Discussion: How Far Can You See?

[Fraser]

SUMMARY: 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.

View full article (http://www.universetoday.com/am/publish/how_far_can_you_see.html)

What do you think about this story? Post your comments below.

[spacepunk]

Quoting from the article

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

Related questions could be asked: are large eyed animals such as elephants are able to see fainter magnitude stars than humans, regardless of their ability to view in color. Also, the eyesight of bees tends towards being in the infrared, so what would they actually see of the night sky, or perhaps there are distance limitations due to their compound eyes?

[TheAstronomer]

Thought provoking and well done!

~T

[antoniseb]

I liked the idea that I could see 3C273 with a six inch reflector. That would certainly be the most distant thing I could see without some incredibly expensive equipment. I think I will try to look at it sometime this year.

[Josh]

This article was a good read! Makes me want to move to the middle of the desert to see what i can see.

Any chance of getting those questions by spacepunk up there answered?

[Near-Sighted Astronomer]

Greetings,

Thank you everyone for your comments on the article "How Far Can You See?".

Here are some off-the-top-of-my-head responses to your comments and questions:



Makes me want to move to the middle of the desert to see what i can see.


Keen eyed observers in the american southwest - well away from city lights
and through very dry skies - report stars to near magnitude 7.5 can be seen.
It's unclear as to whether or not aversion or eye movement is involved but certainly 7.0 mag stars may be held direct under such conditions. I have personally held stars to about magnitude 6.5 at high - though quite wet- elevations.



Are large eyed animals such as elephants are able to see fainter magnitude stars
than humans?"


Assuming all other things being equal (neural sensitivity etc.) every time you double the aperture of the eye or an instrument you get about a 1.5 magnitude increase in theoretical magnitudinal reach. So let's say an elephant's fully dilated eye is 14mm then the elephant would be able to see stars down to magnitude 9.5 in space. However on earth you also need to double the magnification as well (to help dilute light pollution) and I suspect that that's probably the case with an elephants eye as well. So an elephant could conceivable see down to magnitude 7.5 - should they have the same interest in things astronomical as a human...



eyesight of bees tends towards being in the infrared,so what would they actually see of the night sky,


Probably not much - even without compound eyes. There's just too much atmospheric heat which would act as a form of "heat pollution" preventing them from seeing heat radiated by stars...

If you have a telescope and want to see "How Deep it Can Go?" check out:
Limiting Threshold Magnitude Calculator (http://astro.geekjoy.com/calcs/maglim.html)

It can give you an idea of where the edges are for a variety of scope types, apertures, magnifications and seeing conditions.

Carpe Noctem,

jeff

[Josh]

Thanks Jeff!

Elephants (African at least) generally live in zero light pollution areas, wouldn't they? So the should still see down to the mag 9.5 (assuming their eye apertures are that big). It'd be pretty interesting to know how larger eyed animals see the night sky and the world in general compared to us. Thanks again

Josh

[Duane]

It'd be pretty interesting to know how larger eyed animals see the night sky and the world in general compared to us. Thanks again

They would say "hey, wonder why them monkeys are looking up so much, and why do they point those fire sticks at us? Where's lunch?"



:lol:

[Algenon the mouse]

With lasar surgery and the advances in telescopes, we can now see more than we ever have before in the night sky (away the light pollution that is). More and more people are looking and finding new things in the night sky.

However, it has been my understanding that most animals are colored blind even though they can see further.

[spacepunk]

Thanks Jeff for the answers!

In designing 21st century telescopes it may be of some use to understand how animals, not just humans, perceive light.




It'd be pretty interesting to know how larger eyed animals see the night sky and the world in general compared to us. Thanks again

They would say "hey, wonder why them monkeys are looking up so much, and why do they point those fire sticks at us? Where's lunch?"


A compund eye must have its evolutionary advantages, from trilobites 500 million years ago to insects today. Not being an optician myself I always wondered if the compound eye system worked much like having miniature finder scopes surrounding central long range viewing scopes. Or else work very much like a wide array telescope structure with "stacking software" wired in their tiny brains to sort our a clear composite picture from the myriad of optical nerve signals. But there would be a distance/magnitude limation wouldn't there with the extremely small size of the eyelets, unless the long range eyelets were extremely sensitized to light.

Many predator birds and frogs have a double eyelid that folds down on reflex demand to significantly magnify their eyesight. As Duane has indicated I also don't think that these animals nor elephants have the patience to look at the stars!!

[The Near-Sighted Astronomer]

Hi Spacepunk,

It's an intriguing thing to think about sight and cognition.

In the article I mentioned the "superior collicus" in the brain. The collicus
does not in fact "see anything". It does not have a "minds eye". There is no "movie screen of perception" like we see approximating our foreheads in the prefrontal lobes of the brain. The Superior Collicus gives us our "flinch" response. It signals us that something is in motion and gives us a sense of where that motion is coming from directionally. The Collicus explains "blind sight" of the ability some non- seeing humans have that a hand is being waved in front of their face - but without them actually seeing that it is in fact a hand.

There is some thought that bees etc do not actually "see" anything. They do not possess cognition. They have the neural equivalent of a collicus which fires a pattern of neural patterns that correspond to "another bee" or a "nectar filled flower". The bee then responds...

This idea was captured in the movie "Jurassic Park" where if you stood still the T Rex would not "see" you.

As for different kinds of optics there are telescopes now that capture light using separate mirrors - or even telescopes at a distance. In the first instance the mirror cluster focuses light on a single CCD imager. In the second instance the optical interferometer has separate CCD's and the two images are merged by computer.

Hope this clarifies things,

jeff

[Dave Mitsky]

There's a problem with this otherwise excellent article. A number of sharp-eyed observers have seen M81 without optical aid. M81 is considerably more distant than M31. Another galaxy that is also much farther away, NGC 253, has reportedly been detected naked-eye by southern hemisphere astronomers.

For a list of naked-eye deep-sky objects compiled by Lowell Observatory astronomer Brian Skiff see http://www.visualdeepsky.org/netastrocatal...e/msg01732.html (http://www.visualdeepsky.org/netastrocatalog-announce/msg01732.html)

Dave Mitsky

[antoniseb]

Originally posted by Dave Mitsky@Jan 28 2005, 03:04 PM
A number of sharp-eyed observers have seen M81 without optical aid.
Cool. I'll have to find a dark dry sky someplace and look for it.
I've been trying to find some time and place to look for 3C273 with a 10" or bigger telescope, but naked eyeing M81 might be even better.

[Near-Sighted Astronomer]

Hi Dave,



A number of sharp-eyed observers have seen M81 without optical aid.


Thanks for the follow up. I imagine through 7.5 ULM skies at high altitides this is entirely possible. The interesting thing is to determine whether or not M83 could be held direct or spotted on eye movement. I've noticed many visual observers fail to make this important distinction and its something I had to be very carefully about explaining both in the article and when developing the limiting magnitude calculator on astro.geekjoy.

The idea of a list like this is excellent!

Carpe Noctem,

jeff

[Dave Mitsky]

Originally posted by Near-Sighted Astronomer@Jan 28 2005, 11:00 PM
Edit

Thanks for the follow up.* I imagine through 7.5 ULM skies at high altitides this is entirely possible.* The interesting thing is to determine whether or not M83 could be held direct or spotted on eye movement.* I've noticed many visual observers fail to make this important distinction and its something I had to be very carefully about explaining both in the article and when developing the limiting magnitude calculator on astro.geekjoy.

The idea of a list like this is excellent!

Carpe Noctem,

jeff
Jeff,

I assume you meant M81, not M83. IIRC, the accounts that I've read by credible observers have mentioned being able to detect M81 with averted vision and not direct vision.

http://www.visualdeepsky.org/netastrocatal...e/msg01976.html (http://www.visualdeepsky.org/netastrocatalog-announce/msg01976.html)

From the web site of Jerry Lodriguss:

"With a visual magnitude of 6.8, M81 is probably the farthest object that can be seen with the unaided eye at a distance of 12 million light years.

Although this is a most difficult observation, it has been reported on several occasions, particularly by Brian Skiff of Lowell Observatory. He reported glimpsing it as a "threshold" object, detected only 5-10 percent of the time with optimally adapted averted vision under very dark skies at Anderson Mesa where stars down to magnitude 7.9 were visible to the unaided eye. This is no mean feat."

I've been to Anderson Mesa with Brian. It is indeed a very dark site.

Dave Mitsky

[Dave Mitsky]

Originally posted by antoniseb+Jan 28 2005, 03:41 PM--></div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (antoniseb &#064; Jan 28 2005, 03:41 PM)</td></tr><tr><td id='QUOTE'> <!--QuoteBegin-Dave Mitsky@Jan 28 2005, 03:04 PM
A number of sharp-eyed observers have seen M81 without optical aid.
Cool. I&#39;ll have to find a dark dry sky someplace and look for it.
I&#39;ve been trying to find some time and place to look for 3C273 with a 10" or bigger telescope, but naked eyeing M81 might be even better.

[/b][/quote]
It&#39;s possible to see 3C273 with even smaller instruments. This quasar is about a magnitude brighter than Pluto but like most "bragging rights" objects 3C273 is not very impressive visually. Think very dim star...

http://www-2.cs.cmu.edu/~zhuxj/astro/html/...uasar3c273.html (http://www-2.cs.cmu.edu/~zhuxj/astro/html/quasar3c273.html)

http://www.ne.jp/asahi/stellar/scenes/obje...ect_e/3c273.htm (http://www.ne.jp/asahi/stellar/scenes/object_e/3c273.htm)

Dave Mitsky

[Near-Sighted Astronomer]

Hi Dave,



I assumed you meant M81, not M83. the accounts that I&#39;ve read by credible observers have mentioned being able to detect M81 with averted vision and not direct vision.


And I thought you meant M83 not M81&#33;

Anyway you clarified the point - the skies were very deep (7.9ULM) and Brian couldn&#39;t hold M81 direct. This is quite an accomplishment and does demonstrate that the eye can go much deeper than we might suspect when peering out of our backyards in even very rural locales. I wonder sometimes what the fine folks on the ISS can see out there portals...

So yes at magnitude 7 and some 10 MLY distant M81 tops the Andromeda galaxy for unaided perception. But alas most of us are fortunate enough to see M31 these days...

Carpe Noctem,

jeff