Dew Point Calculator

Predict when your optics may fog up and get equipment-specific recommendations to keep your astrophotography session dew-free.

Current Conditions

15°C
70%
2°C/hr
9.6°C
Dew Point
Dew will form
5.4°
Spread
Temp above dew point
2h 42m
Until Dew
~10:13 PM
Low RiskGood conditions with minimal dew concerns

Consider using a dew shield. Monitor conditions as temperature drops.

Equipment Risk Assessment

Refractor Objectivemoderate

Use dew prevention

SCT/Mak Correctorlow

Monitor conditions

Newtonian Primarylow

Monitor conditions

Camera Lens Frontmoderate

Use dew prevention

Eyepiecemoderate

Use dew prevention

Guide Scopemoderate

Use dew prevention

Dew Prevention Tips

🛡️

Dew Shields

Extend 1.5-2x the aperture diameter. Reduces radiative cooling by 50-70%. Essential first line of defense.

🔥

Dew Heaters

Keep optics 14.6°C or warmer. Use PWM controllers for fine temperature control.

💨

Air Movement

Small fans can help prevent condensation by breaking the boundary layer of still air.

Additional Data

Frost Point
Ice forms below this
9.6°C
Wet Bulb
Evaporative cooling limit
11.6°C
Absolute Humidity
Water vapor content
19.5 g/m³

Heater Settings

Primary OpticsLOW

Preventive warming

Secondary/CorrectorOFF

Monitor only

Finder/Guide ScopeLOW

Optional

Target Temperature
14.6°C

Keep optics 5°C above dew point

Temperature Scale

15°CCurrent
Dew Point9.6°C

Understanding Dew Point for Astrophotographers

The Science of Dew

Air can hold a limited amount of water vapor, and this capacity depends on temperature. Warmer air holds more moisture. The dew point is the temperature at which the air becomes saturated and water begins to condense.

For astrophotography, the critical factor is that telescope optics radiate heat into the cold sky, cooling faster than the surrounding air. This means your lens can reach dew point even when the air temperature is still well above it.

Temperature Spread

The "spread" is the difference between current air temperature and dew point. This is the key metric for predicting dew problems:

  • ●10°C+ spread: Very safe conditions
  • ●5-10°C spread: Low risk, monitor conditions
  • ●3-5°C spread: Moderate risk, use prevention
  • ●1-3°C spread: High risk, active heating needed
  • ●<1°C spread: Condensation imminent

Dew Prevention Methods

🛡️

Dew Shields

Simple tube extensions that block radiative heat loss to the sky. Should extend 1.5-2x the aperture diameter. Reduces cooling rate by 50-70% with no power needed.

🔌

Dew Heaters

Heated strips that wrap around optics. Use with PWM controller for temperature regulation. Keep optics 3-5°C above dew point—more causes thermal turbulence.

🌡️

Temperature Sensors

Automated systems use sensors to monitor optic temperature and adjust heater power. Essential for unattended imaging sessions.

💨

Auxiliary Fans

Low-speed fans break the boundary layer of still air, helping prevent condensation and aiding thermal equilibrium of primary mirrors.

Frequently Asked Questions

What is dew point and why does it matter for astrophotography?+
Dew point is the temperature at which water vapor in the air condenses into liquid water. When your telescope or camera lens cools below the dew point (through radiative cooling to the cold night sky), moisture condenses on the optics, ruining your images. Understanding dew point helps you predict and prevent this problem.
How do I measure humidity and temperature for accurate results?+
Use a digital hygrometer or weather station placed near your equipment at the same height. Many astrophotographers use portable weather meters or smartphone-connected sensors. Avoid using indoor readings or weather forecasts, as conditions at your imaging location may differ significantly.
What is a safe temperature spread above dew point?+
A spread of 5°C (9°F) or more is generally considered safe for most conditions. Between 3-5°C is moderate risk requiring monitoring, and below 3°C means dew formation is likely imminent. Optics that radiate heat to the sky (like refractor objectives) need larger margins than protected optics.
How do dew heaters work and when should I use them?+
Dew heaters are resistive heating strips that wrap around your optics to keep them above the dew point. Use them when the temperature spread is below 5°C or dropping quickly. The goal is to keep optics about 5°C above dew point—too much heat can cause thermal currents that blur images.
Why does my equipment fog up even when it's not that humid?+
Relative humidity can be deceiving. Even at 50% humidity, if the temperature drops enough, you'll reach the dew point. Also, telescope optics cool faster than air temperature due to radiative heat loss to the cold sky. This is why a dew shield (which blocks sky exposure) is often the first line of defense.
What's the difference between dew and frost on my optics?+
Dew forms when the dew point is above 0°C (32°F), creating liquid water droplets. Frost forms when the dew point is below freezing, causing ice crystals to form directly. Frost can be more damaging and harder to remove. The calculator shows the frost point when applicable.
How does cooling rate affect dew formation?+
On clear, calm nights, radiative cooling can drop temperatures 2-4°C per hour. This means even safe conditions at sunset can become problematic by midnight. Cloudy or windy nights have slower cooling rates. Use the calculator's cooling rate setting to estimate when you'll need dew prevention.
Which equipment is most susceptible to dew?+
Refractor objectives, SCT/Mak corrector plates, camera lenses, and guide scopes are most at risk because they point at the cold sky. Newtonian mirrors are somewhat protected by the tube. Eyepieces can fog from breath and body heat moisture, not just atmospheric dew.