Description
Cirrus clouds are thin high-altitude clouds, often invisible to the naked eye at night, that let enough light through to make the sky look clear while severely degrading transparency.
In the image, they appear as wide diffuse halos around bright stars, a sky background that brightens inhomogeneously, and above all transparency that varies from one sub to the next as the veils drift through.
This is a purely atmospheric and unpredictable defect: it depends neither on equipment nor on settings, and cannot really be corrected, since each affected sub has lost signal and gained a non-uniform veil.
It differs from dew fogging, which builds up progressively and does not clear without reheating the optic, and from a light-pollution gradient, which is stable and directional. Poor seeing degrades star sharpness; cirrus, by contrast, mainly degrade transparency.
Visual signature
Bright stars are surrounded by wide diffuse halos, and the image loses overall contrast and depth, as if veiled by gauze.
The sky background becomes inhomogeneous, with brighter patches corresponding to cirrus banks, whose shape can change from one sub to the next.
The most telling sign is transparency variation across the series: measuring the flux of the same star across subs shows its brightness fluctuating, whereas a truly clear night gives stable measurements.
Unlike dew, the veil comes and goes: some subs are clean, others milky, depending on whether a cirrus bank was passing through the field at the time of capture.
Differential diagnosis
Not to be confused with dew fogging: dew builds up progressively and does not clear until the optic is reheated, whereas cirrus comes and goes, alternating clean and veiled subs.
To be distinguished from a light-pollution gradient or a moon gradient: these gradients are smooth, directional, and stable from one sub to the next, whereas cirrus veil is irregular and changes over time.
To be separated from bad seeing: poor seeing broadens stars (degraded FWHM) but preserves transparency, whereas cirrus keeps star sharpness while dropping transparency and adding halos.
Not to be taken for a hardware defect: no optics adjustment, focus setting, or sensor configuration reproduces this time-varying signature. It is the weather, not the telescope.
Probable causes
- High-altitude clouds (cirrus) too thin to see with the naked eye at night
- Frontal cloud layer or high-altitude veil forecast but underestimated
- High-altitude humidity forming a diffuse veil
- Intermittent cloud bands drifting through during the session
- Session planned on a night with a low transparency index
- Target low on the horizon, crossing more atmosphere and more cloud layers
Course of action
- Sort subs: reject veiled frames by measuring a reference star's flux or transparency
- Check transparency forecasts (Meteoblue, satellite imagery) before the session
- Reserve deep-sky sessions for nights with genuinely good transparency
- Target objects high in the sky, less exposed to horizon haze
- Extend the session to accumulate enough clean subs despite culling
- Prefer narrowband imaging, which is less sensitive to haze, when transparency is average
- As a last resort, attenuate mild residual haze with gradient removal (DBE, GraXpert)
The Doc's advice
Cirrus clouds are the weather trolling you: the sky looks clear and yet one in three subs is veiled. The winning reflex is ruthless culling: measure the flux of a reference star or the background on each sub and discard the ones that drop off. Stacking veiled subs alongside clean ones just dilutes the signal and skews the color calibration. To anticipate, check transparency maps (Meteoblue, satellite images) before going out. And if the veils roll in mid-night, do not fight it: pack up and save your disk space for a real clear night.
Think you can see this defect in your image?
Run a diagnosisFrequently asked questions
How do I know whether my subs are veiled by cirrus rather than by dew?
The criterion is temporal regularity. Cirrus comes and goes: the series alternates clean and veiled subs depending on cloud band passage, and transparency can improve again in the middle of the night. Dew, by contrast, builds up monotonically and progressively from a corner of the image and does not clear until the optic is reheated. Physical check: lightly touch the lens or dew shield; if the surface is dry but the image is still veiled, it is cirrus, not condensation.
Can images taken under cirrus be salvaged?
It depends on the degree of veil. Barely affected subs can be kept, provided you sort carefully: measure the flux of a reference star or background depth, and discard those that drop off noticeably. Gradient removal (DBE, GraXpert) can attenuate a light residual veil, but cannot recover lost signal or contrast. Heavily veiled subs should be discarded: stacking them degrades the whole integration and biases the color balance. Narrowband imaging is more tolerant and salvages more subs than a broadband color setup.
Is narrowband imaging less sensitive to cirrus?
Relatively, yes. A narrowband filter (Ha, OIII, SII) only passes a narrow slice of the spectrum and rejects much of the parasitic light scattered by cirrus, making the target signal more resilient to the veil. But it is not magic: overall transmission still drops when a thick bank passes, and halos around bright stars persist. Narrowband is a good choice on nights with average transparency; it does not replace a genuinely clear night for broadband targets (galaxies, clusters, RGB).
How do I forecast the presence of cirrus before a session?
Several tools complement each other. Dedicated astro forecast models (Meteoblue Astronomy, Clear Outside) provide an hourly transparency index, distinct from the seeing index. Infrared or water-vapor satellite imagery reveals high-altitude veils that do not always appear on low-cloud charts. Keep the two concepts clearly separate: seeing measures turbulence (star sharpness), transparency measures atmospheric absorption (haze, cirrus). A night can be turbulent but transparent, or steady but veiled.