New telescope owners have a lot to keep track of. And one important but oft-forgotten task is dealing with moisture.
It builds up in all sorts of ways, from getting caught in the rain to dew accumulating overnight.
But is it a problem?
By the end of this article, you’ll understand exactly how getting wet impacts a telescope, and more importantly, what you need to do about it.
Here’s whether telescopes can get wet
Telescopes should not get wet. Water will not damage the telescope, but its presence on lenses and mirrors will badly distort your view. Avoid rain exposure, and use a dew heater or dew shield when observing below the dew point. If moisture does appear, then let it dry indoors, just slightly above room temperature. Never, ever wipe the optics!
This is one of the biggest practical challenges that often surprise new stargazers
And what’s even more surprising?
This happens even clear weather.
You don’t have to be standing outside in the rain for your telescope to become wet. Unfortunately, one of the basic ironies of astronomy is that the precise weather conditions which allow for the clearest and best celestial views are also the ones that facilitate the appearance of dew on your telescope.
You’ll be observing the night sky with perfect clarity…
Suddenly, the dimmer galaxies and stars will start fading out of sight…
And then, you will slowly see that the brightest stars get a hazy halo around them…
At that point, a flashlight check will show you that water is starting to coat your optics.
If you get caught in the rain, your best bet would be to cancel the viewing session right away. Not only is wiping the lens dangerous (as you’ll learn in more detail below) but more water will start forming around the lenses the second you stop.
How does water damage telescopes?
Telescopes don’t just get wet if you put them out in the rain. If the atmospheric conditions are ripe, you may notice a water layer appearing on your lens due to condensation.
But the water does no actual damage to the telescope — at least not directly. After all, it’s just metal, glass, and plastic. (Electronic mounts and computer chips are another story, of course).
So, all you need to do is to let it dry off. But herein lays the real way in which rain and dew can damage your telescope. If you’re an astronomy newbie, you may feel tempted to just wipe the surface of the lens and continue using the telescope. But as you’ll realize now — that would be a grave mistake.
The reason is simple — the surface of the lens is very sensitive. And if there was some dust on it before the condensation of the dew; wiping it may basically result in inflicting hundreds if not thousands of micro-scratches. Those wouldn’t be visible to the naked eye; but once you started using the telescope the next time, you will find that your visibility is severely diminished.
This is a well-known issue in the optics community. If your lens’ temperature goes beneath what’s known as the “dew point,” you’ll basically see the same effect that you see on a bottle of beer taken out of the fridge. Water molecules found in the air surrounding the bottle condense and attach themselves to the glass.
Can you protect a telescope against rain?
If you notice that it’s starting to rain outside, you can definitely pack up your telescope for the night. Chances are that you simply won’t be able to see anything more until you’ve got a dry night again. Indeed, rain is something you simply can’t prevent no matter how hard you try.
On the other hand, there are ways to prevent dew from appearing on your lens. There are specially manufactured dew heaters for your telescope. This amounts to a strip of wire that goes around the lens and slowly heats it using electricity. Also, these dew heaters are most often powered via DC batteries, ensuring that they work in remote rural areas that are the destinations of choice for budding astronomers.
The other option you’ve got is using a dew shield. This is basically a lengthy extension for the tube of your telescope. Its purpose is to reduce the level of exposure that the lens has to the cold night sky. Though, bear in mind that these shields aren’t perfect. They can only prolong your session, but they can’t stave off cold indefinitely.
The shields are basically a low-tech alternative to the heaters. It’s all a trade-off between your needs: On the one hand, the shields don’t need any electricity and they’re less costly. On the other, they’re simply less effective.
Will dew and condensation harm my telescope?
You shouldn’t think that this is a bad question at all. We’ve all been there — you take the telescope out in your garden and start looking at the moon and stars; but you leave it outside as it gets cold. Soon enough, you come back and the thing is positively wet from condensation.
If you take it inside, the best thing you can do is keep it there. You aren’t likely to be able to use it the very same evening, because it won’t slowly dry off until tomorrow. Certainly don’t wipe it at all — if any specks of dust are left on the condensed water and glass, your corrector will probably get scratched. And these fine scratches will completely destroy your surface anti-reflection coatings. So, just point your telescope in a horizontal direction — and slowly let it air dry inside.
What if the condensation froze?
If the condensation on your telescopes freezes, follow the exact same procedure that you’d do if the telescope just got wet. Take it inside, and place it in a mildly warm area where it can thaw and dry off slowly. Don’t attempt to speed up the process by wiping, breaking the ice, or interfering in any other way; all of that is a sure way to risk damaging the lenses.
How do I stop condensation on my telescope?
If you have a refractor telescope, you can always make a dew cap or “dew shield” as we’ve also described it. Here’s how to create your own on the cheap:
- Use cell foam, similar to the material of a sleeping mat (the kind you’d go backpacking with).
- Measure a cylinder about two times as long as the telescope’s front.
- Cut it into the proper shape with a simple utility or X-Acto knife
- Wrap it all around the tube of your telescope, then find a strip of Velcro (or other reusable fastener) to keep it together
These are both durable enough to sustain rural conditions, and lightweight enough to be practical wherever you go.
Will my humid climate hurt my telescope?
Humidity and water doesn’t hurt your telescope directly. So, you can basically drench your telescope in water, and it will still work afterwards; provided you don’t wipe it at all and let it dry itself off in the air instead.
Are any telescopes waterproof?
No. As we’ve outlined above, the telescope itself isn’t damaged by water, so there’s no part that could be waterproof.
However, if you find any advertising that states that telescopes are waterproof; they’re either not really waterproof, or not really telescopes.
The closest you can come to a waterproof model are some monoculars that are designed to be used with your smartphone. However, you do need to remember that this is a far cry from an actual telescope; both in terms of versatility and magnification. (If you’re interested, this article gives some more background on the pros and cons of monoculars for stargazing.)
My telescope got wet! What should I do?
Just take it inside and let it dry itself off completely before using it the next time; potentially by putting it near a mild source of heat. And that’s it — there’s nothing else that you should do! Anything else and you risk damaging your telescope and its sensitive lenses.
And, for next time, consider buying a dew shield/heater or making your own shield as described above.
Conclusion: telescopes & moisture
As we’ve seen, it’s easy but undesirable for telescopes to get wet. The main issue is optical interference, since wet optics distort the image.
Rain is an obvious risk, but dew is a more challenging one, since cool and seemingly dry climates still get below the dew point quite often.
However, that doesn’t mean that they’ll necessarily get damaged — as long as you don’t mistreat them and wipe the water carelessly off the lenses. Letting the telescopes dry naturally is the single most important thing to keep in mind.
And, of course, simple preventive measures like a dew shield will go a long way to keeping your scope dry during a lengthy stargazing session.
