Astronomers use telescopes to study distant objects such as stars and galaxies; these are powerful tools, allowing us to view the universe from Earth, and to observe planets around other stars.
These observations require clear skies. Have you ever wondered whether or not telescopes can see through clouds?
The answer is yes, but only if they are equipped with special filters.
Most telescopes are usually built using large mirrors and lenses which allow light to pass through the telescope and focus onto a camera.
Clouds, however, can block what has the potential to be a stunning view.
Luckily, astronomers have developed specialized techniques for taking high-quality images during cloudy nights.
One method involves placing a special filter in front of the lens on your telescope.
By doing this, it lets more light through while blocking most wavelengths of visible light.
Understanding the different methods that can be used to allow telescopes to see through clouds can help you make the most of your telescopes, and we will explore these in greater detail below.
How Do Telescopes Work?
In order to properly understand the way in which telescopes can see through clouds, we need to have a basic understanding of how they work.
A typical telescope consists of several major parts: a primary mirror (also known as an objective), secondary mirror(s), and an eyepiece.
The primary mirror gathers the incoming rays of sunlight or moonlight into bundles called beams.
At its focal point, where the light converges, lies the second reflector, the secondary mirror.
This reflects the beam back down towards the eyepiece, which makes it possible to view the object through the telescope.
If the image formed by the first mirror falls outside of the field of view of the viewer, then the secondary mirror focuses the incoming light onto the third component, the eyepiece.
Eyepieces come in all shapes and sizes, but some common types include simple magnifying lenses, computerized ‘finderscope’ instruments, and binoculars.
Once light enters the instrument tube of a terrestrial telescope, there are two things that happen.
Firstly, the light passes through lenses made up of glass.
These reduce the size of the light wave so that it may be focused at infinity, i.e., where the magnification factors involved are very close to unity.
Secondly, the light is reflected off of one or more surfaces called mirrors, which increases the angle of incidence.
In general terms, a mirror acts as a prism; by reflecting incident light, it changes the path taken by the light, causing it to enter another portion of the optical system.
The next step after focusing the light is enlarging the image – or reducing the power of the magnification factor – often in combination with a dioptric lens or prisms.
The result is that the image appears closer to the observer than the original target.
However, this process does not increase the wavelength of light.
To determine whether a telescope can see through clouds or fog, we must examine how the instrument works more closely.
If clouds do indeed cause problems for our telescopes, then it must be because of the way in which the components interact, rather than any inherent problem with the technology itself.
So, let’s take a look at what happens when a cloud hits a telescope.
What Are Clouds And What Effect Can They Have?
First off, let’s establish what clouds are. They are usually composed of water droplets suspended within the air.
Water droplets are generally spherical and about 1/30th of a millimeter in diameter.
They become increasingly large over time due to evaporation, and eventually rain out and fall from the sky.
Although clouds don’t always produce precipitation, they still play a fundamental role in regulating Earth systems such as atmospheric circulation, energy budgets, biological growth cycles, ocean circulation patterns, and weather patterns.
When clouds are present near the surface of the Earth, it is common for them to disrupt visibility and introduce problems for viewing stars through telescopes.
Both terrestrial telescopes and space telescopes rely on their ability to capture an image using a bundle of light known as a beam.
This light comes from the Sun or Moon (in the case of space telescopes) and travels directly to us.
Before arriving at the telescope, however, it encounters layers of clouds.
Clouds have three main effects: firstly, they absorb some of the incoming solar radiation, thus lowering the temperature of the atmosphere (which is why you feel cold during a thunderstorm).
Secondly, they block the transmission of light, thereby blurring (and sometimes completely extinguishing) images obtained through a telescope.
Thirdly, they can act as condensation nuclei for moisture, causing particles to form around them.
Once these particles reach a certain mass and become heavy enough, they fall out of the atmosphere and rain down — often bringing with them further cloud formation.
Types Of Cloud
There are four types of cloud commonly encountered by amateur astronomers; each has its own characteristics.
Altostratus clouds occur most often in summer and autumn.
They are characterized by high cirrus clouds located above low-level stratus clouds.
High cirrostratus clouds are formed by ice crystals growing under conditions of very low cloud base temperatures, while altocumulus clouds are produced under similar conditions, but at higher levels.
Cirrus Cumulonimbus is the heaviest type of cloud encountered by observers.
Most frequently found in the winter months, they are massive columns of white cumulus forming between 40 and 70 kilometers above sea level.
Stratus clouds appear low lying.
As the name implies, they are associated with relatively thick layers of cloud, which extend from ground level up to around 10 km.
Stratus clouds tend to be stable, making them ideal observing conditions.
These are large, fluffy clouds that generally contain fair amounts of precipitable water vapor.
The smaller ones appear almost flat-topped, while taller, thicker ones may resemble inverted pyramids or even upside-down cones.
How Do Clouds Affect Telescopes?
The two primary mechanisms by which clouds impact observations with modern telescopes are blurring (or obscuration), and direct scattering (or attenuation).
Blurring occurs when light is scattered back towards the observer and reduces contrast.
Direct scattering refers to the process whereby photons change direction as they pass through the cloud and hence avoid detection by the camera.
Blurring is caused by larger droplets, which scatter more light than smaller particles.
A single cloud particle absorbs all wavelengths of visible light; thus, larger droplets are less effective than small particles at absorbing light.
However, given sufficient time, sunlight will evaporate any significant amount of liquid water within a cloud, so cloud size does not pose a threat to long exposure work.
Direct scattering also depends upon cloud shape.
For example, water droplets that are shaped like spheres or cubes experience no reflection of light.
In this case, the only light losses occur because of absorption and thermal emission.
However, if the shape of the droplet is elongated, then there is significant refraction occurring, as the droplet bends the path of incident light away from the observer.
This results in greater light loss, particularly in blue light where the index of refraction of water is lower than that of air or other gasses.
Issues With Light
Clouds can also cause additional problems.
Since they are opaque to infrared light, an observer might think he sees something glowing inside a cloud, or that it contains much heat.
Both phenomena are actually due to Rayleigh scattering from the droplets, which causes the brightening of a source within the cloud.
This “glow” disappears with time.
More importantly, since optical telescopes detect scattered light, they must have clear skies to produce accurate measurements.
If there is dust in the air, or the view is obstructed by trees or buildings, then the image becomes blurred, as the telescope cannot properly collect all the light falling on its objective lens.
Issues With Colors
Clouds may also affect our perception of colors.
Normally, red light scatters less than violet light, causing the spectrum of the sky to appear bluer.
However, as clouds form, most of their droplets become water ice crystals rather than water molecules.
Thus, they reflect more green light than red light.
Because the human eye is most sensitive to green and yellow wavelengths, the result is that the color of the sky appears darker, even though the number of photons arriving at the retina remains unchanged.
Cloud-filtered images sometimes look whitish, due to a tendency for people to use the word white to mean clear, whereas true whites are often slightly grayish.
As the cloud continues to rain out, the effect improves. With a well-defined cloud base, however, the effect returns after a few minutes.
This may be desirable, as some scenes look better early in the morning, just before dawn, or during sunset when the sun’s rays penetrate deeper into the atmosphere.
Can Telescopes See Through Clouds?
For astronomers, clouds are both a blessing and a curse. When the weather clears, we see the stars and planets clearly.
The lack of haze makes viewing the night sky easy and pleasant.
Unfortunately, when clouds cover the sky, conditions become difficult.
Not only do objects appear dimmer, but stray light from streetlamps, city lights, and moonlight can make things look strange when viewed through dense cloud layers.
Additionally, observing instruments such as photographic plates or CCD arrays require relatively low levels of light.
Most astronomical observatories are far removed from urban areas, and the occasional passing airplane usually doesn’t bother them.
Clouds change these situations. They reduce the useful depth of field and contrast ratio.
By reducing usable light levels, clouds interfere with efforts to take pictures, measure spectra, or even read numbers off scales, and so have a detrimental impact on the quality of your stargazing.
Though telescopes are undoubtedly powerful, the amount of cloud cover present affects what you can observe.
At high elevations and altitudes, clouds tend to linger longer, while thicker clouds will make viewing all but impossible.
If you are looking to really enjoy the night sky, or enjoy a closer, more detailed view of your favorite constellations, it is a good idea to wait for a crisp, clear night to take out your telescope and enjoy the night sky in all its glory.
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