No. Go to your nearest astronomy club, and take a look at any interesting astronomical object through an 8" telescope, then a 16" telescope, at the same magnification. The image is the same size, but the 16" telescope gathers 4x more light, and the image will appear roughly 4x brighter. It is not a subtle effect.
You can see a (simulated, but realistic in my experience) depiction at the bottom of this webpage:
Easy experiment:
Go into a dark room illuminated only by your cell phone about 6 inches away from your face. Now, make an “OK” sign with your fingers, occluding all but a tiny section of your screen.
Now take it away.
The unoccluded display seems brighter, but it’s not. You’re just exposing your retina to more light. The intensity is the same.
Right. But it’s never going to get any brighter than the intensity the naked eye is able to bare alone.
It’s a subtle but crucial point, so we don’t talk past each other.
That’s due to the nature of the object they’re viewing. Like many extended astronomical objects, M13 has a dense core that gets more diffuse farther out. It is also fairly scale-independent. So you go out with a big scope with correspondingly larger magnification, and the core of the object fills a larger area, and so looks brighter overall. That’s what you see in the sequence on that page: the central area is still bright white, but it gets larger. There’s more overall light, but any given point on the object is no brighter.
Of course, you can also put a big magnification on a small scope. In that case you become aperture limited, which means your exit pupil is reduced. The object is actually dimmer than with the naked eye–whether or not you can see it better depends on the nature of the object.
Oh, and there’s another effect (this stuff gets complicated). Some extended objects are just beyond the limits of the eye’s resolution. You see them as dim, fuzzy patches (if at all), because all the detail got blurred away. If you look at them through a telescope, the average brightness hasn’t changed, but because all the light is now concentrated into individual points, it becomes much more visible. It looks brighter in a subjective sense, but isn’t.
It’s like if you take a black image in a photo editor, and add a few sprinkles of white pixels. They’re very easy to spot. But if you then blur out the image with some huge radius, it just looks black, because one white pixel averaged with thousands of black pixels can’t increase the intensity appreciably.
I totally agree. Anyone who has used binoculars on a boat at night knows that you can see things which you cannot see with the naked eye. That’s the whole point of using binoculars at night.
Binoculars and telescopes are used to make things more visible, not to make them brighter. Being brighter is just one way of making things more visible. There are also many ways for things to appear subjectively brighter without actually being brighter, and for there to be an increase in light entering the eye, even when the viewed object has the same surface brightness.
The point is that we have to be clear about our terminology here. With passive elements, the OP’s request is not possible. All of the advantages of binoculars and telescopes in some way come down to magnification (when used with the human eye).
No, this still makes no sense. I guess I have been convinced by cmyk’s explanation, that the object can never be brighter than it appears with the naked eye, but what you are saying makes no sense.
First, if you look at that sequence of image and pick a pair of stars in the first one, you can clearly identify the same stars in the other images. The scale is the same. The images are simply brighter.
I have a question for you. I have a 8" aperture telescope with 1600mm focal length, and an eyepiece with a 20mm focal length. This gives a magnification of 80x. I also have a 16" telescope, with a 1600mm focal length, and the same eyepiece, for the same magnification.
I point both of those telescopes at the same star cluster, or galaxy, or nebula. What is the difference in the image?
Regarding cmyk’s explanation, if I understand you correctly, what you’re saying is that a telescope cannot increase the perceived brightness per steradian in your visual field compared to the unmagnified, naked eye image.
However, if a telescope magnifies an image, then the perceived brightness of any particular static feature on the object in view increases (since the brightness per steradian is at best constant, but the solid angle occupied by that feature is now larger), and this is the mechanism by which a telescope lets you see dim objects better.
Is that it?
We’re just playing semantic games. Suppose a point of light in the distance which I cannot see with the naked eye. Now I look through binoculars and I can see it. I think I can say it is brighter and anything else is just playing with words.
Suppose I remove the binoculars. Now I can’t see it again. then the source increases in intensity and I can see it without binoculars. I think I can say it is brighter from my point of view.
I can’t see why I can’t say it is brighter with binoculars than without binoculars. It is brighter and that is why I can see it.
Assume a point of light in the distance. The rays of light arrive at the observer virtually parallel. If the diamater of the pupil is 5 mm then the rays (photons) entering the eye and concentrating at one point in the retina are those arriving in a circle of that diameter.
If now I have a lens of 50 mm diameter concentrating all the photons which arrive parallel and sending them into the eye, then I am catching 100x more photons. (The response of the eye is not linear but logarithmic so the apparent brightness will not be 100x.)
In summary, for me it is very clear that using binoculars at night makes lights “brighter” just like telescopes make stars “brighter” because you can see stars with a telescope which you cannot see with the naked eye. If that is not making things “brighter” I don’t know what is.
That’s not true. The magnification is clearly increasing from left to right. Look at the 22" image on the edge of the disk at maybe the 3:10 position. There’s a little cluster that looks a bit like a reversed “C”. The same cluster is off the edge in the 25" image, close to the same position in the 20" image, but visibly displaced in the 18", 15", and 12.5" images. I can highlight this tonight if you want.
Your 8" scope has an exit pupil of 2.54 mm, which is smaller than a normal human pupil. The resulting image will be dimmer than with the naked eye. With the 16" scope, the exit pupil is 5.08 mm. This is pretty close to a fully dilated human pupil. Therefore, the 16" scope will be actually brighter, but only because the 8" scope was restricted in the first place (as compared to the naked eye).
If you put a 40 mm eyepiece on your 8" scope, it ends up with the same 5.08 mm exit pupil, but with correspondingly lower magnification.
More or less, yep.
The OP asks if there is a purely optical way of peering through something akin to binoculars, increasing the absolute magnitude of your field of view, but without altering the magnification in any way. Can’t be done, since any time you magnify, light’s intensity will drop (since the photons are spread over a larger area, they’re more dilute). We can compensate for this with larger objective lenses to catch more light to project onto the retina and by doing so bring the absolute magnitude very close to what we perceive with the naked eye, but never above it due to the law of physics.
In that sense, there’s no possible way to increase only the intensity greater than what the naked eye can detect.
So, In regards to sailor and your point, yes, magnification allows you to see in greater detail, and flooding your field of vision, physiologically, with more light, giving you a sense of much greater vision in apparent magnitude. But the image, though larger and clearer, is always below that naked vision threshold in absolute magnitude.
*I use absolute and apparent in looser terms here, absolute meaning an objective measure of the intensity of light, over a subjective, apparent one.
Sure. “Brightness” isn’t actually a well-defined scientific term. Instead we use things like “radiant intensity”. As a purely subjective and unscientific term, you’re free to use the word however you wish. But if we’re explaining why the OP can’t get a scope that increases brightness without increasing magnification, then we have to define brightness to be something very specific (i.e., radiant intensity, or watts/steradian, or the perceptual equivalent). And in that specific sense, we can say that no scope can increase brightness (though it can decrease it).
Ok, I’m with you. Thank you for taking the time explain this, it is a subtlety I had never appreciated in amateur astronomy.
The point is that, for telescopes below a certain aperture size, the exit pupil at typical magnifications (e.g. magnifications achievable with inexpensive eyepieces and suitable for typical objects) will be smaller than your dilated pupil. As you go up in telescope size, the exit pupil will get larger, and so the image will get brighter, until the exit pupil meets or exceeds your actual pupil size. At this point, the image brightness will be as close as possible to the perceived naked eye brightness, and increasing telescope size will not increase the image brightness any further, but you will be able to go up magnification by a certain amount without compromising apparent brightness either.
Very interesting.
What if I took a device with multiple lenses like a binoculars but focused the output of each lens in the same place? Would that increase the intensity of the output?
Yep; you’ve got it. Glad to help.
I did a bit of additional reading and it appears that even in astronomy circles, there is a bit of confusion about the nature of magnification and exit pupils, but I think most of it can again be traced to perceptual effects.
For instance, it is sometimes claimed that a small exit pupil is better under some conditions. The reason, as best I can tell, is that if the stars are already bright enough to saturate a given photoreceptor, then a larger exit pupil won’t improve them any. However, it will increase the brightness of the background, and the net result is a reduction in contrast.
There may well be more of these kinds of things that I’m not familiar with. It’s fairly clear what’s going on from a physics point of view, though.
The short answer is… you can’t do that. If you could, you could just build a bigger lens in the same place.
It’s a little hard to explain why it doesn’t work. I don’t know if you followed the discussion above about exit pupils, but the quick version is that the exit pupil is the diameter of the focused beam of light that goes into your eye. If that diameter is bigger than your actual pupil size, then the extra light is wasted: it just hits your iris or the white of your eye.
Building a multi-scope would necessarily mean increasing your aperture, which means increasing your exit pupil size. It’ll help if your exit pupil is too small, but not if it’s already sufficient.
This all seems to assume a simple two-lens eyepiece. There are eyepieces such as the naglers that have more than two lenses and provide more eye relief than a cheap plossl lens at the same magnification. If you can increase eye relief at a given magnification, why can’t you decrease it?
Though you’ve provided the best technical explanation by far, let me take a shot at a more fundamental reason why the laws of physics prevent it…
Optics work by taking the available amount of intrinsic light and by reflection or refraction, bend this light to suit one of two purposes: magnification or reduction.
Focal lengths and apertures aside, magnification will spread this light out across a large surface area, dimming the image in proportion to the magnification over surface area.
Reduction is the inverse of this. Smaller image, but more intense in brightness, since the intrinsic amount and intensity of light is crammed into a smaller area. This is why kids can fry ants with a magnifying glass, they’re actually focusing the intrinsic light of the sun to a teeny-tiny image on the ants body, and with light, comes heat…
That said, you can try and arrange lenses and mirrors in any arrangement you can possibly devise, but you’ll never get anything brighter than what you see intrinsically with the naked eye if there’s no bending and focusing of light involved into a smaller projection. Like mechanical advantage, you’re always trading off one aspect to enhance another.
Again, playing with words but the way it is stated I disagree. I have already explained it and I’ll say it again: a telescope or binoculars allow the observer to see objects which he could not see with the naked eye and this is because more light from the same source reaches the eye. This is a fact and I call this “brighter”.