RigTonightHow to stop dew ruining your astrophotography: shields, heaters and power
You set up on a perfect clear night. Two hours later every star has a soft halo, then the frames go grey. The objective lens has fogged over. Dew is the most common way a good night ends early, and it's easy to prevent.
Why optics dew up
Dew forms when a surface cools below the dew point, the temperature at which the air's moisture condenses. On a clear night your telescope's front glass radiates heat straight out to the cold sky, so it cools a few degrees below the air around it. That's why the lens can drip with dew while the air itself is above the dew point. Clear, still, humid nights are the worst: exactly the nights you want to image.
Dew shields
A dew shield is a tube that extends beyond the front of the telescope. It blocks the lens's view of most of the sky, so it radiates less heat and cools more slowly. It also cuts stray light.
- Refractors usually come with one. Extend it fully.
- SCTs and other closed tubes need one added; the corrector plate faces the sky directly and dews up fast.
- A shield delays dew. On a humid night it won't stop it on its own.
Heater strips
A dew heater strip wraps around the tube just behind the front glass and keeps it a few degrees above ambient. Run it from a controller that sets its power, so you don't waste battery heating glass that's already warm enough.
Where to put them:
- Refractors: around the objective, just behind the dew shield.
- SCTs and Maksutovs: around the corrector plate.
- Newtonians: the primary sits at the bottom of the tube and rarely dews, but the secondary can. A small heater on the secondary holder helps on damp nights.
- Guide scope: easy to forget, and when it fogs your guiding fails. Give it a small strip too.
- Camera lenses: a strip around the front of the lens barrel.
Start heating as soon as you set up, not when you see dew. Clearing a fogged lens takes far longer than keeping it clear.
How much power dew heaters use
Dew heaters are often the biggest draw on a rig after a cooled camera. A strip for an 80 mm refractor typically draws around 5–8 W at full power, more for larger apertures, and a guide-scope strip a couple of watts more. Run through a controller at about half power, as on a typical night, that roughly halves.
Over a 10-hour winter night, that adds up. On a small power station it can be the difference between imaging until dawn and running out at 2 am. Powering your rig in the field shows how to budget it.
Let RigTonight budget it
The rig builder's Power panel includes dew heaters for your main scope and guide scope by default, sized to the aperture, and shows how many hours your battery lasts against tonight's length. Untick Dew heaters to see the difference they make.