Description
Dew (or condensation) appears when an optical surface cools below the dew point of the ambient air: water vapor then deposits on it as fine droplets.
In astrophotography, optics pointed at the sky lose heat through radiation and drop a few degrees below air temperature. The front lens of a refractor, the meniscus of a Schmidt-Cassegrain, or the secondary mirror of a Newtonian are the first victims.
The result is a diffuse veil that reduces contrast, causes stars to swell and bleed, and drowns faint details. Its characteristic signature: it intensifies progressively throughout the night as the temperature drops, and often starts from one edge or corner of the image.
This is a purely environmental defect, unrelated to equipment quality, and it cannot be corrected in post-processing: once a frame is fogged, the lost signal is gone for good. It differs from a light pollution gradient or residual vignetting, both of which are stable from one frame to the next.
Visual signature
A milky veil invades the image, most often starting from an edge or corner, where the optics cool fastest.
Contrast collapses: the sky background rises, faint nebulae disappear, and bright stars are surrounded by a diffuse halo, the signature of scattering by the droplets.
The most reliable sign is read in the chronology of frames: scrolling through the raws in acquisition order, one sees the veil appear then spread over the course of the night, while it was absent at the start of the session.
In advanced stages, focus appears to degrade and overall transparency drops, as if under a light fog, even though the sky is actually perfectly clear.
Differential diagnosis
Do not confuse with a light pollution gradient: the latter is stable from frame to frame and oriented toward the horizon, while dew grows over the course of the night and starts from a corner of the optics.
Distinct from residual vignetting, which darkens the corners in a constant way: dew, on the contrary, brightens the image by adding a veil.
Nothing to do with an internal reflection or a filter halo: those remain localized around a bright source and are identical on all frames, whereas a dew veil is global and evolving.
Do not mistake for passing cloud cover: a cloud attenuates the image briefly and irregularly, while dew settles in slowly and does not clear until the optics have warmed up.
Probable causes
- Radiative cooling of the front optic, which drops below the dew point
- High ambient humidity, close to saturation in the late night
- No dew heater band on the lens or corrector
- No dew shield: the optic radiates directly toward the cold sky
- Rapid temperature drop in the second half of the night
- Condensation on the sensor window or a filter (poorly sealed or unheated camera)
Course of action
- Install a dew heater set 2 to 3 degrees C above ambient temperature
- Mount a dew shield to reduce radiation toward the sky
- Control heating with a dew controller (Pegasus, ZWO) slaved to the dew point
- Monitor the gap between temperature and dew point during the session
- Against internal camera dew: desiccant packet and sensor window heater
- Sort frames chronologically and discard fogged raws before stacking
- Allow equipment to dry before storing, to avoid dew and frost
The Doc's advice
Dew is the defect that eats your late-night session without warning: everything is fine until 1 a.m., then the veil settles in and you discard half the session. The solution is simple and inexpensive: a dew heater around the front lens, set to keep the optic 2 to 3 degrees C above ambient air, plus a dew shield to limit radiation toward the sky (and do not forget the secondary mirror on a Newtonian). Watch the gap between temperature and dew point: when it drops below 2 to 3 degrees, increase heating. On the processing side, do not try to recover a fogged frame; sort and stack only the clean ones.
Think you can see this defect in your image?
Run a diagnosisFrequently asked questions
How can I tell if my image was ruined by dew rather than something else?
The decisive criterion is evolution over time. Scroll through your raws in acquisition order: if the veil is absent at the start of the session and then appears and spreads as the hours pass, it is dew. A light pollution gradient or vignetting, by contrast, are present in identical form from the very first frame. Another clue: dew causes stars to swell and surrounds them with a diffuse halo, even though the sky visually appears clear. And if you gently touch the lens or dew shield and the surface is damp, the diagnosis is confirmed.
Is a dew heater band enough to prevent dew?
On the front lens or corrector, a properly sized dew heater is sufficient in the vast majority of cases, provided its power is set to stay just above the dew point. It benefits from being complemented by a dew shield, which reduces radiation toward the sky and therefore the heating power required. Watch out for classic omissions: the secondary mirror of a Newtonian, which also gets dew, and the sensor window inside the camera, which requires internal heating or a desiccant, not the front band.
Can a frame fogged by dew be recovered in post-processing?
No. Once the frame is fogged, local contrast and signal-to-noise ratio are irreparably degraded: the veil adds a non-uniform spurious signal and raises the background. A gradient removal tool (DBE, GraXpert) can attenuate a very light veil on a barely affected frame, but the extra noise and loss of detail remain. The right approach is sorting: identify the frame where dew begins, discard all subsequent ones, and stack only the clean raws. Two clean hours are worth more than four hours where half are fogged.
At what power should the dew heater be set?
The correct setting keeps the optic 2 to 3 degrees C above the dew point, no more. Too much power creates convection currents in front of the lens (tube currents) that degrade FWHM and therefore star sharpness: one then trades a dew problem for a turbulence problem. The ideal is a dew heater controller (Pegasus PPB, ZWO, etc.) that slaves power to the measured gap between temperature and dew point. Otherwise, start low and increase in steps until dew no longer returns.