How to prevent dew on a telescope

How to Prevent Dew on a Telescope

Dew can be a very serious issue when it comes to stargazing, astronomy, or astrophotography. In fact, it has ruined many-a-night for me personally, this is why you must be aware of what causes dew and how to prevent dew on your telescope.

To be clear, dew is the most common equipment hassle that evening observers face, no matter how good or expensive your telescope and binoculars are. When dew happens, you might first notice the stars you are observing becoming harder to see, and then you might see fuzzy halos around the brighter stars. In severe cases, dew can also soak the whole tube of your telescope.

Worse still, most new-comers are tempted to wipe the dew away. This will only make things worse as it is not effective solution, in fact, more water will simply condense the moment you stop wiping.

So how do you prevent dew from forming on your telescope? The simple answer is to first educate yourself on the following topics:

  • What causes dew and what is a dew point.
  • Understand why dew forms on telescope mirrors and lenses.
  • How to properly acclimate your telescope.
  • Why a dew shield can be effective at minimizing dew formation.
  • Why you should consider investing in a dew heater.
  • How to properly store your telescope after use.

In this guide we’ll explore all the aforementioned points in explicit detail so you have everything you need to extend those evening outings far into the wee hours of the morning.

What is Dew and What is the Dew Point?

A very common misconception is to think that dew “falls” from the sky, but this is incorrect.

Dew, on the other hand, is water moisture that is condensed from the surrounding air onto the surfaces of an object that is colder than the air’s dew point.

This commonly happens at night or early morning. Condensation, in physic’s terms, is the transformation from the gaseous form of water (that is contained in our surrounding air) to liquid form.

Specific in astronomy, the dew phenomenon will cause a fogging up of telescope lenses, which in turn will disrupt our visibility in observing the celestial bodies.

The telescope’s (or binoculars’) lenses or mirrors are better thermal conductors than air, causing heat to be removed when the air makes contact with  said lens. This will transform water vapor in the air into tiny droplets of water (in liquid form), which will create fog on the lens.

Dew point, in a nutshell, is the minimum temperature where the air cannot hold more water in gas form (assuming the air pressure remains constant). If the temperature is lowered to the dew point, water vapor in the air will be transformed into liquid form, usually in the form of precipitation or fog.

The dew point would also affect the surrounding air’s overall humidity: the higher the dew point, the more water moisture would be contained in the air. A dew point of lower or equal to 55 and the air is considered “dry”, while a dew point greater or equal to 65 is considered “heavy” or “humid”.

To measure the dew point, you will need to use a device called hygrometers. These devices can measure dew point over a wide range of different temperatures and are a handy instrument to have around.

Why does dew form on telescope mirrors and lenses?

Based on the discussion above, it’s important to understand that dew will not always form on the telescope’s optics (mirrors and lenses). Dew will only form when the telescope’s optics are colder than the surrounding air’s dew point.

To explain this phenomenon, we can use the analogy of taking a can of cold beer out from the refrigerator. As soon as the can is out in the open, condensation will form on it. This is because the can is colder than the air’s dew point. Your telescope’s lens is the beer can in this example.

How do you acclimate a telescope?

A common misconception is that just because the telescope feels warmer than the surrounding air when it is brought outdoors, it won’t get dew since it can’t get colder than its surroundings. This belief is based on something called The Second Law of Thermodynamics (the total entropy of an isolated system can never decrease over time).

The thing is, your telescope and the surrounding air is not a real closed system. Your telescope won’t only exchange heat with the surrounding air but also with other objects in the night sky.

The true temperature of the dark night sky is just a few degrees above absolute zero, and your telescope’s optics are also exposed to this cosmic coldness. This is why we will need to properly acclimate a telescope to prevent dew from forming, but also to prevent tube currents.

Tube currents is a phenomenon that happens when warmer air is formed inside the tube of the telescope as they lose heat by convection. These tube currents would stay inside the telescope’s tube and would cause differing delays to light passing through compared to the cooler air, causing disruptions to the telescope’s ability to focus properly.

As you can see, we’ve come to a dilemma: if the temperature goes lower than the dew point, it can cause a dew issue. On the other hand, if the telescope is too warm compared to the ambient temperature, it can cause the tube currents issue.

So, the basic idea is to acclimatize your telescope to cool down sufficiently to match the ambient temperature, ideally within 0.5°C difference to each other, but at least less than 1ºC difference. However, it’s not always easy to do, and the larger the telescope, the longer it will take to acclimatize.

Here are some tips for acclimating your telescope:

  • Allow sufficient time for the telescope to acclimatize itself by leaving it outdoors before use. For small to medium-sized telescopes, generally leaving it outdoors for 30 minutes will suffice, but you’ll need more time for larger telescopes.
  • You can use a free-standing fan (of any kind) to blow on the mirror/scope end of your telescope. This can help the telescope get closer to the ambient temperature.
  • In cases of severe tube currents, you can fit the scope with aluminized radiator foil to reduce the effect during cold nights. Aluminum foil can help the exposed parts of the telescope to stay closer to ambient temperature. Alternatively, you can use a mylar space blanket to wrap the body of your telescope for the same purpose.

How does a dew shield work?

After you’ve properly acclimatized your scope, the basic approach to defend against dew is to shield the optics from as much exposure to the night sky as you can to stave off the dew point.

It’s likely that your telescope already has a part of the tube extending just beyond the lens (or refractor lens). This is called the dew cap, which can be thought of as a built-in dew shield.

The longer this dew cap, the better it is in keeping the lens warm. A dew shield essentially works by extending this dew cap (or replacing the cap’s function when there isn’t any).

How a dew shield work is actually pretty simple: it physically shields the lens from the cold sky’s temperature. This is why the longer the shield is, the lower the amount of cold sky the lens is exposed to.

Also, the dew shield can help in blocking stray light to enter the light path from oblique angles, providing a better focus for your telescope.

It is quite easy to make your own dew shield with foam rubber that is at least ⅝-inches thick or flexible plastic that is thick enough. You’d want to paint the inside of the shield jet black to prevent any reflection of light from entering the lens (which will disrupt focus).

If you not a do-it-yourselfer, we’d recommend Astrozap Flexible Dew Shields.

How long should a dew shield be?

As a general rule of thumb, a dew shield should be at least 1.5 times the telescope’s aperture width. If your telescope’s aperture is 2.8 inches (70 mm), then you’d need a dew shield that is at least 4.2 inches in length.

However, the longer the dew shield, the less light that will enter the scope, which might make observation much more difficult. So, finding the right balance is important.

The issue with dew shields, however, is that it can’t block the sky’s cold temperature from directly above the lens, and since we’d want to point our telescopes much higher to the sky in our observations, dew shields won’t be 100% effective. This is where active dew heaters come in (as we will discuss below).

It is worth noting, however, that dew shields are still pretty effective in extending your observation sessions by at least a few hours. Considering they are also pretty affordable, getting at least one is a must, especially if you live in areas with colder climates.

How does a dew heater work?

If a dew shield is passive dew protection for your telescope, a dew heater is active anti-dew protection.

Dew heaters typically come in the form of a strip of a resistive electrical band that will fit around the circumference of the lens. There are electrical wires inside the band, that are typically driven with a DC battery to produce heat, which will warm up the lens.

The basic idea of a dew heater is that it will warm the lens enough to keep it above the surrounding air’s dew point at all times, so unlike dew shields that are passive, it can actively prevent the formation of dew entirely.

How to Use a Dew Heater?

You can wrap the band of a dew heater around the end of your telescope or finder-scope. Alternatively, you can also use the dew heater around the telescope’s body, shortly behind the objective lens (or corrector plate).

The dew heater can be powered on its own, but ideally, you should use a dew heater controller that can adjust the amount of heat generated by the heater strap.

Since the dew heater is a portable rig, power consumption is going to be the main factor when choosing between different products. If you’re not looking to spend a lot, we’d recommend the CooWoo Lens Heater that comes with a built-in temperature controller with a low, medium, and high settings. This will allow you to adjust the temperature according to the current conditions.

While the inexpensive CooWoo heater is a solid choice, if you’re an avid astronomer in colder climates, you may want to invest in multiple heating bands with an external controller.

For this, we recommend the Orion 3517 Dew Zapper Pro 4-Channel Control Module. With it, you have the ability to control up to four different heater bands simultaneously and individually. So, for example, you can use the low-temperature setting on your guide scope and medium setting on your imaging lens.

DIY Dew Management: Hairdryer

If you don’t have a dew heater protection system yet, you can use your household hairdryer to help to prevent dew buildup if you are desperate.

Obviously, this is not an ideal option especially if you won’t have access to electricity during your astronomy observations. Hairdryers will use a very high amount of power that can quickly drain your battery, so using batteries or power banks might not be ideal (and can be dangerous if you are not careful).

Also, using hairdryers to blow your lenses might cause dust and debris to be blown from the hairdryer, which might stick to the optics and can be hard to clean.

Last but not least, hairdryers can run really hot quickly, so if you are not careful, it might create the tube currents issues instead.

To summarize, while it is certainly possible to use hairdryers or any other household heating equipment you might have at home to prevent dew buildup, they are not ideal and you should be extra careful while using them.

How To Store Your Telescope?

A very important consideration is that dewing can happen when the telescope is in storage, and it is actually much more dangerous than when dew happens at night while you’re making your observations.

During storage, water forming from dew can’t move anywhere, and this could permeate optical coatings and potentially damage your lens and the telescope’s internal parts. Additionally, condensation and evaporation can happen repeatedly in the tube’s sealed environment, which can lead to damage over time.

The bottom line is we shouldn’t underestimate dew just because the telescope tube is an airtight space (that is completely dry) when you store the telescope. Remember that air always contains water vapor, therefore, the air inside your telescope’s tube does as well. This is why it’s quite common to find water stains on the inside surfaces of the telescope’s lens.

Here are some tips to prevent this issue:

  • Never move a telescope from warm to cold storage when sealed. In general, completely sealing the telescope during storage is not recommended. It’s better just to cover the telescope with cloth or blanket to allow air circulation while protecting it from dust.
  • Don’t store a telescope in a humid/damp place, make sure it’s dry with standard ambient temperature (just like your living space).
  • You can heat the telescope with a dew heater around the objective lens during storage (which is relatively cost-efficient), make sure to set it to low or low-medium temperature.
  • Alternatively, you can position a 4-watt lightbulb just below the objective lens. You can also attach a smart humidistat to this bulb if you only want it to turn on when it’s damp.
  • Silica gel can also be a great solution. Get some bags and attach them to your lens caps to help absorb excess moisture.

Some closing tips to prevent dew on a telescope

Well we’ve given you a number of strategies to make your next stargazing outing a long and pleasurable one. There’s nothing worse than heading home early because of dew. Before we leave you, here are a couple more tips that will help prevent dew on a telescope:

  • Make sure to point your telescope (or binoculars) downwards between each observation. This can be a simple but effective way to prolong your sessions.
  • Either make your own, or invest in a longer dew shield. This can be a very inexpensive way to extend your viewing session and protect your telescope from dew.

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