When we’re talking about extended objects (galaxies and nebulae), telescopes help by enlarging objects that are too small to see with your naked eyes. A tiny dim blob becomes a huge dim blob. You’d want to use a magnification that gives you a large exit pupil (as large as your eye’s pupil), because if you use a higher magnification, you are turning a tiny dim blob into a huge but even dimmer blob.
It’s a little different for point sources (stars and perhaps small bright nebulae). You are still magnifying the image, but if a tiny unresolved dot becomes a larger but still unresolved dot, then you are getting more light into one receptor in your eye. So a telescope allows you to see dimmer stars than you can with naked eyes.
p.s. You could try for yourself - take a small telescope or binoculars and look at a nearby uniform surface (e.g. side of a building). Maybe look through it with one eye and open the other eye to look directly at it. The wall will be magnified in the telescope/binoculars, but it won’t look any brighter.
So the objects I could see through the glasses but not with the unaided eye was only because they were magnified with a proper-sized aperture? I’m not buying it; if magnification (with the proper aperture) was the only factor I’d still be able to see them unaided, just smaller. The Dobsons I linked to are large aperture, low magnification telescopes with the express purpose of looking at deep sky, dim objects. No advantage is gained over a smaller, easier to transport telescope in using them to look at the planets or the moon because there’s plenty of brightness anyway.
In the Sky and Telescope article I linked to:
Here is a chart that shows what magnitude you can see with a particular aperture. Notice, magnification is not a factor considered. Obviously, it plays a part; otherwise peering through a toilet paper tube would magically make me see a 12-magnitude star I couldn’t see before. As I kind of said in my first post, magnification is what lets the light gathered by the larger aperture get shrunk down to an exit pupil that can get past the 7mm aperture of your eye.
scr4, sorry for not answering your earlier reply to me just after Chronos, I missed it.
You got it right on the second part. True, a telescope doesn’t amplify light; it collects it through a larger aperture than your eye, then the magnification lets it fit through your eye’s aperture.
Think of it this way. You’re looking at Sumdim Galaxy and 10,000 photons a second are getting to your retina, not enough to trigger the rods (never mind the cones). Now, you raise your 7x50 binos. Its aperture is 51 times that of your eye, collecting 510,000 photons a second all of which, thanks to the 7x magnification, are getting into your eye; now you can see it*. Next you turn to your 10-inch Dobson which is gathering 2,030,000,000 photons a second. Anything above 25x magnification will let them all into your eye and, boy, can you see it; the more photons smacking your retina, the brighter it looks. Are the instruments amplifying the light? No, they are gathering it into a bunch and stuffing it into your eye. Does Sumdim look brighter? You bet it does.
See my parable above.
For all intents and purposes, stars are point sources of light. Any enlargement you see of a magnified star is due to the limitations of the optics, not the star’s disk getting larger. Otherwise, looking for exoplanets would have been a snap. Yes, we’ve been seeing some distant stars resolved into disks, but that’s with equipment and image processing far beyond an amateur astronomer peering through her telescope, no matter what the aperture is.
That’s right, it won’t because there’s plenty enough light that when you peer though the eyepiece, your iris contracts even further than it does when you’re gazing directly. I haven’t tried it, but I’d bet an exposure meter held up to the eyepiece would read higher than one aimed at the wall.
When I go bird watching I take along the 8x35s rather than the 7x50s because there’s no advantage for the weight.
*I’m making the figures up; I have no idea how many photons a second constitute a magnitude 7 object, never mind getting into the whole wavicle thing.
If this galaxy is so tiny that it’s filling illuminating only a fraction of one rod receptor in your eye, then yes, a telescope will collect more photons and shove it into one receptor in your eye.
But most galaxies are not that small. Each rod cell subtends about 1/2 arcminute. M104, for example (which I think is one of the smaller galaxies on the Messier catalog), is about 9 x 4 arcmin.
So Chronos is correct. The telescope is just magnifying the view, not making it brighter. A 7x50 telescope may collect 51 times more light than your naked eye, but that light is spread over a larger area (51 times larger) of your retina. So each rod cell isn’t receiving any more light than without the telescope.
The reason M104 is visible through a telescope but not with naked eyes is because the human brain has some ability to average the signals from multiple receptors. If 10 adjacent rod cells receive 100 photons / second each, that might be dismissed by your brain as noise. But with 7x50 binoculars, you now have 510 adjacent rod cells each receiving 100 photons/second, and now that is seen as a feature. (The actual numbers may not be accurate, they are to illustrate the math.)
No, it has nothing to do with your iris contracting. Try it. Put an exposure meter behind the lens, or point a camera into the eyepiece. You will never register a higher surface brightness.
Or for that matter, just try holding up a magnifying glass up against the blue sky (nowhere near the sun) and see if the blue sky looks any brighter through the magnifying glass. It won’t.
p.s. A more formal way to say “a telescope cannot increase apparent surface brightness” is to say etendue is conserved. An optical system can increase etendue but it can never decrease it.
Well, I spent an hour creating the reply to your and Chronos’ opinions, complete with [del]appeals to authority[/del] citations. I know what I have seen. Perhaps is it some sort optical illusion, bigger apertures allowing you to see objects too dim to be seen unaided. If so, it’s been a pervasive illusion, persisting since 1608 and leading deluded astronomers to make bigger and bigger telescopes, poor sods.
Nothing Chronos and I wrote imply that large telescopes are useless. Of course they serve a function. Even for visual use, a large telescope allows you to see dimmer point sources (stars), and enlarges the view of extended objects, allowing you to see dimmer objects (because your eye/brain has a lot more illuminated rods to work with). Which is why I own a pair of 100mm ED-Apo binoculars and a 14-inch Dobsonian.
And for non-visual (i.e. film, CCD and other sensors), aperture is even more important because film & sensors are not limited by a pupil - you can make them as large as you want, and collect more light. My colleagues and I once did a concept study for an 8-meter solar space telescope for this reason.
But no matter how you look at it, it’s impossible for an optical instrument to increase the apparent surface brightness of an extended object. You can calculate the flux based on geometric optics, or you can argue from the standpoint of etendue, or consider the laws of thermodynamics (i.e. if a telescope could amplify the apparent surface brightness, it would be possible to make a telescope whose focal plane receives more light than the original object, i.e. heat flows from a cooler surface to a hotter surface.)
A star is a disk, whether you can resolve it or not. A telescope aimed at a star will turn a disk that’s too small to see into a disk that’s larger but still too small to see. You won’t see evidence of the magnification from an increase in size of the disk, but you will see evidence of the magnification from the increase in brightness.
Here is a telescope & eyepiece calculator spreadsheet, which produces a graph of apparent magnitude as seen through a telescope. Note as magnification is increased, the blue line (apparent magnitude of an extended object) stays flat until the exit pupil matches the eye’s pupil. After that the apparent magnitude increases (gets darker). Point sources don’t get darker because the apparent size of a point source stays the same. (At least it’s approximated as such. In reality it increases slightly due to atmospheric distortion and diffraction.) The page also explains:
p.s. I can appreciate this is counter-intuitive to amateur astronomers who have always been taught that aperture is everything, and experienced first-hand how telescopes and binoculars allow us to see deep-sky objects that are invisible to the naked eye. But it’s all an illusion, mainly stemming from the fact that the human eye does a very good job of filtering noise and integrating signal. A large dim object appears brighter than a smaller and equally dim (same surface brightness) object.
Back to the OP, fellas! Sorry to be late to the party.
How closely does this image replicate a naked-eye view of the Milky Way from space? From Earth, I believe you would only get those colours and intensity with a long exposure/stacked images. The source doesn’t say anything about the exposure/manipulation of the image (although it would be hard to do a long exposure, with that clarity, moving 7 kps, no?)
A lot of people’s answers assume we’ve got our little feets planted on the earth.
If we garbed ourselves in an appropriate space-suit and positioned ourselves in the darkness between galaxies and stared at, for instance, the one mentioned by the OP, with no bright stars anywhere nearby to mess up our night vision and no atmosphere to obscure stuff, what would our human eyeballs be able to see?
Well, one thing to remember is that the human eye is many times more sensitive than a typical camera. I’m sure you’ve had the experience of looking at clouds or sunsets or distant mountains, you try to photograph it and in the picture the features are completely washed out and have none of the detail you saw with your naked eye. Or you’re in dim light and take a picture of an object you can see easily and the photo is pretty much just black.
So…apparent brightness is a tricky thing. An object can be very very dim, and if there’s no other light source you can see it clearly, even though if you tried to take a picture you’d see nothing.
So if you’re hovering in intergalactic space looking at a nearby galaxy with no other light sources, you’ll see something like that spiral galaxy in your picture. But that galaxy will actually be much dimmer than the picture. You’re in a bright room right now and you can easily see the galaxy in the picture on your monitor, right? But if you were in a bright room and tried to look at the real galaxy you wouldn’t be able to see it because it would be many times dimmer than that image on your monitor.
Yup, that’s the OP all right. I think way up someone mentioned the posish: sitting on my armchair armed only with a beer perched somewhere in intergalactic space, what or where could I get the best postcard-pretty view.
The atmosphere hardly makes any difference. It absorbs very little visible light - only a couple of percent at most. See here for example - keep in mind that the human eye is only sensitive from 400nm to 700nm or so. That’s just the first 3 tick marks in the plot.
The image you linked to looks impressive because it’s a long-exposure photo. You can get very similar photos from the ground under a dark sky with a similarly long exposure time.
Actually, modern digital camera sensors are far more efficient than the human eye. By some estimates the human eye is only 1-2% efficient (cite. Even a basic CCD in a digital camera has >10% efficiency, and modern back-illuminated sensor may have >80% efficiency.
The human eye does have a huge dynamic range, so you can look at bright & dark features at the same time. Digital cameras can only match this by taking multiple images at different exposure times and then combining those images (HDR imaging). Most high-end cameras and phones have this feature now.
If your camera isn’t as good at imaging in dim light, it’s because your eyes have very good image stabilization, and your brain is very good at ignoring noise and detecting patterns in what you see.
No it won’t. There is a hard limit on how far your pupils can open up, no matter how dark it is. It’s around 7mm. You can reach it on earth in a dark site, far away from city lights, on a moonless night. Going into intergalactic space won’t make your eyes any more sensitive than that.
The answer is, it won’t look any more impressive than our own galaxy (the Milky Way) does to your eyes, from a dark sky site (no light pollution, no moonlight). Which I think is pretty impressive, but you won’t see vivid colors like those Hubble photos. Because the cones (the color-sensitive light receptors in our eyes) only respond to bright light. In dim light, our eyes only work in monochrome.
You’ve got that right! I’m heading to our cottage next week, and one of my favourite things about it is it is dark enough to see the Milky Way. I can’t wait!
There’s something like 2-4 thousand stars visible to the naked eye from the Earth’s surface. That’s about half the sky.
Your photo shows quite a bit more than that in a smaller segment.
So not completely accurate but maybe off one order of magnitude. (I did a sample count on one small square but the motion blur really hurts the count. A more stable image would should quite a few more.)
