Understanding Your SkyQI Results
A complete guide to interpreting your sky quality measurements
You've uploaded your night sky photo to SkyQI. Now you're looking at numbers like "SQM: 18.5" and "Bortle: 7" - but what do they actually mean?
This guide explains every metric in your results and what they tell you about your sky.
The Key Metrics
When SkyQI analyzes your photo, you receive several measurements:
- SQM Value - The scientific measurement
- Bortle Scale - The practical classification
- Star Count - What the algorithm detected
- Light Pollution Level - Simple rating
- NELM - Naked Eye Limiting Magnitude
- Confidence Score - How reliable the results are
Let's break down each one.
SQM: Sky Quality Meter Value
What It Is
SQM measures sky brightness in magnitudes per square arcsecond (mag/arcsec²). This is the same unit used by professional astronomers worldwide.
How to Read It
Higher numbers = darker skies
This is counterintuitive! Here's why: astronomers measure brightness on a logarithmic scale where brighter objects have lower numbers.
| SQM Value | Sky Condition |
|---|---|
| 22.0+ | Pristine dark sky (rare) |
| 21.5-22.0 | Excellent dark site |
| 21.0-21.5 | Good rural sky |
| 20.0-21.0 | Rural/suburban transition |
| 19.0-20.0 | Suburban sky |
| 18.0-19.0 | Bright suburban |
| 17.0-18.0 | Urban sky |
| 16.0-17.0 | City center |
| <16.0 | Heavily light-polluted |
What Your Number Means
SQM 21.5+ - Excellent! You're at a genuinely dark location. The Milky Way is impressive, many deep-sky objects are visible to the naked eye. Treasure this site.
SQM 20.0-21.5 - Good A decent dark sky. Milky Way visible but may be washed out near horizon. Good for astronomy and stargazing.
SQM 19.0-20.0 - Moderate Typical of rural areas near towns or outer suburbs. Milky Way faintly visible overhead on good nights.
SQM 18.0-19.0 - Poor Suburban conditions. Only bright stars and planets clearly visible. Milky Way invisible or barely perceptible.
SQM <18.0 - Very Poor Urban/inner-city conditions. Only the brightest stars visible. This is where most urban Indians live.
The Math Behind It
Each 1.0 difference in SQM represents about 2.5x difference in brightness.
- SQM 20.0 is 2.5x darker than SQM 19.0
- SQM 21.0 is ~6x darker than SQM 19.0
- SQM 22.0 is ~16x darker than SQM 19.0
So a pristine sky (22.0) is roughly 40 times darker than a typical suburban sky (18.0).
Bortle Scale: The Practical Classification
What It Is
The Bortle Dark-Sky Scale was created by amateur astronomer John Bortle in 2001. It classifies sky darkness on a scale of 1-9 based on what you can actually see.
The Scale
| Class | Name | Description | What You See |
|---|---|---|---|
| 1 | Excellent dark-sky site | Best possible | Zodiacal light, gegenschein, Milky Way casts shadows |
| 2 | Typical dark site | Very dark | Milky Way highly structured, zodiacal light obvious |
| 3 | Rural sky | Dark | Milky Way still impressive, M33 visible to naked eye |
| 4 | Rural/suburban transition | Fairly dark | Milky Way obvious but lacks detail |
| 5 | Suburban sky | Limited | Milky Way very weak, only visible overhead |
| 6 | Bright suburban | Light-polluted | Milky Way invisible, sky glow obvious at horizon |
| 7 | Suburban/urban transition | Poor | Only brightest stars visible, sky appears grey |
| 8 | City sky | Very poor | Only brightest stars and planets visible |
| 9 | Inner-city sky | Worst | Only Moon, planets, and handful of stars visible |
What Your Class Means
Bortle 1-2 - World-class dark site These locations are increasingly rare. Think remote deserts, mountain observatories, or protected dark-sky reserves. If you're getting this reading, you're somewhere special.
Bortle 3-4 - Good observing conditions Typical of rural areas far from cities. Amateur astronomers travel to reach these conditions. Stargazing is rewarding here.
Bortle 5 - The suburban divide This is where the Milky Way starts to disappear. Many people in smaller towns or outer suburbs live here. You can see main constellations but miss the fainter wonders.
Bortle 6-7 - Light-polluted suburban/urban Most suburban India falls here. Sky has noticeable glow, only bright stars visible. This is what most people think of as "normal" - but it's far from natural.
Bortle 8-9 - Urban core Major city centers. Delhi NCR, Mumbai, Bangalore, Chennai city centers typically measure here. You might see 20-50 stars on a good night instead of thousands.
SQM to Bortle Conversion
| SQM Range | Bortle Class |
|---|---|
| 21.99+ | 1 |
| 21.89-21.99 | 2 |
| 21.69-21.89 | 3 |
| 20.49-21.69 | 4 |
| 19.50-20.49 | 5 |
| 18.94-19.50 | 6 |
| 18.38-18.94 | 7 |
| <18.38 | 8-9 |
Star Count
What It Is
This is the number of stars our algorithm detected in your photo. It's not a direct measure of sky quality, but it provides supporting evidence.
How to Interpret It
More stars = darker sky (generally)
But star count depends heavily on:
- Your camera quality
- Exposure settings
- How much sky is in frame
- Photo sharpness
Typical Ranges
| Star Count | Interpretation |
|---|---|
| 100+ | Dark sky, good camera |
| 50-100 | Good conditions |
| 20-50 | Moderate light pollution |
| 5-20 | Significant light pollution |
| <5 | Heavy light pollution or poor photo quality |
Limitations
- Phone cameras capture fewer stars than DSLRs
- Short exposures show fewer stars
- Some faint stars may be missed
- Noise can be misidentified as stars (we filter this, but imperfectly)
Use star count as supporting information, not the primary metric.
Light Pollution Level
What It Is
A simple, human-readable classification of your sky quality.
The Categories
| Level | Meaning |
|---|---|
| Excellent | Pristine or near-pristine conditions (Bortle 1-2) |
| Good | Good dark sky (Bortle 3-4) |
| Moderate | Some light pollution but still decent (Bortle 5) |
| Poor | Significant light pollution (Bortle 6-7) |
| Very Poor | Heavy light pollution (Bortle 8-9) |
This is essentially a friendlier way to understand your Bortle class.
NELM: Naked Eye Limiting Magnitude
What It Is
NELM estimates the faintest star you could see with your naked eye under these sky conditions.
How to Read It
Higher numbers = fainter stars visible = darker sky
| NELM | What You Can See |
|---|---|
| 7.0+ | Extremely faint stars, exceptional sky |
| 6.0-7.0 | Faint stars visible, good dark sky |
| 5.0-6.0 | Average stars visible, moderate conditions |
| 4.0-5.0 | Only bright stars visible, light-polluted |
| <4.0 | Only the brightest stars, heavy light pollution |
Reference Points
| Object | Magnitude | Visibility |
|---|---|---|
| Sirius (brightest star) | -1.46 | Always visible |
| Vega | 0.03 | Always visible |
| Polaris (North Star) | 1.98 | Usually visible |
| Faintest star in Big Dipper | 3.31 | Visible in cities |
| Faintest Pleiades star (naked eye) | 5.6 | Needs good sky |
| Faintest visible in perfect conditions | ~7.0 | Rare |
Confidence Score
What It Is
This indicates how confident our algorithm is in the results, based on photo quality and analysis consistency.
Interpreting Confidence
| Score | Meaning |
|---|---|
| High | Clear photo, consistent analysis, reliable results |
| Medium | Decent photo, results should be accurate |
| Low | Photo quality issues or inconsistent signals |
What Affects Confidence
Higher confidence:
- Sharp, clear photos
- Good exposure (not too bright or dark)
- No obstructions
- Multiple detected stars
- Consistent measurements across different analysis methods
Lower confidence:
- Blurry photos
- Overexposed sky
- Clouds or obstructions
- Very few stars detected
- Photo processing artifacts
If you get a low confidence score, try taking another photo with better conditions or settings.
Comparing Your Results
To Other Locations
Use the SkyQI map to compare:
- Your location vs. nearby readings
- Urban vs. rural differences
- Your city vs. other cities
Over Time
Track your location over time:
- Is light pollution getting worse?
- Do results vary by season?
- Does moon phase affect your measurements?
To Expectations
| Your Location | Expected Range |
|---|---|
| Major Indian metro center | Bortle 8-9, SQM 16-18 |
| Metro suburbs | Bortle 6-8, SQM 17-19 |
| Small city | Bortle 5-7, SQM 18-20 |
| Rural area | Bortle 3-5, SQM 20-21.5 |
| Remote/mountain | Bortle 2-4, SQM 21-22 |
What Can You Do With This Information?
If Your Results Are Poor (Bortle 7+)
- Advocate locally - Share data with RWA, local representatives
- Make personal changes - Reduce your own light pollution contribution
- Find darker spots - Use the map to find better locations nearby
- Track changes - Monitor if things improve or worsen
If Your Results Are Moderate (Bortle 5-6)
- Protect what you have - These locations are increasingly rare
- Document - Regular measurements create valuable records
- Educate neighbors - Small changes could push you into "good" territory
If Your Results Are Good (Bortle 4 or better)
- Celebrate! - You're in a minority of humanity
- Protect the site - Advocate against new lighting
- Share - Invite others to experience dark skies
- Contribute data - Dark-sky measurements are valuable for research
Frequently Asked Questions
Q: My reading seems wrong - what happened? Check: clouds, moon, camera settings, obstructions in frame. Try again on a clear, moonless night.
Q: Why do I get different results from the same location? Moon phase, clouds, humidity, and camera settings all affect results. For best comparisons, control these variables.
Q: How does this compare to professional SQM devices? SkyQI is calibrated against professional equipment. For typical photos, expect accuracy within ±0.5 mag/arcsec².
Q: Can I improve my location's results? Not alone - but collective action (better municipal lighting, reduced commercial lighting) can improve area-wide conditions.
Next Steps
Now that you understand your results:
- Share your reading - Add to the global map
- Compare - See how your location ranks
- Learn more - Read about reducing light pollution at home
- Take action - Join the movement for darker skies
Questions about your results? Contact us through skyqi.in.
This is part of StarQI's educational series. Visit www.skyqi.in to measure your sky.
Featured Image: images/featured_6_understanding_results.jpg
Tags: #SkyQI #Results #SQM #BortleScale #Tutorial #HowTo
Category: How-To
Reading Time: 8 minutes
Slug: understanding-your-results