their observing capabilities. The scale ranges from Class 1 (an "ultra-dark" or "pristine" dark sky) to Class 9 (an urban-lit sky, comparable to that of an average residential area). Each class has a brief description of the visible stars, the Milky Way, nebulae, and other celestial objects.

Description per Class

1 Pristine Dark Sky Site: Completely dark sky. The Milky Way is a broad, transparent band stretching across the entire sky. Mild light pollution is barely noticeable; even the faint particles of the Milky Way's core are clearly distinguishable. The Andromeda Galaxy appears as a faint, wispy burst of light.

2 Typical Truly Dark Site: Very little light pollution. The Milky Way is still clear and rugged, but the edges are starting to fade slightly. 6th-magnitude stars are clearly visible everywhere.

3 Rural Sky: A bright, but still dark, horizon. The Milky Way is still clear, but the edges are less sharp and the core appears slightly diffuse. The M31 Nebula (Andromeda) is still visible as a faint, hazy patch.

4 Rural/Suburban Transition: The Milky Way is visible but starting to fade. The hazy edges and dark areas are still distinguishable, but stars of magnitude 6-7 are becoming more difficult to spot. 5 Rural Suburban Sky The Milky Way is faint and faint, clearly visible only in the darkest parts. The M31 nebula is sometimes just visible, but only under favorable conditions.

6 Suburban Sky The Milky Way is barely visible; only the brightest core is faint. The M31 nebula is almost invisible. Stars of magnitude 6 are still visible, but magnitude 5 becomes difficult.

7 Bright Suburban Sky The Milky Way is practically invisible. Only the brightest star clusters near the celestial pole (e.g., the Crux region) are still visible. Stars up to magnitude 7 are still visible, but magnitude 6 is fleeting.

8 City Sky The Milky Way has completely disappeared. Only the brightest stars (Sirius, Vega, etc.) are visible. Most "nebulae" have been suppressed by light pollution.

9 Inner City Sky A very brightly illuminated sky. Only the densest stars are visible; The Milky Way's faintest stars, M31, or the Orion Nebula are elusive. Even Jupiter's brightest planets are often difficult to distinguish.

How do you determine your own Bortle class?

Look at the Milky Way Is it a broad, transparent band (class 1-2) or just a faint spot (class 4-5) or not visible at all (class 6-9)?

Find the Andromeda Galaxy (M31) Can you see it as a faint light? In classes 1-3 yes, in classes 4-5 sometimes, ab ab.

Count the faint stars Can you see stars of magnitude 6 or 7 in the dark areas of the sky? The less you can, the higher the class.

Look at celestial objects Such as the Horn Nebula, the Orion Nebula, the Pleiades. Are they bright or faded?

Use a light pollution meter There are simple “Bortle apps” or paper forms where you can check off answers (e.g., “stars with a white band = class 1”; “no Milky Way = class 9”).

Measure horizontal light pollution (SQM) If you have a light meter (Sky Quality Meter), you can measure a quantitative value in magnitudes per square second (mag/arcsec²). An SQM value of 21.7-22.0 indicates approximately class 1, while ≈18.0 indicates class 8-9.

Practical applications

Change your location If you want to take a photo or do a spectrophotometric observation, find a location with a lower Bortle class (e.g., a nature reserve, nature reserve, or dark sky park).

Plan your observations For faint deep-sky objects (e.g., the Andromeda Galaxy, faint or nebulous objects), you need at least a class 3-4.

Compare with others By describing your location with a Bortle class, you can easily make comparisons with other observers or with previous observations at the same site.

Understand the limitations of your equipment ​​A high-ISO camera can still “feel” something at a class 6-7, while a visually pristine DSLR image without long-exposure light pollution only works at class 1-4.

Tips to improve your Bortle class

Move to a darker area A few kilometers outside the city can result in a jump of 2-3 classes.

Use light crates Turn off all excess outdoor lighting (streetlights, car headlights, billboards).

Cover your windows and roof Sealed windows or skylights inside or directly above the observation site can create significant pollution. Use blackout curtains or blackout screens.

Plan for moonless nights A full moon can illuminate the background; Choose a night with a new or dwarf moon.

Choose a clear sky Cloud cover reduces the visibility of faint objects, but can also "pollute" if a lot of light is scattered by the atmosphere.

Arrange a dark sky permit Many nature parks and reserves have special dark sky zones where you are officially allowed to camp and take photographs.

Useful resources

John Bortle's original article "The Bortle Dark Sky Scale" (Sky & Telescope, 2001).

Online Bortle calculators Search for "Bortle scale calculator" or "dark sky rating" there are several web tools where you can determine your rating with simple questions.

Dark sky park websites For example, the International Dark Sky Association (IDA) has lists of recognized dark sky reserves.

Sky-Quality-Meter (SQM) Handheld meters from companies like Unihedron (SQM-Lite, SQM-Pro) that allow you to measure a quantitative magnitude value.

Summary

The Bortle scale ranges from 1 (ultra-dark) to 9 (brightly illuminated).

It's a visual instrument; you primarily look at the Milky Way, Andromeda, the faint star cluster, and how well you can see star magnitudes.

With a simple observation, you can categorize your location and determine whether it's suitable for deep-sky photography, planetary observations, or casual stargazing.

Small adjustments (turning off the lights, moving the distance, darkening the windows) can make a huge difference in the Bortle scale.

The Antoniadi Scale - Determining the seeing

The Antoniadi Scale is a five‑point rating system that amateur astronomers (and many professionals) use to describe the atmospheric “seeing” that is, how steady and sharp the image of a celestial object appears through a telescope at a given moment.

It was introduced in the early 20th century by Serbian‑French astronomer Eugène Antoniadi (1870‑1944), who needed a simple way to convey the quality of his planetary sketches. 

Over the decades it has become the de‑facto standard for quick seeing reports on observing logs, forums, and even in many variable‑star and planetary‑imaging programs.

What the numbers mean

1 Perfect

The image is crystal‑clear. No wobble or scintillation is noticeable. Fine planetary details (cloud bands on Jupiter, lunar rilles, double‑star separations) are sharp across the entire field. Ideal for high‑resolution work such as planetary imaging or precise double‑star measurements.

2 Good

The image remains steady; a very slight tremor may be detectable if you stare for a while, but it does not impair the view of fine details. This is the most common rating for a pleasant night. Planetary features are still well resolved, though the very best stacked images may need a few extra frames.

3 Average

A gentle shake is evident. The jitter begins to smear the smallest details, especially at high magnifications, but the major structures are still recognizable. Suitable for most visual work on planets, the Moon, and moderate‑magnification deep‑sky objects. Imaging will benefit from a higher frame rate to freeze the turbulence.

4 Poor

The image trembles noticeably; the jitter is fast enough that you must constantly re‑center the target, and fine detail is consistently washed out. Only the brightest, largest features (major cloud belts, broad lunar maria) are clearly visible. Imaging requires very short exposures and extensive post‑processing to recover anything useful.

5 Very Poor (Bad)

Atmospheric turbulence is severe. The target appears to “dance” rapidly, and fine contrast is lost. Only the brightest, most prominent structures survive; stars may twinkle intensely even through the telescope. Visual observation becomes largely qualitative, and imaging is generally futile without adaptive optics or a rare “lucky” moment.

Key point

The scale is subjective, based on what you can see with your own eyes (or sometimes the eye of a colleague). It’s not a physical measurement like a Fried parameter (r₀), but a practical shorthand that works across all telescope apertures.

How to Estimate the Seeing

Pick a familiar reference object Jupiter, Saturn, the Moon, or a bright double star (e.g., Albireo).

Observe at a moderate magnification (≈ 100 × for planets, 200–300 × for the Moon) and note the steadiness of the image.

Ask yourself:

Do the fine details hold their shape for a few seconds without “wiggling”? → 1 or 2.

Is the tremor evident, but you can still follow the features with your eye? → 3.

Does the image jitter rapidly, forcing you to constantly re‑center? → 4.

Is the target essentially a blurry blob that only the brightest outlines survive? → 5.

Record it in your observing log with a time stamp, airmass, and instrument used. 

Over many nights you’ll start to see patterns (e.g., time‑of‑night or weather‑related trends).

Why the Antoniadi Scale Still Matters

Quick communication A one‑digit code tells a fellow observer instantly whether a night is worth trying a high‑resolution planetary sketch or whether the focus should shift to brighter, low‑magnification targets.

Planning imaging sessions When you log seeing as “2” you know you can probably obtain usable planetary video frames at 100–150 mm focal length; a “4” suggests you should drop to 30 mm or wait for a better window.

Correlating with instruments Modern tools like Sky Quality Meters (SQM) or Fried‑parameter sensors can be cross‑referenced with Antoniadi ratings to build statistical models of a site’s atmospheric stability.

Historical continuity Thousands of observing reports from the 1900s onward use the same five‑point scale, allowing modern observers to compare past and present conditions on the same site.

Relation to Other Seeing/Transparency Scales

Antoniadi

• • Seeing (image steadiness)

  • Focuses on seeing the steadiness of an image

  • 1–5

  • Planetary work, double‑star observation

Fried Parameter (r₀)

Fried Parameter (r₀) is a physical measurement of turbulence in meters, used mainly in professional site testing and adaptive‑optics design.

• Physical turbulence 

• size (meters)

• 0.05 – 0.30 m (typical)

• Professional site testing, adaptive optics

CN (Clear‑Night) Scale

CN (Clear‑Night) Scale addresses transparency how clear the air is, i.e.

• • How much faint detail can be seen.

  • Transparency (how clear the sky is)

  • 1–5

  • Deep‑sky observations, photometry

Bortle 

Bortle Dark‑Sky Scale measures light‑pollution how bright the night sky background is.

• Dark‑Sky Scale

• Light pollution/sky brightness

• 1–9

• Site selection, astrophotography planning

While the Bortle Scale you asked about earlier quantifies background sky brightness, the Antoniadi Scale deals only with the dynamic component of the atmosphere. Good transparency (low Bortle number) does not guarantee good seeing; a dark, clear night can still have a turbulent boundary layer, and vice versa.

Tips for Improving Your Seeing Rating

Observe from a stable, low‑turbulence site a hilltop away from urban heat islands, or a desert location where the ground cools slowly after sunset.

Allow the telescope to thermally equilibrate give the instrument at least 30 minutes to match ambient temperature; otherwise internal convection adds “seeing” noise.

Use a short focal length for “lucky imaging” the shorter the focal length, the less a given angular jitter translates into image motion on the sensor.

Employ an atmospheric “filter” a low‑power (≈ 2 ×) Baader‑de​​focator or a shallow focus mask can reduce the effect of high‑order turbulence on the final image.

Schedule observing during the night’s “seeing window” many sites show the best steadiness roughly 2–3 hours after astronomical twilight, when the surface layers have cooled but before the midnight temperature minimum begins to cause ground‑level convection.

Logging Example

• 2026‑02‑25 22:17 UT 8

• Dobsonian (f/5) Jupiter (mag 2.1)

• Antoniadi: 2 (steady, fine band detail visible)

• FWHM (video stack 10 s): 1.4″

• SQM‑Lite reading: 20.9 mag/arcsec²

• Notes: Slight wind gusts at 12 km/h, airmass 1.6, temperature 5 °C, dew point 2 °C.

By pairing the Antoniadi rating with numeric data (FWHM, SQM, airmass) you create a dataset that can later be plotted to see how “seeing” varies with temperature, wind, or time of night.

Quick FAQ

Q: Can I use the Antoniadi scale for deep‑sky observing?

A: Not directly. Deep‑sky work is limited more by transparency (Bortle) and sky background than by short‑timescale image jitter. However, a very poor Antoniadi rating (4–5) often coincides with poor atmospheric stability that can also affect guiding accuracy.

Q: Is there a “decimal” version of Antoniadi?

A: Some observers write “2‑” or “3+” to indicate halfway points, but the formal scale stays whole numbers. If you need finer granularity, record a separate “seeing estimate” (e.g., 2.3) alongside the official rating.

Q: How does the scale apply to modern adaptive‑optics (AO) telescopes?

A: AO systems translate raw turbulence into a Strehl ratio; the Antoniadi rating can still be useful for visual correlation (e.g., “the AO loop stayed locked because the natural seeing was around a 2”). It’s a handy bridge between human perception and instrument metrics.

Bottom Line

Antoniadi = 5‑point seeing rating (1 = perfect, 5 = very poor).

It’s a subjective, visual gauge but highly valuable for planning, reporting, and comparing observations.

Pair it with other quantitative tools (SQM, FWHM, r₀) for a more complete picture of night‑time conditions.

Pickering Seeing Scale Plain‑Text Summary

What the scale measures

The Pickering scale is a 10‑point visual rating system for atmospheric seeing, i.e., how steady and sharp the image of a star appears when viewed through a telescope. It was introduced in the early 1900s by William H. Pickering of the Harvard College Observatory. Unlike the broader‑stroke Antoniadi scale, Pickering provides finer granularity (0 – 10) by describing the appearance of the star’s diffraction pattern (the “Airy disk” and the surrounding rings).

Rating definitions (0 – 10)

0 Excellent (Perfect)

The star’s Airy disk is a crisp, bright, perfectly round point with several concentric diffraction rings clearly visible. No tremor is detectable; the image remains perfectly steady for many seconds.

1 Very Good

The Airy disk stays sharp; the first diffraction ring is distinct, the second may be faint but still recognizable. Tiny, almost imperceptible tremor may be present, but it does not blur the rings.

2 Good

The central disk and the first ring are crisp; the second ring is just about visible. A slight, slow wobble can be felt if you stare for a while, but the overall shape stays defined.

3 Fairly Good

The central disk is round and stable; the first ring is clear, the second ring is faint or intermittently broken. Small tremors become more apparent, but the star still looks “tight.”

4 Fair

The Airy disk remains fairly concentrated, but the first ring shows noticeable distortion or intermittent breakup. The second ring is generally invisible. Gentle jitter is obvious.

5 Moderate

The central spot is still discernible, but it begins to elongate or “smear” briefly. The first diffraction ring is either faint or broken into blobs. The image feels “bouncy” and requires frequent recentering.

6 Poor

The star appears as a short, fuzzy blob that occasionally flickers. The diffraction pattern is essentially gone; you may see a vague halo. The image trembles rapidly, and fine detail is lost.

7 Very Poor

The star is a large, blurry patch that constantly shifts. No diffraction structure is visible. Only the brightest part of the seeing disc can be followed.

8 Bad

The star is a bright, irregular smudge that rapidly moves across the field. It is difficult to keep it centered for more than a second.

9 Very Bad

The target is a vague, constantly dancing spot. You can barely distinguish it from the background sky; guiding is almost impossible.

10 Extremely Bad (Unusable)

The star is essentially invisible through the turbulence; you only see a diffuse glow that flares in and out. No useful visual or imaging work can be done.

How to estimate the Pickering rating (step‑by‑step)

• • Select a bright, isolated star preferably magnitude 2 – 3, high in the sky (airmass < 1.5) so that the telescope’s diffraction pattern is easily seen.

  • Use a moderate to high magnification about 100 × for a 4‑inch refractor, 150–200 × for a similar‑aperture reflector. The goal is to resolve the first few diffraction rings.

  • Observe the Airy disk and its rings:

  • If you can count at least three clear concentric rings, you are at 0–1.

  • If you see a solid central disk plus a well‑defined first ring, you’re around 2–3.

  • When the first ring is faint, broken or intermittent, you’re in the 4–5 range.

  • When the diffraction pattern disappears and you only see a fuzzy blob, you’re in the 6–10 range, moving toward higher numbers as the blur and jitter increase.

  • Note any tremor the speed and amplitude of the wobble help push the rating a step higher. A slow, gentle jitter might keep you at a lower number; rapid, erratic shaking moves you to higher numbers.

  • Record the rating in your log along with the time, airmass, and telescope details. Over many nights you will see trends (e.g., “most evenings in early summer hover around 3–4”).

Why the Pickering scale is still valuable

• • Fine granularity: The 0‑10 range lets you distinguish subtle improvements or degradations in seeing that a 1‑5 system might mask.

  • Diffraction‑pattern focus: Because the scale is anchored to the visibility of Airy rings, it directly reflects the optical quality of the image, which matters for high‑resolution planetary imaging and precise astrometry.

  • Historical continuity: Many classic observational journals (e.g., Pickering’s own “Harvard College Observatory Circulars”) used this scale, allowing modern observers to compare their nights with historic records.

  • Complement to other metrics: Pairing a Pickering rating with a Fried‑parameter measurement (r₀) or a video‑derived FWHM gives a more complete picture of the atmospheric state.

Relationship to other seeing scales

• • Antoniadi (1‑5) a coarser, more subjective scale focusing on overall steadiness rather than diffraction detail.

  • Pickering (0‑10) emphasizes the visibility of the Airy pattern, giving a finer gradation of image quality.

  • Fried parameter (r₀) a physical measurement (in centimeters/meters) of turbulence strength; larger r₀ corresponds to lower (better) Pickering numbers.

  • Strehl ratio used in adaptive‑optics contexts; a high Strehl ratio typically coincides with a low (good) Pickering rating.

Practical tips for obtaining better Pickering scores

• Let the telescope acclimate to ambient temperature for at least half an hour; internal tube currents otherwise degrade the diffraction pattern.

• Observe from a site with minimal ground‑level heat a hillside or desert location reduces the low‑altitude turbulent layer that most often destroys the Airy rings.

• Use a stable mount and avoid vibrations even slight mechanical shaking can masquerade as poor seeing.

• Choose nights with low wind at the observatory level; wind shear tends to break up the diffraction rings.

• Observe early in the night when the ground has cooled but before the midnight temperature dip can generate low‑level convection.

• If possible, employ a small “mask” (e.g., a 2‑inch aperture stop) to simplify the diffraction pattern; a clean, unobstructed pupil helps you see the rings more clearly.

Sample log entry (plain text)

2026‑02‑26 21:42 UT 6‑inch f/8 Dobsonian Star: Altair (mag 0.77), airmass 1.3

Pickering rating: 3 (central Airy disk steady, first ring clear, second ring faint, slight wobble)

Telescope cooled for 45 min, ambient temp 4 °C, wind 5 km/h, humidity 28 %

Video capture (200 ms exposures) FWHM: 1.6″, Strehl estimate ~0.4

Notes: Low wind, no obvious dome seeing. Seeing may improve after midnight as ground stabilizes.

Quick FAQ

Q: Do I need a perfect star for Pickering, or can I use planets?

A: The scale is defined for point sources (stars) because the diffraction pattern is only visible for unresolved objects. Planets can give a rough sense of steadiness but are not appropriate for a formal Pickering rating.

Q: Is a “0” really achievable with ordinary equipment?

A: It is rare, but on exceptionally good nights at high‑altitude, low‑light‑pollution sites, a well‑collimated telescope with a clean aperture can produce a visible second diffraction ring and earn a 0 or 1.

Q: Can I use the scale with a camera instead of the eye?

A: Yes. Record a short video (≥30 fps) of a bright star and examine individual frames. If you can resolve the rings in single frames, you can assign a Pickering number similar to what you would perceive visually.

Q: How does the Pickering scale relate to modern “lucky imaging”?

A: Lucky imaging relies on capturing moments of excellent seeing (often corresponding to Pickering 0‑2). By monitoring the Pickering rating in real time, you can decide when to start or pause video acquisition to maximize the fraction of usable “lucky” frames.

Bottom line The Pickering seeing scale offers a ten‑step, diffraction‑pattern‑based gauge of atmospheric steadiness. Use it to log nights with finer detail than a coarse scale, to plan high‑resolution visual or imaging work, and to compare your conditions with historical observations.

Atmospheric Transparency

What “transparency” means 

Transparency (often called  Sky Transparency  or the  Clear‑Night (CN) Scale ) describes how *clear* the atmosphere is to faint light. It is a measure of the amount of extinction caused by particles, water vapour, thin clouds, and aerosols that scatter or absorb incoming photons. Good transparency lets you see very faint stars, nebulae, and the outer reaches of the Milky Way; poor transparency washes those out even on a dark site.

The standard Clear‑Night (CN) Scale (1 – 5)

 1 Excellent (Crystal‑Clear)  

The sky is virtually free of any haze, dust, or thin cloud. Stars fainter than magnitude +9 are readily visible to the naked eye; the Milky Way is a deep, rich band. Very low surface‑brightness nebulae (e.g., the North America Nebula) are discernible without averted‑eye techniques. Extinction is typically < 0.1 mag per airmass.

 2 Good  

A slight, almost invisible haze may be present. Stars down to magnitude +8½ are easily seen; the Milky Way is still prominent but its faintest extensions are a bit subdued. Low‑surface‑brightness objects are visible with averted‑eye observation. Extinction ≈ 0.1 – 0.2 mag per airmass.

 3 Average (Moderate)  

A thin veil of moisture, dust, or faint cirrus is noticeable. Stars to magnitude +7½–+8 are clearly seen; the Milky Way is present but its outer skirts look washed out. Only the brighter parts of faint nebulae are detectable. Extinction ≈ 0.2 – 0.4 mag per airmass.

 4 Poor  

Significant thin cloud or high humidity creates a noticeable “washed‑out” look. The naked eye comfortably reaches only magnitude +6½–+7. The Milky Way is a vague glow, and most faint deep‑sky objects disappear. Extinction ≈ 0.4 – 0.8 mag per airmass.

 5 Very Poor (Bad)  

The sky is heavily veiled by thin cloud, smoke, dust, or high humidity. Only the brightest stars (≤ mag +5) are obvious; the Milky Way is invisible. Even the brightest nebulae (e.g., Orion Nebula) lose contrast. Extinction > 0.8 mag per airmass, sometimes approaching 1.5 mag.

*The scale is deliberately coarse; most observers will spend the night in a single rating, but brief “lucky” moments can briefly improve the score.*

How to estimate transparency in the field (step‑by‑step)

1. Pick a set of reference stars  spanning a range of magnitudes (e.g., +5, +6, +7, +8, +9). Choose a region high in the sky (airmass < 1.5) to avoid extra extinction from the lower atmosphere. 

2. Look with the naked eye, then with averted‑eye  (hold your gaze slightly off the object to make faint stars pop). Note the faintest magnitude you can reliably recognise. 

3. Assess the Milky Way  (if you are on a dark site): is the band bright and detailed (CN 1), just a faint glow (CN 2–3), or absent (CN 4–5)? 

4. Check a low‑surface‑brightness nebula  (e.g., the Veil Nebula, the North America Nebula, or the diffuse outer parts of M33). Can you see any structure without a camera? 

5. Record the rating  together with the time, airmass, temperature, humidity, and any known local sources of aerosol (e.g., forest fire smoke, sea‑salt spray). 

Over several nights you’ll develop a “transparency baseline” for your site, which helps you decide whether a night is worth attempting deep‑sky imaging or whether you should focus on bright planets instead.

Why the transparency rating matters 

• • • Target selection:  High‑resolution planetary work can be done under any transparency, but faint deep‑sky imaging (galaxies, nebulae) requires a CN 1–2 night. 

    • Exposure planning:  Knowing the extinction (approximate from the CN rating) lets you adjust exposure times or ISO settings before you start a long imaging run. 

    • Comparing sites:  Transparency is independent of light pollution (Bortle) and seeing (Antoniadi/Pickering). Two sites may have the same Bortle class but very different CN numbers, leading to dramatically different deep‑sky performance. 

    • Historical continuity:  The CN scale has been used by amateur clubs and professional observatories for decades, making modern logs comparable with older records.

Relationship to other atmospheric metrics

• • • Bortle Dark‑Sky Scale:  Measures background sky brightness (light pollution). You can have a Bortle 1 site (very dark) but a CN 4 night if a thin cloud layer rolls in. 

    • Antoniadi / Pickering Seeing Scales:  Measure image steadiness (turbulence). Good seeing does not guarantee good transparency; a calm night can still be hazy. 

    • Extinction coefficient (k)  a physical value expressed in magnitudes per airmass. Roughly: CN 1 ≈ k < 0.1, CN 2 ≈ 0.1‑0.2, CN 3 ≈ 0.2‑0.4, CN 4 ≈ 0.4‑0.8, CN 5 > 0.8. 

    • Sky Quality Meter (SQM) readings:  Primarily gauge sky brightness, not transparency, but a sudden rise in SQM during a night often coincides with a drop in transparency (e.g., thin clouds reflecting city light).

Practical tips to maximize transparency

• • Observe after the midnight temperature dip  the atmosphere tends to settle and moisture condenses out, improving clarity. 

  • Choose high, dry locations  altitude reduces the amount of air (and thus particles) the light must travel through; deserts and mountain tops are classic transparent sites. 

  • Stay away from local pollution sources  even a distant highway can loft dust that degrades transparency. 

  • Avoid nights with high humidity or recent rain  water droplets in the air scatter light strongly. 

  • Let the telescope equilibrate  a cold tube can create local “misting” that mimics poor transparency. 

  • Use a clean optical train  dust on lenses or mirrors adds scattering that reduces perceived transparency, especially for faint targets. 

Sample log entry (plain text)

• 2026‑02‑26 20:15 UT Dark‑Sky Reserve (elevation 1800 m)

• CN rating: 2 (good; faintest naked‑eye stars to mag +8.5, Milky Way bright but outer edges faint)

• Ambient temperature: 2 °C, relative humidity: 25 %, wind: 4 km/h from SW

• Extinction estimate (from standard star measurements): k ≈ 0.15 mag/airmass

• Notes: Light breeze, no visible cirrus. Expect decent imaging of low‑surface‑brightness galaxies with modest exposure times.

Quick FAQ (plain text)

Q: Can I use a DSLR or CCD to measure transparency?  

A: Yes. Take short exposures of a standard star field at low airmass, compare the measured instrumental magnitudes with catalog values, and compute the extinction coefficient. That gives a quantitative counterpart to the CN rating.

Q: Does a high Bortle number always mean poor transparency?  

A: No. A city (high Bortle) can have a clear night with CN 1–2 if there is no haze or fog; however, the background sky brightness will still drown out faint objects. Conversely, a remote dark site (low Bortle) can suffer from a low‑lying haze and be stuck at CN 4.

Q: How often does transparency change during a night?  

A: It can be fairly stable for several hours, but thin cirrus, humidity spikes, or aerosol plumes can appear abruptly. It is useful to re‑assess the CN rating every 30–60 minutes if you are planning long imaging sessions.

Q: Is there a “decimal” version of the CN scale?  

A: Some observers write “2‑” or “3+” to indicate a rating that sits between whole numbers, but the formal system uses integers 1‑5. If you need more precision, log the measured extinction coefficient alongside the CN rating.

Bottom line

Atmospheric transparency, expressed on the Clear‑Night (CN) 1‑5 scale, tells you how much faint light can penetrate the atmosphere. Use it to choose targets, set exposure parameters, and compare nights or sites. Pair it with seeing (Antoniadi/Pickering) and light‑pollution (Bortle) ratings for a full picture of the night sky’s quality.