Why do telescope optics dew when the air has not?

Optical surfaces facing the open sky radiate heat away and cool a few degrees below the surrounding air. If they drop to the dew point before the air does, moisture condenses on the glass even while the air itself stays clear. That is why a dew heater keeps optics a degree or two above ambient.

How is the dew point calculated?

The tool uses the Magnus-Tetens formula. It converts temperature and relative humidity into the actual water vapour content, then finds the temperature at which that vapour would saturate the air. That saturation temperature is the dew point.

What dew margin is risky?

When the air is within about 3 degrees of the dew point, radiating optics will very likely dew over within an hour or two. A margin of 5 degrees or more is comfortable, but high humidity can erode it quickly as the night cools, so watch the trend.

Do I need a dew heater or just a dew shield?

A dew shield reduces sky exposure and buys time, often enough on dry nights. On humid nights with a small margin, only a powered dew heater that holds the optics above the dew point reliably prevents condensation. Refractors and corrector plates are especially prone.

Does this apply to my camera lens too?

Yes. Any exposed glass facing the sky behaves the same way, so the dew point and margin apply equally to camera lenses, finder scopes, and eyepieces. Wide front elements with a clear sky view dew fastest.

Dew Point Risk Calculator for Optics

Dew forms on telescope optics when their surface temperature falls to the dew point. Because optics radiate heat to the cold sky, they cool below the surrounding air and can dew up while the air stays clear. This calculator finds the dew point from temperature and humidity and tells you how much margin you have before the optics fog.

How it works

The dew point comes from the Magnus-Tetens approximation. With temperature T in Celsius and relative humidity RH as a fraction:

a = 17.27,  b = 237.7
γ  = (a × T) / (b + T) + ln(RH)
Td = (b × γ) / (a − γ)

Td is the dew point. The dew margin is simply T − Td. Because exposed optics cool a few degrees below ambient, a margin under about 3 degrees means condensation is likely soon, while a larger margin gives you breathing room.

Example and tips

At 12 degrees Celsius and 85 percent humidity, the dew point is about 9.6 degrees — a margin of only 2.4 degrees, so a dew heater is essential. Drop the humidity to 55 percent and the dew point falls to around 3 degrees, a comfortable 9-degree margin. Watch the trend through the night: as temperature falls toward a fixed dew point, the margin shrinks, so set up dew control before the margin closes rather than after the optics have already fogged.