Monday, November 12, 2018

How far can our eyes see?



-  2163  -  How far can our eyes see?  -  With the newest backyard telescope it is possible for amateur astronomers to see 2,000,000,000 years back in time and 11,760,000,000,000,000 miles distance.    The human eye is an amazing instrument and working together with modern telescopes our brains perceive far back in time and far, far away. 
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 ------------------  2163  -  How far can our eyes see?
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-  How far can your eyes see?  You may be surprised.  How far back in time can your eyes see?  With a 10-meter telescope you can see quasars and galaxies that are 12 billion lightyears from Earth.  These views are also 12 billion lightyears back in time.  What you see happened then and that is what it looked like 12 billion years ago.  These galaxies look totally different if you could see them as they are today.
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-  What you would see is similar to what the Milky Way Galaxy probably looked like when it first formed about 12 billion years ago.  The Earth in our Solar System first formed about 6 billion years ago.
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-   In distance measured in miles 12 billion light years is:
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-----------  70,560,000,000,000,000,000,000,000 miles          (70.56*10^21 miles)
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-  But, what may surprise you is that you don't need to go to the Mauna Kea Telescope in Hawaii.  From your own backyard you can see 17% of that distance. 
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-  Even with the naked eye you can see a fuzzy ball in the night sky that is the Constellation Andromeda.  In the north east just above the horizon.  Use the corner of your eye, that part of the back of the eye is more sensitive to dim light.  The Andromeda Galaxy is 2,500,000 light years away.  (14.7 * 10^18 miles).
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-  When we see the Andromeda Galaxy we are seeing what it looked like when the Earth was populated with only bacteria, Achaeans, algae, jellyfish, and worms.  (the Proterozoic Eon).  The Earth will become a snowball 200 million years later after that era.   The first animal life will not appear on the Earth for another one billion years later.
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-  So, the Hawaii telescope will let you see 4,800 times further.  In effect the telescope is giving your eye a magnification of 4,800 to one.
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-   Andromeda looks like a little cloud and is at first disappointing to many astronomy buffs.  The Hubble Space Telescope will restore your wonder of this beautiful galaxy.  Check the pictures out on the internet.
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-  All the rest of the stars you see with the naked eye are in the Milky Way galaxy.  The Milky Way is 100,000 lightyears across. We are located  30,000 lightyears out from the center.  So the stars we see are no further than 70,000 lightyears away,  still that is 41.2 * 10^16 miles away.
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-  The ancients recorded 6,000 stars in the night sky and that was in 150 B.C.  Their nights were darker and clearer than the ones we live in today.  Air pollution and light pollution have decreased our seeing ability to about 3,000 stars in a typical night sky. (See Review 535  "Hipparchus - sky maps in 150 B.C.)
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-  The human eye sees using four main properties of light.  Light photons are refracted, reflected, diffracted, and absorbed.  Light enters the eye in parallel beams from the distance of stars.  Because the eye has a limited aperture it is only able to collect a very small portion of the rays coming from any one astronomical object.  The eye's collection area is about 38 square millimeters, when fully dilated and darkness adapted.  This means you can see stars with a light intensity of magnitude 6.  (See Review 535 for a full discussion of light magnitude scales).
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-  The eye is limited by diffraction, light waves are bent when passing near an object's gravity or through a hole.   It is a form of refraction where light is bent when passing through different mediums.  Diffraction in the iris creates a diffusion that allows photons to come only so close together.  These characteristics of light limit our ability to see in any greater detail.  Things tend to get fuzzy.
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-  The eye uses refraction to bend photons entering the cornea and the lens behind it.  The cornea does 67% of the focusing of light and the eye lens does the remaining 33%.  The lens can change its curvature to focus an image on the retina where the tiny neurons convert light energy into nerve signals the brain can interpret.  It is the occipital lobes at the back of the head that do the image processing.  The brain gives coherence to the steady stream of signals arriving from the eye's retina.
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-  To detect the light the retina uses absorption.  Photons landing on the neurons of the retina depolarize.  Depolarization moves chemo-electrical signals from axons to dendrites in the  brain.  The retinal neurons are made up of rods and cones.  There are about 220,000,000 of them in the back of your eyes. 
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-  The rods detect light of any color and are more sensitive than the cones.  The cones detect specific colors and are only found along the main axis of the eye.  The rods on the other hand are mainly off - axis.  This is the reason the averted eye can see stars 2.5 times fainter than if you are looking directly at the star.
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-  To get from the retina to the brain neural signals pass through the superior collicus.  The collicus can detect even fainter sources of light, but, only if it is in apparent motion.  This is what causes a flinch response.  You can actually flinch before the brain sees what you are flinching about.
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-   You can avoid the strike of the rattlesnake before your brain realizes the snake is even there.  To use this capability in night vision sweep your eye across the night sky and you can see stars 4 times fainter than straight on viewing.
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-  The eyes naturally adapt to night vision in about 30 minutes.  First, the fine muscles retract the iris located between the cornea and the eye lens to admit as much light as your eye can allow.  Second, the visual purple (rhodopsin)  on the retina rods takes on a rosy red color.  This change increases the sensitivity of the rods to the point where a single photon of visible light can be detected.  A single photon, amazing!
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-  The neurons in the eye can reside only so close together so the eye can only have a 1 x magnification at 25 millimeters focal length.  Since the eye can only open to 7 millimeters, it becomes an effective equivalent pair of binoculars of 1 x 7 millimeters.
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-  With the eye's 1 x 7 millimeter binoculars you are limited to 8th magnitude star brightness.  This is viewing under the best conditions.  The 8th magnitude brightness is 1,500 times fainter than the brightest star Vega in the southern sky.  Your eye is able to resolve binary pairs of stars to about 2 arc minutes of angular separation.  This is 1/15 the apparent size of the Moon which is 30 arc minutes, or 1/2 a degree of arc.
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-  CCD, charge coupled devices, offer telescopes a enormous advantage.  CCD's can accumulate photons over a long period of time, something the eye cannot do.  CCD's can look at the same image over a very long period of time, months, and simply add up all the photons over that period of time.  As long as nothing moves side to side relatively,  a clear image will materialize.  This is how the Hubble Deep Field telescope gets its far away images. 
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-  Of course telescopes can also have much larger apertures to capture light and they are not limited to only collecting visible light.  They can collect images in the ultraviolet and in the infrared.  In fact telescope detectors can see the entire electromagnetic spectrum from radio waves to microwaves to gamma rays.
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-  With the newest backyard telescope it is possible for amateur astronomers to see 2,000,000,000 years back in time and 11,760,000,000,000,000 miles distance. 
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-  The human eye is an amazing instrument and working together with modern telescopes our brains perceive far back in time and far, far away. 
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-  One light year is 5.88 * 10^12 miles
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-  3C273 quasar is 2 billion lightyears away.  It has a variable visual magnitude of 12.8 magnitude.  It can be seen with a 6 inch, 150 millimeter aperture telescope at 150 power.  The night sky needs to be clear enough for naked eye visibility to 5.5 magnitude.
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-  November 12, 2018.   536    from  January 4, 2005   An Index of recent Reviews is available.
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 ---------------------   Monday, November 12, 2018         -------------------------
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