During your observing sessions, you may have wondered how far a telescope can peer into the cosmic depths or how far we can see with only our eyes. I was wondering about this recently and thought I would investigate it and provide information that others might find helpful.
Also included in the article is how far we can see with the naked eye.
Understanding the Observable Universe
The observable universe represents everything we can potentially see from Earth, stretching approximately 46.5 billion light-years in any direction. However, what we can observe varies dramatically depending on our viewing method – from the naked eye to the most powerful space telescopes humanity has created.
We mentioned light-year and should explain what this is. People are sometimes confused by this term and think it is a time measurement. It is a measurement of distance. A light-year is the distance that light travels in a vacuum in an Earth year. This distance is about 6 trillion miles (9 trillion km).
Key Features of the Observable Universe
The universe is approximately 13.8 billion years old and spans a diameter of about 93 billion light-years. Within this vast expanse, we find hundreds of billions to trillions of galaxies organized in a cosmic web of galaxy clusters and superclusters. This intricate structure forms the largest known pattern in nature, with galaxies arranged along cosmic filaments.
One of the most remarkable aspects of observing the universe is that we’re looking back in time. When we observe an object that’s 1 million light-years away, we see it as it appeared 1 million years ago! That is because it takes light that long to reach us.
What Can See With the Naked Eye?
Before we examine how far we can see with a telescope, let’s examine what our naked eyes can see.
Under clear, dark skies, the unaided human eye can achieve remarkable views of the cosmos. On clear, dark nights, the most distant object we can see with our eyes is the magnificent Andromeda Galaxy (shown below), which is 2.5 million light-years away.
Visible from the Southern Hemisphere, the Magellanic Clouds can be observed at distances between 160,000 and 200,000 light-years. Under dark skies, various star clusters and nebulae within our galaxy also present themselves to the patient observer.
Stars are visible up to about 16,000 light-years away. Every star that we see in our sky is in the Milky Way.
How Far Can a Telescope See: Amateur Telescope Capabilities
The primary function of a telescope is to collect light from cosmic objects. Because they have a larger aperture than our eyes they can collect much more light. This allows us to see fainter objects.
The viewing power of amateur telescopes varies primarily based on their aperture. With larger apertures, we can observe farther objects. The following table approximates how far you can see given an aperture.
Telescope Aperture
Approximate Maximum Range
4″ (100mm)
10 million light-years
8″ (200mm)
30 million light-years
12″ (300mm)
50 million light-years
16″ (400mm)
70 million light-years
The numbers given above are general. Possibly the most distant object that may be visible to amateur astronomers is the quasar called PG 1634+706. This object has variable brightness and whose light has travelled a whopping 8.8 billion years to reach us (Halfway to the edge).
What Limits How Far Can We See With Our Eyes?
There is no limit on how far our eyes can see so why can’t we see any farther into space than we can? The answer lies in the nature of light and the limit of our eyes.
As light travels away from the emitting object, it spreads out more and more. It follows what is called an inverse square law. The light intensity decreases in proportion to the square of the distance from its source. This is a complicated way of saying that if light travels a certain amount, it will dim by a square of that distance. If you double the distance between you and the light source, the light will be a quarter the brightness it was.
Light is composed of photons. The more photons that reach your eye, the brighter the light will be. The cells in your eyes must receive enough photons to see the light. Here lies the problem with seeing very distant objects.
Because light from distant objects gets very dim, there are not enough photons from the objects that are very far away for your eyes to see. Basically, the light becomes too dim for our eyes to detect.
Even if our eyes could see very dim objects, another factor restricts how far we can see. Because the Universe is expanding, the light that travels also becomes stretched out. With very far objects, the light is affected so much that it moves from visible to infrared wavelengths, which we can not see.
The next sections will examine professional ground-based Observatories and space telescopes. The space telescopes are equipped with cameras that can detect infrared light.
Professional advanced telescopes are not used visually. Instead, they use CCD cameras to take images. These offer many advantages over visual observations. Other than the possibility of deeper investigation after they are taken, they are more sensitive than our eyes. Also, long exposures can be taken, which reveal dimmer objects.
How Far Can a Telescope See: Professional Ground-Based Observatories
At this stage, we should clearly distinguish between visual amateur observations and those of professional telescopes (including space telescopes). The distances we have quoted so far are for visual observations. The use of cameras allows for the collection of more photons to detect dimmer objects at farther distances.
The world’s largest ground-based telescopes push the boundaries of what we can observe from Earth’s surface. The Very Large Telescope (VLT) in Chile’s Atacama Desert consists of four 8.2-meter telescopes that can work together, effectively creating a 16.4-meter telescope. When operating in interferometric mode, the VLT can distinguish details 25 times finer than individual telescopes, allowing it to observe objects up to 13 billion light-years away.
The Gran Telescopio Canarias (GTC) in Spain’s Canary Islands features a 10.4-meter primary mirror, making it the world’s largest single-aperture visible-light telescope. The GTC can detect objects approximately 500 million times fainter than what the human eye can see, reaching distances of about 13 billion light-years.
The twin Keck Telescopes in Hawaii, each with 10-meter primary mirrors, have made ground-breaking discoveries about distant galaxies and quasars. Using adaptive optics to compensate for atmospheric distortion, these telescopes can observe objects at the edge of the observable universe, approximately 13 billion light-years away.
How Far Can a Telescope See: Space Space Observations
As mentioned in an earlier section, cameras must be able to detect infrared light to see objects at extreme distances. The expansion of the Universe has stretched light from distant objects so much that it is invisible at visual wavelengths. This is where Space telescopes come into their own.
The Hubble Deep Field: A Window into Deep Space
The Hubble Deep Field images provided a wonderful insight into deep space. Using long multiple exposures, Hubble revealed thousands of galaxies billions of light-years away. The Hubble Ultra Deep Field (see image below) contains galaxies up to 13.4 billion light-years distant, providing an unprecedented view into the early universe.
Hubble Ultra Deep Field (ESA/Hubble)
James Web Space Telescope
The James Web Space Telescope (JWST) is widely considered as Hubble’s replacement. This powerful telescope pushes the distance we can observe farther away.
At the time of writing, the most distant object imaged by JWST is JADES-GS-z14-0, which was 3.5 billion light-years away when the light left it. Due to the expansion of the Universe, it is now 33.7 billion light-years away.
JADES-GS-z14-0 (ESA/Webb)
Gravitational Lensing: Nature’s Telescope
Gravitational lensing is an important phenomenon for observing distant objects.
This phenomenon occurs when massive objects bend light from distant sources, creating a natural magnifying glass in space. This remarkable phenomenon has enabled astronomers to observe galaxies up to 13.4 billion light-years away. Gravitational lensing has also allowed scientists to make discoveries about the distribution of dark matter throughout the universe. The technique has also been used to study extremely distant supernovae and early distant galaxies, providing insights into the universe’s earliest history.
Notable Celestial Objects Within Reach
The Orion Nebula: Our Cosmic Nursery
Located approximately 1,344 light-years away, the Orion Nebula (M42) is one of the most spectacular objects in the night sky. This stellar nursery is visible to the naked eye and has become one of the most photographed objects by amateur astronomers worldwide. The nebula spans approximately 24 light-years and is an active star formation region where new stars are continuously being born.
Andromeda Galaxy: Our Cosmic Neighbor
The Andromeda Galaxy (M31) presents amateur astronomers with an awe-inspiring view of the nearest major galaxy to the Milky Way. The galaxy spans an incredible 220,000 light-years and contains over one trillion stars. Andromeda is accompanied by multiple satellite galaxies, creating a rich field of study for astronomers of all skill levels.
From Earth, Andromeda fills a little more than three degrees of our sky, the same width as six full Moons. However, as only the central bulge is visible to the naked eye, it will appear smaller.
Other Deep-Sky Wonders
Amateur astronomers regularly observe several fascinating deep-sky objects. The Pleiades Star Cluster (M45) shines brightly at 444 light-years away, while the Whirlpool Galaxy (M51) can be spotted at an impressive distance of 23 million light-years. At 2,283 light-years from Earth is the Ring Nebula (M57), which is a planetary nebula. The sublime Sombrero Galaxy (M104) captivates observers from 29 million light-years away.
The Evolution of Telescope Technology
Historical Milestones
Astronomy observations using telescopes began in 1608 with Hans Lippershey’s invention of the refracting telescope. Isaac Newton revolutionized astronomy by creating the first astronomical reflecting telescope in 1668. William Herschel pushed boundaries in 1789 by building the largest telescope of his era. The 100-inch Hooker Telescope made history in 1917 by revealing the true nature of galaxies. Finally, 1990 marked a new era with the launch of the Hubble Space Telescope.
The Hubble Space Telescope’s Impact
The Hubble Space Telescope has transformed our understanding of the cosmos by observing objects up to 13.4 billion light-years away. It has been instrumental in helping determine the universe’s age with unprecedented accuracy and discovering thousands of galaxies. The telescope has provided evidence for the existence of dark energy.
The James Webb Space Telescope: A New Era
The James Webb Space Telescope (JWST) represents the next generation of space observation. This revolutionary instrument has detected galaxies that formed soon after the Big Bang and has been used to study exoplanet atmospheres. Its advanced capabilities allow it to see through cosmic dust clouds that block visible light, allowing us to see earliest stars in the universe.
Future of Land-Based Observation
The future of land-based astronomical observation is also developing. The Extremely Large Telescope (ELT) promises to revolutionize ground-based astronomy. The Giant Magellan Telescope (GMT) will offer unprecedented views of the cosmos. The Thirty Meter Telescope (TMT) will push the boundaries of what we can observe from Earth.
Key Takeaways
Amateur telescopes can observe objects millions of light-years away
Larger apertures generally mean better deep-space viewing capability
Space-based telescopes can see billions of light-years into the universe
Gravitational lensing helps us see even further
The future of astronomy promises even deeper views into space
Frequently Asked Questions
How far can the naked eye see in space?
Under ideal conditions, you can see the Andromeda Galaxy, about 2.5 million light-years away.
What determines how far a telescope can see?
The main factors are aperture size, optical quality, atmospheric conditions, and the brightness of the observed object.
Can amateur telescopes see the Hubble Deep Field?
No, the galaxies in the Hubble Deep Field are too faint for amateur telescopes to detect.
Why can’t we see further back than 13.8 billion years?
This marks the age of the universe; before this time, there was nothing to observe.
How does atmospheric turbulence affect telescope viewing?
Atmospheric turbulence (seeing) can blur images and limit the effective magnification of ground-based telescopes.
Can telescopes see the flag on the Moon?
Even the Hubble Space Telescope cannot resolve objects as small as the lunar landing flags.
What’s the difference between magnification and aperture?
Aperture determines how much light a telescope can collect, while magnification simply enlarges the image. Larger apertures generally allow you to see fainter, more distant objects.
Will bigger telescopes always see further?
While larger apertures generally see fainter objects, factors like light pollution, atmospheric conditions, and optical quality also play crucial roles. Remember, the journey into astronomy is as much about understanding these limitations as it is about pushing them. Whether you’re using a small beginner’s telescope or dreaming of the next great space observatory, there’s always something fascinating to discover in the night sky.
I found astronomy while working in dark rural locations. Initially, I explored the night sky and learnt the constellations before purchasing a pair of binoculars to further my knowledge of the sky.
My first telescope was a 200 mm Newtonian reflector on an equatorial mount. I found that this telescope had a steep learning curve but was a rewarding experience.
As time progressed, I became interested in astrophotography. This resulted in purchasing a 110 mm refracting telescope and a dedicated monochrome-cooled astronomical camera. This resulted in another very rewarding steep learning curve that far surpassed the experience with my first telescope.
I have joined Telescope Guru to share my knowledge of telescopes and astronomy.
