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Dew Heaters

Dew heaters should be considered an essential piece of gear unless you live on top of a desert mountain. Simply put, dew heaters keep your optics slightly above the ambient temperature outside to prevent condensation on your lens, mirror, or front element.

The Problem

As the night cools down and your lens or scope is pointed up at the clear sky, moisture in the air can condense on your optics. This is more likely as the humidity increases. You can check your forecast to see how humid it is likely to be by watching the “dew point spread” or how far away the temperature and dew point are from each other.

When dew forms on your optics, it ruins your images. There is no way to recover from this with editing, the data is trash. Trying to wipe away dew with a cloth often leads to smears at best and scratches at worst. The best way to deal with dew is to prevent it from forming in the first place! That is what the dew heater do.

The Solution

To prevent dew from forming, the front element needs to be warmed just enough to keep it above ambient temperature. Almost all dew heaters accomplish this by heating up some sort of resistor with electricity and conducting that heat into the glass. For a dew heater to be effective it needs to be as close to the actual lens or mirror as possible.

Types

There are a few different kinds of dew heaters.

  • The most basic is a cheap “lens warmer” that plugs directly into a USB port. Some have a control dial to adjust how high it runs, while others are simply “always on” as soon as they are plugged in.
  • Astrophotography specific dew heaters usually consist of a soft, flexible, Velcro-adjustable band with a resistor element that heats up when electricity passes through it. The main difference between these and cheap lens warmers, is the plug is typically RCA – the style you may be familiar with on old TVs and VCRs – which plugs into a separate dew heater controller.
  • A third type is for Newtonians and can be as simple as a few resistors wired together, or a silicone patch that looks like an oil pan heater. These are typically affixed to your newt’s secondary mirror with some sort of permanent glue. The plug style is RCA as well.

With AP-specific dew heaters, a controller is needed. This can also be a simple DIY dial or multiple dials for multiple bands. It can also be a software-controlled box like the popular Pegasus Power Box series. These use a temperature and humidity sensor to adjust the amount of heat so that the bare minimum effective amount is used.

This has two advantages:

  • It reduces power draw which is very helpful for remote imagers running off battery power.
  • It also reduces the potential for tube currents. Tube currents are burbles of hot air that blur the view directly in front of your scope. Think of the mirage over an asphalt parking lot, but right in front of your telescope.

Summary

Always use a dew heater to prevent dew forming on your scope. It's an easy problem to prevent but nearly impossible to fix after it's happened. There are good solutions for different gear types and budgets so don't let dew ruin your night of imaging.