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.
Why optics cool faster than air
The physics behind telescope dewing is radiative cooling. A glass surface facing the open sky continuously emits infrared radiation upward. When the sky is clear, that radiation is not returned from above — it escapes into space, and the glass surface loses energy. The surrounding air cannot conduct heat to the glass fast enough to compensate, so the optical surface temperature drops below the ambient air temperature.
The rate of cooling depends on the solid angle of sky the surface “sees.” A primary mirror in an open-tube reflector sees a very large patch of sky and cools aggressively. A lens at the front of a refractor sees a smaller angle (limited by the tube walls) and cools somewhat slower. A dew shield extends the effective tube length, reducing the sky angle the front element sees, which slows radiative cooling and buys time before the surface reaches the dew point.
Interpreting the dew margin throughout the night
The dew point is roughly constant through a night (it changes only as humidity changes). The ambient air temperature, however, typically falls as the night progresses. This means the dew margin usually narrows through the observing session, even without a change in absolute humidity.
A useful habit is to check the margin at the start of the session and estimate when it might close, given the forecast temperature drop. If the margin at midnight is 4 degrees and the temperature is expected to fall another 5 degrees overnight, the margin may close well before dawn. Deploy dew heating before the margin reaches 3 degrees — not after dew appears, since re-drying optics in the field is difficult and waiting damages image quality.
Dew shields vs. powered dew heaters
A dew shield addresses the root cause — radiative cooling — by limiting the sky angle the optic faces and trapping a layer of warmer air around the glass. On nights with a large dew margin (above 8 degrees), a shield alone is often sufficient and introduces no electrical complexity at the telescope.
A powered dew heater targets the symptom: it adds just enough heat to keep the glass surface temperature slightly above the dew point, regardless of radiative losses. On humid nights with a small margin (under 5 degrees), a heater is necessary. Many observers use both: a shield to reduce how hard the heater must work, and a heater as a reliable backstop on difficult nights. Temperature-controlled heater controllers (which modulate power based on measured surface temperature) are more efficient than fixed-power straps and avoid the image degradation that comes from over-heating optics.