Light Pollution Impact Calculator Formula
Understand the math behind the light pollution impact calculator. Each variable explained with a worked example.
Formulas Used
Sky-to-Object Flux Ratio
flux_ratio = pow(10, (object_brightness - sky_brightness) / 2.5)Object Contrast (%)
contrast_pct = 100 / (1 + pow(10, (object_brightness - sky_brightness) / 2.5))Variables
| Variable | Description | Default |
|---|---|---|
sky_brightness | Sky Brightness(mag/arcsec2) | 19 |
object_brightness | Object Surface Brightness(mag/arcsec2) | 22 |
How It Works
Light Pollution and Contrast
Light pollution raises the sky background, drowning out faint objects.
Key Relation
The flux ratio of sky to object is:
ratio = 10^((mag_obj - mag_sky) / 2.5)
A higher ratio means the sky overwhelms the target. Object contrast is approximately 1 / (1 + ratio).
Darker skies (higher mag/arcsec2) dramatically improve contrast for faint targets.
Worked Example
Sky = 19 mag/arcsec2 (suburban), object = 22 mag/arcsec2.
- 01ratio = 10^((22 - 19) / 2.5)
- 02ratio = 10^1.2 = 15.85
- 03contrast = 100 / (1 + 15.85) = 5.94%
- 04The object has only 5.9% contrast against the bright sky.
Frequently Asked Questions
What is a good sky brightness for deep-sky imaging?
Bortle 3 or darker (about 21.5 mag/arcsec2 or higher) is excellent. Bortle 7 suburban skies (about 19 mag/arcsec2) are challenging for faint nebulae.
Do narrowband filters help?
Yes. Narrowband filters reject broadband light pollution while passing specific emission lines (H-alpha, OIII), dramatically improving contrast.
Can I subtract sky background in software?
Partially. Software can subtract the smooth background, but the shot noise from bright skies remains and limits the achievable SNR.
Ready to run the numbers?
Open Light Pollution Impact Calculator