The forecast says clear all night, you set up, and two hours in the stars start to swell and fade. Wipe the front of the scope and your finger comes away wet. Dew ends a lot of imaging nights early, and it is one of the easiest problems to predict.
The dew point
Air can only hold so much water vapour, and the amount depends on temperature. The dew point is the temperature at which the air you have right now would be holding all it can. Any surface colder than that collects water from the air around it.
Weather apps report temperature and relative humidity, and the dew point follows from those two numbers with the Magnus formula:
g = ln(RH / 100) + 17.625 T / (243.04 + T) Td = 243.04 g / (17.625 - g) (T in °C)
At 12 °C and 75% humidity the dew point is 7.7 °C. At 18 °C and 60% it is 10.1 °C. The dew point usually stays close to level through the night while the air cools toward it, unless a front or a sea breeze brings in different air.
Why the telescope dews up before anything else
On a clear night, any surface facing the sky radiates heat out to space and gets almost nothing back. Glass is good at this. The front element of a refractor, the corrector plate of a Schmidt-Cassegrain or a camera lens pointed up can sit 2 to 4 °C below the air on a still night. Wind and cloud reduce the effect, because they bring heat back to the surface.
So the question is not whether the air reaches the dew point, but whether your optics do. With the air at 12 °C and the dew point at 7.7 °C, glass running 3 °C below the air sits at 9 °C, only 1.3 °C from the dew point. A little cooling later in the evening and the front lens fogs over while the air is still well above the dew point.
Below freezing it is frost
When the dew point is below 0 °C, moisture collects as frost, and it starts a little earlier. The frost point, the temperature at which the air saturates over ice, is always slightly above the dew point. At -2 °C and 85% humidity the dew point is -4.2 °C but frost starts forming at -3.7 °C. Frost is also slower to clear than dew, because the heater has to melt the ice before it can dry the glass.
What to do about it
Dew shield
A tube that extends past the front of the scope or lens. It narrows how much sky the glass sees, so the glass loses heat more slowly and dews later. It costs little, also blocks stray light, and on a dry night it can be all you need. On a humid night it only delays the problem.
Dew heaters
Resistive straps that wrap around the front of the tube or lens, just behind the glass, and keep it a few degrees warm. Schmidt-Cassegrains also have heater rings that replace the retaining ring at the front of the tube. Refractor users often need a second strap for the guide scope. A camera lens shooting a night-long timelapse needs one too.
Controllers
Running a heater at full power wastes battery and can set up heat currents in front of the optics that soften stars. A controller sets the power. The better ones read the air temperature and humidity, work out the dew point and hold the glass just above it, so they use only as much power as the night needs.
A simple plan for a night out
- Check the temperature, humidity and forecast low before you leave.
- Work out the dew point and how close your optics will get to it. If the glass will be within about 2 °C of it at any point, plan to run heaters.
- Put the heaters on before you start imaging, not when you see dew. Clearing a dewed corrector takes much longer than keeping it dry.
- Fit the dew shield regardless. It helps the heaters as well.
With the air at 18 °C, 60% humidity and a forecast low of 9 °C, the dew point is 10.1 °C. Glass 3 °C below the air reaches it once the air cools to about 13 °C, so the heaters should be running by the time the air reaches 15 °C.
How accurate is this
The Magnus formula with these constants is within about 0.35 °C of the full calculation between -45 and 60 °C. The real uncertainties are your humidity reading, since many cheap sensors are off by 5% or more, and how much your particular optics cool, which depends on the design, the wind and the sky. Treat the result as a guide to when to act, and leave a margin.