How Does a Telescope Dew Shield Work | A Step By Step Guide




How Does A Telescope Dew Shield Work

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The dew shield works by reducing the amount of heat radiated out into space from the optics. Whenever the glass temperature falls below the dew point, dew develops on the surface. This is only a few degrees warmer than the surrounding air. Any glass exposed to the sky will rapidly lose its heat and drop below the surrounding air’s temperature

Maybe you have heard the word dew shield being thrown around especially among the stargazers community and you are wondering, “How does it work?” Well, you are in the right place.

Read through this article to understand more about dew, telescopes, and dew shields. Shall we?

To fully understand how a dew shield works, we first have to understand how dew is formed.

What Causes Dew to Form?

You must first grasp the dew point to better comprehend how dew forms.

Dew Point Formation 

Even though you cannot see it, the air around you includes a significant amount of water in the form of vapor. Water vapor may be held in an atmosphere for a maximum of ten hours depending on its temperature:

The lower the temperature, the less water vapor can be contained in the atmosphere. The temperature at which the air can no longer retain any more water vapor and is so saturated with water vapor is the dew point.

Dew Formation 

Droplets of liquid water will emerge out of the air due to cooling it below the dew point, which will compel water to be lost from the air.

However, when the ambient air temperature is above the dew point, it is still feasible for dew to develop on your telescope. One of the reasons for this is a phenomenon known as the “chilling effect.”

Dew Formation on your Telescope 

Depending on how cold your telescope is, it can be significantly colder than the surrounding air. If your telescope is colder than the current dew point, it can cause water to be lost from the surrounding air and deposited as droplets of dew.

In rare circumstances, astronomical telescopes will dew up due to the cold impact, but this will not occur on other occasions. It also aids in our understanding of how to deal with the dew.

A clear night sky emits relatively little thermal (heat) radiation, and its radiative temperature might be many tens of degrees lower than the surrounding air temperature. The chilling effect is caused by this phenomenon.

Surfaces exposed to this chilly night sky radiating their heat away receive little heat back from the sky in return. As a result, they can cool down by several degrees compared to the temperature of the surrounding ambient air.

Only the warmth reflected back into them from their surroundings prevents them from cooling much further.

What Is The Purpose Of Using A Dew Shield With Your Telescope?

Dew is perhaps the most vexing problem that amateur astronomers have to deal with.

When this aqueous film builds on the inside of your telescope, annoyance and observation difficulties follow rapidly. Depending on how severe the impacts are, they might be as simple as dimming star charts or partially blocking the view through your finder scope.

Alternatively, they can be more severe: even a tiny misting of your telescope’s optics can dull the view and deprive your images of critical contrast and sharpness.

Water has even been known to find its way into pricey multi-lens optics. It can also cause rusting and seizing of some sections of your mount.

How Does a Dew Shield Prevent Dew from Forming On Your Telescope? 

The dew shield has two purposes: first, it reduces the amount of heat lost by the lens to space for dew, and second, it prevents dew from forming on the lens. 

As your lens is pointed toward the sky, it emits heat outward, and, in short, the order will get cooler than the surrounding environment, causing dew to collect on the lens surface.

The shield will only slow down dew accumulation; it will not prevent it entirely, and if your lens is already dewy, it will not make it disappear on its own.

You’ll need either a heater strip or a hot air cannon to do this. The second application for the dew shield is to prevent stray light from entering the room at an acute angle. 

As far as the material goes, anything will do, even paper, although it will not survive for very long at all.

Is It Necessary To Use A Dew Shield?

Although a dew shield will almost always be required for your evening viewing, there are a few exceptions to this general rule.

This is particularly true when the temperature of the environment surrounding your device is lower than the temperature of your lens. Generally speaking, if you find yourself in this circumstance, the dew point has not been reached.

Dew may also fail to develop if the air around you is extremely dry, which is another possibility. Dry air suggests that the temperature is unlikely to drop to a point where condensation of water vapor will not occur, resulting in another observation period with no dew.

If you want to prevent dew from accumulating, you may also use your surroundings. A real-world example is when the grass in a field has white frost or is just moist with dew, yet the area under a tree appears to be completely clean.

As a result, the tree effectively functions as a giant dew cap, significantly slowing down the dew-forming process. So it’s possible that employing a tree to prevent dew from accumulating rapidly on your telescope is a strategy worth experimenting with.

On top of that, trees help to lessen the amount of wind that blows over your region, which is another advantage of spending time under them.

Of course, you shouldn’t expect a tree to be the most effective dew-prevention option available to you. However, it will ultimately cause the dewing process to be slowed.

5 Ways to Keep Your Telescope From Dewing-Up?

Preventing dew typically entails a series of procedures guided by a knowledge of why dew occurs.

  1. Using a Dew Heater 

A dew heater band can also be wrapped around the end of your finder, around the end of your telescope, or around the end of your telescope a little distance back from the objective lens or corrector plate, depending on your preference.

  1. Using Low Remittance Metal

Alternatively, you may use low-remittance, glossy metal to cover everything from your finderscope to your dew shield. Aluminized Mylar space blankets or central heating radiator foil are excellent choices for this purpose.

  1. Using a Telescope Head Extension

An extension at the head of the scope tube will function as a dew shield and prevent your secondary mirror from dew-forming, especially if the rear of the secondary holder is made entirely of low-emissivity aluminum, as is the case with Newtonian telescopes.

  1. Keeping Your Eyepieces In The Pocket 

Keep your eyepieces in your pocket to keep them just a little bit warmer. In addition, chemical hand warmers may be strapped to the end of your finderscope to provide a rapid anti-dewing remedy.

  1. Using An Hair Dryer

Wiping dew optical surfaces is pointless since you must elevate their temperature over the dew point to achieve success. One method of accomplishing this is by gradually heating your telescope directly with a hairdryer. 

It is recommended that you use the hairdryer on the lowest heat setting possible and that the air is blasted horizontally over the corrector plate rather than directly onto it.

Telescope Dew Shield FAQs


The only thing that must be done to maintain something dew-free is to keep it above the dew point; this does not imply that it must be warm to the touch. As long as your telescope maintains a temperature even a few degrees above the surrounding environment, it will remain dew-free.

Dew shields serve this purpose by maintaining a temperature gradient.

Perhaps this article has provided you with some useful information, be sure to check other similar articles on telescopes to enhance your astronomy journey. I recommend Collimation Cap and the Cheshire which is better?

It will help you choose the correct eyepiece for you and inform you about the differences.

Please be careful and use at your own risk
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