What dew is and why it forms on optics
Dew forms when a surface cools below the dew point, the temperature at which water vapour in the surrounding air starts to condense. On clear, calm nights, exposed surfaces radiate heat to the sky and can become colder than the air. A telescope’s front lens or corrector plate, an eyepiece, or a finder scope can cool enough for moisture to gather on them, even when the air itself feels dry.
Humid nights with little wind tend to be worst. Dew can start within an hour of setting up, or not at all, depending on conditions.
Signs of dew
- Stars and planets slowly lose contrast and brightness.
- The view looks soft or hazy, as if there is a film in the way.
- Droplets or a sheen appear on the front glass when you shine a torch on it, using a dim red light to protect your night vision.
- The eyepiece or finder fogs when you breathe near it.
Dew is easy to confuse with focus problems or poor seeing. If the view degrades gradually and a quick check of the front of the telescope shows moisture, dew is the likely cause. The broader telescope troubleshooting guide can help you rule out other causes.
Passive prevention
Passive methods slow down cooling or shelter the optics:
- Dew shield. A tube extension on the front of the telescope that reduces the optic’s exposure to open sky. It limits radiative cooling, so dew forms later or less. Many refractors and catadioptric telescopes have them as accessories, and a simple one can be made from flexible foam sheet. A longer shield generally helps more, although a very long one can catch the wind.
- Lens cap between targets. Capping the front when you are not observing helps a little on short breaks.
- Setting up in sheltered spots. Trees, walls or an awning can reduce exposure to the open sky, but make sure they do not block your view or cause turbulence.
- Keeping spare eyepieces in a warm pocket or case. Swap them when one fogs.
Active prevention
Active methods add a small amount of heat:
- Dew heater strips. Flexible heater bands wrap around the telescope near the front lens, or around an eyepiece or finder, and run from a battery or power pack through a controller. Keep the power low, since the aim is to hold the optic just above the dew point rather than warm the glass noticeably. Excess heat can create tube currents that blur the image.
- Hand warmers. Disposable chemical hand warmers can be placed near the front of the shield or finder as a low-tech option. Keep them from touching glass directly and follow the manufacturer’s directions.
Always follow the instructions that come with any heater product, and keep cables away from the telescope’s moving parts.
Dew already formed
If dew has already condensed on the glass, you have a few options:
- Move it under cover. Bringing the telescope under an awning or into a porch for a short while, with the front uncapped, lets moisture evaporate faster than it does under open sky.
- Use a hair dryer on a low, cool or warm setting. Hold it at a distance and keep it moving, rather than concentrating heat on one spot. Avoid hot air on plastic parts. Let the telescope settle afterward because warm air in the tube may distort the image.
- Do not wipe wet glass. Wiping droplets can drag grit across the surface. Let the moisture evaporate or use gentle warming where possible. If you must clear a smear later, see cleaning telescope eyepieces.
Be especially careful with open-tube reflectors. Dew can also form on the primary mirror, but there are few safe quick fixes, so prevention and a dew-free storage routine matter more. A mirror cleaning is a last resort; see cleaning a telescope mirror.
Do not forget eyepieces, finders and cameras
Dew often hits the smallest items first. Finder scopes and red-dot finders, eyepieces and camera lenses may be exposed to the sky and cool quickly. A small heater strip or dew shield on the finder, and swapping eyepieces from a warm case, can save a session.
After the session
Even with good protection, equipment can be damp at the end of the night.
- Cap the telescope and bring it inside, but do not put it straight into a sealed case while wet, as trapped moisture can encourage mould on coatings.
- Leave the covers and caps off in a dry, dust-free room for some hours so the optics can dry gradually.
- Wipe the outside of the tube and the mount with a dry cloth.
If you are considering leaving your equipment outside, read can I leave my telescope outside first.
Frequently asked questions
How does a dew shield work? It shades the front of the telescope from open sky, which reduces how much heat the glass radiates away and so how quickly it cools below the dew point.
Can a telescope look through clouds? No. Clouds block visible light, so a telescope shows little more than a grey or brighter patch. Thin high haze may allow some bright objects such as the Moon to be seen, but with reduced detail.
Will a dew heater damage my telescope? Used at low power as directed, heaters are designed for this purpose. Following the instructions is important, since too much heat can distort the view.