Dew Heater

Specifications

Observing parameters

Overview

Dew forms when a surface falls below the dew point, and a telescope pointed at a cold sky radiates heat away until it does exactly that. A dew heater is a resistive band or collar that puts just enough power into the optic to hold it above the dew point — typically a few watts, far less than enough to change the figure of a mirror.

A dew shield helps passively by slowing radiative loss, but on a humid night it only delays the problem. Heaters are the actual solution, and a controller that measures ambient humidity and regulates power is the difference between "on" and "right".

Place heaters on the corrector plate or front lens, on the secondary mirror of a Newtonian, and on the guide scope — the guide scope usually fogs first, and losing your guide star ends an imaging run as thoroughly as fogged main optics.

Where it fits

Practical notes

• Fit a heater to the guide scope first — it is small, fogs earliest, and costs you the guiding.
• Use a humidity-sensing controller; a manual heater left on high all night wastes power and can cause tube currents.
• A dew shield plus a low-power heater is far more effective than either alone.
• Do not over-power a heater. A warm mirror creates its own seeing problems.

Pairs with

• Catadioptric Telescope
• Guide Scope
• CMOS Camera
• Refractor Telescope