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How Far Can You See?
Any idea how deep the eye could go if there was no atmosphere to scatter starlight? Ever wonder what optical and physical principles limit the eye's ability to see small, faint things? Have you given thought to how the "why" of astronomy changed before and after the telescope? In this article Jeff Barbour explores the limits of human sight - with and without the telescope. Learn more about the equipment available to contemporary amateurs making backyard astronomy the "extra-terrestrial" hobby of choice.
Amateur astronomy isn't for everyone. But unlike other interests, it could be! After all, there's plenty of sky to go around. And to enjoy the sky doesn't take much. To start, just the power of human sight and the ability to "keep looking up".
Appreciating the night sky and its numerous denizens is akin to enjoying any great work of art. Anyone captive to a painting by Van Gogh, statue by Roden, sonata by Beethoven, play by Shakespeare, or poem by Tennyson, can certainly appreciate a constellation wrought by nature's sculpting hand. So like such great works of art, a fine appreciation of the night sky can be cultivated. Yet unlike such works, there is something far more primordial and immediately evocative about the heavens - a thing that defies any need for profound study or inculturation by others.
While it is true that some ingenious devices (such as the quadrant) were developed early on in the history of astronomy, it wasn't until the time of Galileo (the early 17th century) that Astronomers began probing the universe in detail. Before that time, the human eye placed such constraints on what could be seen that all we knew of the heavens was limited to two large bright bodies (Sun and Moon), numerous faint lights (the fixed stars and infrequent novae), and an intermediate group (the planets and occasional comets). Using instruments such as the quadrant (for position), and waterclock (for time), it became possible to predict the movements of all such bodies. And it was prediction - not understanding - that drove observation using the human eye alone.
Ultimately it was the telescope that made discovery - rather than measurement - the driving force behind the science of astronomy. For without the telescope, the universe would be a far smaller place and populated by far, far fewer things. Consider that at 2.3 million light-years, the most distant celestial object visible unaided - the Great Galaxy of Andromeda - could never have been so-named at all. In fact, it might not have even received its older name: The Great Nebula in Andromeda. First noted in the 10th century text "Book of Fixed Stars", sharp-eyed Abd-al-Rahman Al Sufi described the Great Galaxy as "a little cloud". And that - without the telescope - is all we would ever have seen of this:
Because of the telescope, we now know far more about Sun, Moon, planets, comets, and stars than simply where they might be found in the sky. We understand that our Sun is a nearby star and that our Earth, the planets, and those "harbingers of doom" - the comets - are all part of a solar system. We have detected other such stellar systems beyond our own. We know we live in a Galaxy that - from a distance of two million light-years - would appear much like M31. We have determined that several billion years hence, our Galaxy and M31 will embrace spiral arms. And we recognize that the universe is extraordinary in its vastness, diversity, beauty, and harmony of inter-connectedness.
We know all this because we possess the telescope - and similar instruments - that can sound the depths of the Cosmos across numerous octaves of spectral vibrancy.
But it all begins with the human eye...
The working of the human eye is based on three of the four main properties of light. light may be refracted, reflected, diffracted, or absorbed. light enters the eye as parallel beams from the distance. Because it is limited in aperture, the eye is only able to collect a very small proportion of the rays coming from any one thing. That collecting area - roughly 38 square millimeters (fully dilated and dark-adapted), allows the eye to normally see stars down to about magnitude 6. Ancient Astronomers - free of the effects of modern sources of atmospheric illumination (light pollution) - were able to catalogue about 6000 individual stars (with a sprinkling of other objects). The faintest of these were classed of the "sixth magnitude", and brightest of the "first".
But the eye is also limited by the principle of diffraction. This principle prevents us from seeing exceedingly fine details. Because the eye is limited in aperture, parallel beams of light begin to "spread out" or propagate after entering the iris. Such diffusion means that - despite the use of refraction to focus - photons can only come so close together. For this reason, there is an ultimate limit to how much detail may be seen by any aperture - and that includes the eye itself.
The eye, of course, exploits the principle of refraction to organize beams of light. Photons enter the cornea, bend, and pass to the lens behind it. (The cornea does the bulk of the focusing and leaves about a third up to the lens.) The lens itself adjusts ray angles to bring things - near or far - to focus. It does this by changing radius of curvature. In this way, parallel rays from a distance or diverging rays from nearby may project an image on the retina where tiny neurons convert light-energy into signals for interpretation by the brain. And it is the brain - primarily the occipital lobes at the back of the head - that does the "image processing" needed to give coherence to that steady stream of neural signals arriving from the eye.
To detect light, the retina employs the principle of absorption. Photons cause sensory neurons to depolarize. Depolarization projects chemo-electrical signals from axons to dendrites deeper in the brain. Retinal neurons may be rod-shaped or conical. Rods detect light of any color and are more sensitive to light than cones. Cones detect specific colors only and are found in greater concentration along the main axis of the eye. Meanwhile rods dominate off-axis. The averted eye can see stars roughly two and half-times fainter than those held direct.
Beyond aversion, neural signals passing from the retina (via the optical chiasm) are first processed by the superior collicus. The collicus gives us our visual "flinch" response - but more importantly - it does less filtering of the visual field than the occipital lobes. Because of this, the collicus can detect even fainter sources of light - but only when in apparent motion. Thus the discerning observer can detect faint stars - and faintly glowing objects - some 4 times fainter than those seen through ordinary "straight-on" viewing. (This is done by sweeping the eye across the night sky - or across the field of view of the telescope.)
In addition to aversion and eye movement, the eyes increase sensitivity by adapting to low light conditions. This is done in two ways: First, fine muscles retract the iris (located between cornea and lens) to admit as much light as possible. Second, within roughly 30 minutes of exposure to darkness, "visual purple" (rhodopsin) on retinal rods takes on a transmissive rosey-red color. This change increases the sensitivity of rods to the point where even a single photon of visible light may be detected.
Aside from limitations imposed by diffraction, there is a second natural limit to how much detail can be seen by the eye. For neurons can be made only so small and placed only so close together. Meanwhile at about 25mm's in focal length, the eye can only see "1x". Add this to the fact that the greatest opening achieved by the eye (the entrance pupil) is 7mms and human eyes become the effective equivalent of a pair of "1x7mm" binoculars.
All these factors limit the eye - even under the best observing conditions (like the vacuum of space) - to seeing stars (using direct vision) of the eighth magnitude (1500 times fainter than the brightest stars) and resolving close pairs to about 2 arc-minutes of angular separation (1/15th the apparent size of the Moon).
Observational astronomy begins with the eyes. But new instrumentation evolved because some eyes have difficulty focusing light. Because of human near- and far- sightedness, the first spectacle lenses were ground. And it was only a matter of experimentation before someone combined one of each type lens together to form the first telescope or "instrument of long seeing".
Today's Astronomers are able to augment the human eye's capacity to the point where we can almost peer back to the beginning of time itself. This is done through the use of chemical and solid-state principles embodied in photography and charge-coupled devices (CCDs). Such tools are able to accumulate photons in a way the eye can not. As a result of these "visual aids", we have discovered things once unimagined about the universe. Many of these discoveries were unknown to us - even as recently as the beginning of the era of the Great Observatories (the early twentieth century). Today's astronomy has expanded the range of cosmic vision across numerous bands of the electromagnetic spectrum - from radio to X-rays. But we do far more than simply find stuff and measure positions. We seek to grasp more than light - but comprehension as well...
Today's amateur Astronomers - such as the author - use hand- and mass-produced Telescopes from all parts of the world to peer billions of light-years into the depths of the Universe. This type long-seeing is possible because the eye and telescope can work together to collect "more and finer light" from on high.
How far can you see?
According to NASA the Milky Way Galaxy would appear very much like 15.3 MLY distant barred spiral M83 found in the constellation Hydra (as seen at right). A human being in space would just be able to hold the bright central portion of this 8.3 magnitude Galaxy as a "fuzzy star" using averted vision. M83 can easily be found using low power binoculars from Earth.
-2 Bearing a variable visual magnitude of 12.8, 2 billion-light year distant quasar 3C273 can just be held direct by the human eye when augmented by a six-inch / 150mm aperture telescope at 150x through night time skies of 5.5 unaided limiting magnitude and 7/10p seeing stability. A pair of 10x50mm binoculars would reveal 3C273 as a faint star from Earth orbit.
Inspired by the early 1900's masterpiece: "The Sky Through Three, Four, and Five Inch Telescopes", Jeff got a start in astronomy and space science at the age of seven. Currently he devotes considerable time maintaining the website Astro.Geekjoy.
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