THE ASTRO MANUAL
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Plan tonight

Dew point calculator

Enter the temperature and humidity from your weather app. See the dew point, how close your optics are to it, and when to run the dew heaters.

Read the guide: Why your telescope dews up, and how to stop it

Weather
Units
°C
%
°C
The dew point usually holds steady overnight while the air cools, so the low decides whether dew arrives later.
Optics
°C
Glass facing a clear sky loses heat by radiation and runs below the air temperature, often by 2 to 4 °C on a still night. Use less under cloud or in wind, and 0 for a sealed camera window with its own heater.

Dew point

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Temperatures tonight

How to use the dew point calculator

  1. Read the current temperature and relative humidity from a weather app or a sensor at your site.
  2. Add the forecast overnight low if you have it.
  3. Leave the optics cooling at 3 degrees for a clear, still night, or lower it for cloud or wind.
  4. Follow the verdict. Switch the heaters on before the air reaches the temperature shown, since dew is easier to prevent than to clear.

Questions

What is the dew point?

The temperature at which the air holds all the water vapour it can. Any surface colder than the dew point collects dew. Below 0 °C the moisture forms frost instead, starting at the frost point, which is a little above the dew point.

Why do my optics dew up when the air is above the dew point?

On a clear night, glass that faces the sky radiates heat to space and runs a few degrees colder than the air around it. The corrector plate of a Schmidt-Cassegrain or the front element of a lens can reach the dew point well before the air does.

How warm should a dew heater keep the optics?

Just above the dew point is enough. A couple of degrees of margin is safe, and more only wastes power and can add heat currents in front of the optics. Controllers that read a sensor at the optics do this for you.

Does a dew shield replace a heater?

A dew shield cuts how much sky the glass sees, so it cools less and dews later. On humid nights it delays dew rather than preventing it, so most imagers use both.

How accurate is the dew point here?

The Magnus formula with these constants is within about 0.35 °C of the full thermodynamic value from -45 to 60 °C. The bigger uncertainty is your humidity reading, since cheap sensors are often off by 5% or more.

Method

Dew point

The Magnus formula with the constants of Alduchov and Eskridge (1996), the set most weather services use:

g  = ln(RH / 100) + a T / (b + T)
Td = b g / (a - g)

a = 17.625      b = 243.04 °C      T air temperature, °C

Worked example: 20 °C at 50% gives g = -0.693 + 1.340 = 0.647 and Td = 9.3 °C, matching published dew point tables.

Frost point

Humidity is reported against water even below freezing. The actual vapour pressure is e = 6.1094 exp(a T / (b + T)) x RH / 100 hPa, and the frost point is the temperature where ice saturates at that pressure, using the WMO Magnus constants over ice (6.112 hPa, 22.46, 272.62 °C). Below 0 °C the frost point is always a little above the dew point, so a sub-zero surface collects frost first. The tool uses the frost point whenever the dew point is below 0 °C.

Optics temperature and the verdict

  • Optics temperature is the air temperature minus the cooling you set. This is an estimate: real cooling depends on sky clarity, wind, the optical design and any dew shield.
  • At or below the dew or frost point: dew or frost is forming now.
  • Within 2 °C of it: dew is close, so run heaters from the start.
  • Above that, but within 2 °C at the forecast low: dew is likely later in the night.
  • The tool assumes the dew point stays level overnight, which holds while the same air mass stays in place. A front or a sea breeze can change it quickly.