Temperature spread
The air-temperature minus dew-point spread shows how much cooling is available before saturated conditions are reached.
Astronomy Tools
Free observing-planning tool
Dew Risk Calculator
Estimate when lenses, corrector plates, eyepieces and camera equipment may cool below the dew point, then plan sensible dew control before condensation forms.
Air conditions
Use current conditions, or values from the Clear Sky Indicator for the hour you plan to observe.
Used to estimate the risk later in the session, assuming the amount of moisture in the air remains broadly similar.
Optical surface
Exposed glass can radiate heat to the clear sky and become cooler than the surrounding air.
This is an estimate, not a measured value. Clear, calm nights can cool exposed optics more strongly.
The selected cooling allowance is a practical planning estimate. Wind, cloud, dew shields, telescope orientation and local ground conditions can change the actual optic temperature.
Understanding the result
Dew forms when an optical surface reaches the dew point—not simply when relative humidity is high.
Radiative cooling
An optic facing a clear sky can cool below ambient air temperature, so it may dew before nearby metalwork or the weather station.
Gentle heat works best
A dew heater should normally keep the optic only a few degrees above the dew point. Excess heat can create local air currents and degrade images.
Forecast, not a guarantee
Humidity, fog, wind and cloud can change quickly. Recheck conditions during the session and inspect exposed optics regularly.
Calculation method
The calculator uses the Magnus approximation for dew point. Projected humidity assumes broadly unchanged water-vapour content as the air cools.
Dew point = --