I know many spectacular space images are composites of all sorts of spectra “colorized” for our visual understanding. And any image is taken from a certain vantage point and it’s telescopic magnification. And even within those spectral ranges, the field of view is insufficient for a detailed overall picture, and the resultant image is a composite.
But where would I, could I, using human optics ever be in a position, theoretically, where I could get such a knock-out view of this or any of all our other beautiful galaxies?
An optical telescope (i.e. without electronic amplification) can only enlarge the view. It cannot amplify the surface brightness of the object you are looking at. So no matter how big your telescope, the dimmer parts of the galaxy wouldn’t be visible. You probably can’t see any color, because the cones are much less sensitive than the rods.
M51 is about 12 arcmin (1/5 degree) long. So at 300x magnification, it would fill up much of your field of view (60 degrees across) - not out to your peripheral vision, but much of your central vision. At that magnification, you need a 2-meter aperture telescope to provide full illumination (no loss in surface brightness). With this setup, M51 should look as bright as the Milky Way does to your naked eyes. Except a bit more blurry, because of atmospheric distortion. You should see dark lanes and other features.
If you look at the Andromeda Galaxy with your naked eye, all you can see is a dim blob about as big as the Moon; this isn’t the whole galaxy, but only the central hub. The spiral arms are about three times as wide, but you can’t really see them.
If you could see M51 close up from a million light years or so, the central hub would look fairly impressive, but the spiral arms would be disappointingly dim.
Yeah, the telescope is an advantage, but it isn’t actually the biggest advantage. The biggest advantage comes from long-duration exposures, which only became possible with photography (and now, with digital sensors).
That said, while the Hubble pictures you see are all false color because they choose filters based on scientific interest rather than to correspond with the human cones, they do still pick false colors that at least somewhat approximate the true colors. So if your eye could integrate over long exposures, you would see something at least resembling the pictures.
If you’re ever out on a very clear dark night and look up and see the Milky Way, you can see it very clearly but it’s also very dim. So that’s the view from inside a galaxy. If you were outside a galaxy looking at it it would be even dimmer.
If you were inside a brightly lit room inside a spaceship looking out the window you would not see anything. The galaxy in the distance would be much too dim. If you turned off all the lights and told all the other ships in the fleet to turn off all light sources and looked out the window you’d see the galaxy…but it would very very dim. It wouldn’t look bright like that at all.
Your eyes are really very sensitive and can see things that are incredibly dim, but only when there aren’t other bright objects nearby.
My experience has been that even with very little light pollution, the central hub of M31 appears to be far smaller than the Moon; also, if we could see the spiral arms the galaxy would appear to be seven Moon diameters wide.
Although I’m pretty sure about both of these statements, I won’t mind being corrected if I’m wrong.
With a good telescope with a human eye at the eyepiece, you can see a fair bit of structure in M31. You still certainly can’t see all of it, but you can see enough to see the dust lanes against the bright disk quite clearly.
I speak from experience, having looked through a 20-incher in one of the very few truly dark sky locations left in North America.
The portion that’s visible will be the same, with a completely naked eye or with an eye at an eyepiece. The telescope will, however, make it a lot easier to see detail in that visible portion.
You can expose a plate of film (or a photo-receiver array) to a long (long!) exposure, and collect a lot more light that way, over time. That’s one of the big things that we’ll never get to do with naked-eye telescopy.
A lot of the Hubble images were collected with time exposures, so, kinda no, nothing we can see will live up to that (miraculous!) quality.
I think you are almost certainly correct; I was speaking very roughly. It is difficult to estimate how big a thing would look to the naked eye if you can’t actually see it with the naked eye.
Technically true but not relevant. A telescope not only enlarges the view, it gathers in more light than can enter your Mk I eyeball and squeezes it down so it can enter. The sweet spot is when the aperture divided by the power equals about 7. For binoculars, 7x50s (power x objective diameter in mm) are popular because the exit pupil is 7.12 (close enough), they are light enough to hold steady for a while, and the magnification is not excessive. 7mm is about as big as your pupil can get so all the light gathered gets in. 100x7 is a waste because its exit pupil is 14.2; they would be way heavier and not all the extra light the bigger objectives gather can get in.
The Navy’s “big eye” bridge binoculars are 20x120 with an exit pupil of 6 The sweet spot for 20 power would be 140mm objectives but you can see how 120 is unwieldy enough as it is.
With amateur telescopes, presumably not hand held any more, it’s a pull between magnification and aperture. Here is a Sky & Telescope article about when to favor one over the other.
When I was living at my dark sky house in Carson City, I went out one night with my 8x35 “day glasses” and my 7x50 "night glasses. The difference was astonishing. Even the day glasses showed me more than my unassisted eyes could discern, but with the night glasses, many more objects could be seen.
This, which is why I specified “amateur telescopes” above. When you can expose a plate for a half hour or more, you’re going to come up a lot more than applying your eye to the eyepiece will (besides making a record). The pros rarely do this, applying some sort of instrument or a camera instead. A couple weeks ago at Westercon, one of the speakers was an astronomer at Kitt Peak. He mentioned how a couple months before on a ‘parents night’ sort of thing, he’d actually looked through the eyepiece of the 1.3m telescope. “It was really cool.”
It only gives you more light in the sense that the intensity is covering a larger area. It does not “concentrate” or “squeeze down” the intensity at all: Doing so would violate the Second Law of Thermodynamics.
Don’t feel too bad-- This is a very easy misconception to hold. I myself held it, until CalMeacham disabused me of it some years back.
How is it not relevant? The question was whether any telescope could make a distant galaxy look as bright as it does in a Hubble photo. The correct answer is no, because no optical telescope can amplify the apparent surface brightness of an extended object. It follows from the laws of thermodynamics (if a telescope could amplify light, it would allow heat to flow from a colder object to a warmer object, which will reduce entropy.)
Yes, I’ve heard many times that “a telescope is not a device for magnifying the view, it’s a device for collecting light.” While this works as a way to emphasize the importance of aperture, it is highly misleading. The fact is, a telescope is a device for mangnifyin the view - because if it didn’t magnify, then it wouldn’t do anything. A telescope magnifies the view and collect enough light that the surface brightness is not reduced (or not reduced too far). As you say, maximum surface brightness is achieved when the exit pupil of the telescope is at least as large as the pupil diameter of the observer (generally assumed to be 7mm, though it depends on the person, age, etc), e.g. the ubiquitous 7x50mm binoculars.
Crafter_Man, did you quote the wrong one of my posts, there? Because the only elaboration I can add to that post is that the location was the Lewis and Clark National Forest, in Montana; the telescope was a 20" Dobsonian owned by the Southwest Montana Astronomical Society, and the view of M31 was real, and it was spectacular.
I’m not grasping what you mean by “the intensity is covering a larger area.” All I know is, when I tried my Carson City experiment mentioned in my first post I could see more objects in the night glasses than the day glasses, even though the magnification was less. With a 12-inch Dobsonian there would have been even more. They’re called “light buckets” for a reason.
It’s easy to magnify an object while decreasing the surface brightness. That’s what you’ll get if you use too small an aperture. Use the proper size aperture, though, and you can magnify an object while maintaining the same, or very nearly the same, surface brightness. So the view through large-aperture binoculars will be brighter than the view through small-aperture binoculars. Neither will be brighter than the naked-eye view, but the naked-eye view might well be small enough (from lack of magnification) to not be useful.
_prepares_to_get_underway_for_the_first_time_after_a_six-month_dry_dock_period.jpg){kind=link}