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Sky-to-Object Flux Ratio
15.8489
Sky-to-Object Flux Ratio vs Sky Brightness
Formula
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.
Esempio Risolto
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.
Domande Frequenti
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.