Description
The moon gradient is a sky-background brightness gradient oriented toward the Moon. Moonlight scattered by the atmosphere illuminates the sky non-uniformly and drowns the faint signal on the side closest to the Moon.
Its intensity depends on the phase (maximum around full Moon, negligible near new Moon) and on the angular distance between the target and the Moon: the farther away the target, the cleaner the background becomes.
It primarily affects broadband imaging (galaxies, clusters, RGB), while narrowband (Ha, OIII, SII) resists it very well because the narrow filter rejects most of the scattered lunar spectrum.
Mechanically identical to a light pollution gradient, it differs by its orientation (toward the Moon) and its variability from one night to the next. Not to be confused with cirrus haze.
Visual signature
A soft gradient sweeps across the image, the background being noticeably brighter on the Moon's side and darkening toward the opposite edge.
The effect is low-frequency and textureless, like a pollution gradient, but its orientation follows the Moon: the same target imaged at two different phases shows a gradient of different intensity.
The background often picks up a bluish or grayish cast (solar spectrum reflected by the Moon), and the contrast of faint nebulosity collapses on the illuminated side.
In narrowband, the same sky yields a far more neutral and darker background: this is a useful test for confirming the lunar origin of the gradient.
Differential diagnosis
Not to be confused with a light pollution gradient: the scattering mechanism is the same, but the moon gradient points toward the Moon and varies with its phase, while the pollution gradient points toward the city and remains constant from night to night.
Distinct from residual vignetting: vignetting is radial and symmetric around the optical center; the moon gradient runs in one direction.
Separate from cirrus haze: cirrus produce a non-uniform background that varies from one frame to the next; the moon gradient is smooth and stable throughout the night.
Simple test: suspend broadband imaging while the Moon is high and bright; if the background clears once the Moon has set, the diagnosis is confirmed.
Probable causes
- Moonlight scattered by the atmosphere
- Phase close to full Moon (sky background heavily illuminated)
- Target too close angularly to the Moon
- Broadband imaging, sensitive to the lunar spectrum
- Target low in the sky, passing through more scattering atmosphere
- Mist or humidity amplifying the scattering of moonlight
Course of action
- Schedule broadband targets around the new Moon
- Reserve moonlit nights for narrowband work (Ha, OIII, SII)
- Keep the target at least 60-90 degrees away from the Moon
- Remove the gradient during processing (DBE, GraXpert) before stretching
- Place sample points only on the sky background
- Favor targets high in the sky
- Use a narrowband or dual-band filter to resist moonlight
The Doc's advice
The Moon is your worst enemy in broadband and almost irrelevant in narrowband. Start with the calendar: plan your RGB targets (galaxies, clusters) around the new Moon, and keep the full-Moon nights for Ha/OIII/SII, which are largely unaffected. If you must image with the Moon up, keep your target at least 60-90 degrees away from it -- that alone makes a huge difference to the gradient's severity. During processing, the same approach applies as for light pollution: DBE or GraXpert before stretching, sample points on the background only. And keep in mind that a very strong moon gradient permanently buries faint signal -- no amount of processing brings back contrast that was drowned out at acquisition.
Think you can see this defect in your image?
Run a diagnosisFrequently asked questions
Can you image deep sky with the Moon above the horizon?
Yes, but it depends on what you are imaging. In narrowband (Ha, OIII, SII), the Moon is barely a nuisance: you can photograph emission nebulae even under a full Moon. In broadband (galaxies, clusters, RGB), it is much harder: the sky background rises, contrast falls, and the gradient sets in. If you must image in broadband with the Moon up, keep the target as far as possible from the Moon (60-90 degrees minimum) and prefer nights around the crescent rather than the full Moon.
Is narrowband immune to the moon gradient?
Very resistant, but not completely insensitive. A narrowband filter passes only a thin slice of the spectrum and rejects most scattered moonlight, making the background far more neutral and stable even under a full Moon. Dual-band filters offer a good compromise for color cameras. The limits: halos around very bright stars may persist, and a residual haze remains possible if the target is right next to the Moon. For broadband targets, narrowband is not an option.
Moon gradient or light pollution: how to tell them apart?
Look at orientation and variability. The moon gradient points toward the Moon and changes from one night to the next depending on phase and position; it disappears once the Moon has set. The pollution gradient points toward the city or ground and stays constant night after night, regardless of the Moon. Simple test: image the same target on a moonless night. If the gradient disappears, it was lunar; if it persists in the same direction, it is light pollution.
How do I remove a moon gradient during processing?
Exactly like a light pollution gradient: with a gradient-removal tool (DBE, ABE, GraXpert) applied to the linear image, before stretching and before color calibration. Sample points are placed on the sky background only. This corrects the gradient well, but bear in mind that a strong moon gradient means the background has buried part of the faint signal, and no processing recovers the lost contrast. Removal cleans up the appearance; it does not resurrect the faint detail erased by moonlight.