APOD: Thor's Helmet (2024 Jan 09)

[Chris Peterson]

I'm saying that every (extended) object that is bright enough to see visually through a telescope eyepiece is bright enough to see with the naked eye. All a telescope can do is make it bigger, not brighter. Bigger can make it more visible because more of the retina is involved... but that is exactly what happens when you get closer, as well.

I really have to query this. The bigger the telescope aperture, the fainter the objects that you will be able to observe. An 8 inch diameter mirror will have almost twice the light gathering power as a 6 inch mirror, so if the same magnification is used on both then the image will be brighter with the 8 inch telescope.

Question: why not make a 1-meter diameter telescope with a 1-meter focal length, and use it with an eyepiece having a 1-meter focal length. That would have a magnification of one. It should let us view the sky as with our eyes, but hundreds of times brighter, right?

[johnnydeep]

And if we're being very technical, that system with the camera isn't even using a telescope in the optical sense. A telescope requires an objective and an ocular, and is classified as an afocal system. One that has no focus. A camera on a "telescope" is just an objective, which brings an image to focus on the camera sensor.

Hmm. So what would "using a telescope in the optical sense" mean? Does that only happen when you use your eye to look through the eyepiece? And if so, why? Just because your eye's lens is used to focus the incoming (already) parallel light rays?

Exactly. At its most formal, a telescope is an afocal optical system. Your eye is a focal optical system, and that's what brings the image to focus. Of course, in practical usage "telescope" has come to mean "big tube with lenses and/or mirrors used to examine astronomical objects... even if it's just a single objective. (And I'm not arguing against that usage, just pointing out an interesting technicality.)

Ok, thanks.

[johnnydeep]

I'm saying that every (extended) object that is bright enough to see visually through a telescope eyepiece is bright enough to see with the naked eye. All a telescope can do is make it bigger, not brighter. Bigger can make it more visible because more of the retina is involved... but that is exactly what happens when you get closer, as well.

I really have to query this. The bigger the telescope aperture, the fainter the objects that you will be able to observe. An 8 inch diameter mirror will have almost twice the light gathering power as a 6 inch mirror, so if the same magnification is used on both then the image will be brighter with the 8 inch telescope.

Question: why not make a 1-meter diameter telescope with a 1-meter focal length, and use it with an eyepiece having a 1-meter focal length. That would have a magnification of one. It should let us view the sky as with our eyes, but hundreds of times brighter, right?

So why do visual astronomers looking though an eyepiece prefer larger aperture telescopes? Are they not the proverbial "bigger light buckets"? Is this all about the difference between "diffuse extended light sources" and "point sources"?

[Chris Peterson]

I really have to query this. The bigger the telescope aperture, the fainter the objects that you will be able to observe. An 8 inch diameter mirror will have almost twice the light gathering power as a 6 inch mirror, so if the same magnification is used on both then the image will be brighter with the 8 inch telescope.

Question: why not make a 1-meter diameter telescope with a 1-meter focal length, and use it with an eyepiece having a 1-meter focal length. That would have a magnification of one. It should let us view the sky as with our eyes, but hundreds of times brighter, right?

So why do visual astronomers looking though an eyepiece prefer larger aperture telescopes? Are they not the proverbial "bigger light buckets"? Is this all about the difference between "diffuse extended light sources" and "point sources"?

As you increase the aperture, you can operate at higher magnifications, which generally let us see more detail even though the surface brightness is the same (or usually a little less) than we would have with our eyes alone. And for any given magnification, there is a point where increasing the aperture makes no difference at all.

[johnnydeep]

Question: why not make a 1-meter diameter telescope with a 1-meter focal length, and use it with an eyepiece having a 1-meter focal length. That would have a magnification of one. It should let us view the sky as with our eyes, but hundreds of times brighter, right?

So why do visual astronomers looking though an eyepiece prefer larger aperture telescopes? Are they not the proverbial "bigger light buckets"? Is this all about the difference between "diffuse extended light sources" and "point sources"?

As you increase the aperture, you can operate at higher magnifications, which generally let us see more detail even though the surface brightness is the same (or usually a little less) than we would have with our eyes alone. And for any given magnification, there is a point where increasing the aperture makes no difference at all.

Ok, let's see. Surface brightness would be the photons per second coming from every point of something being observed, whether extended or point (though I think you've said in the past that there are no true point sources). So clearly the photons/sec won't change depending on the aperture of the objective. That I understand. But, there are other subtleties in your explanation that aren't totally clear to me...yet.

[Chris Peterson]

So why do visual astronomers looking though an eyepiece prefer larger aperture telescopes? Are they not the proverbial "bigger light buckets"? Is this all about the difference between "diffuse extended light sources" and "point sources"?

As you increase the aperture, you can operate at higher magnifications, which generally let us see more detail even though the surface brightness is the same (or usually a little less) than we would have with our eyes alone. And for any given magnification, there is a point where increasing the aperture makes no difference at all.

Ok, let's see. Surface brightness would be the photons per second coming from every point of something being observed, whether extended or point (though I think you've said in the past that there are no true point sources). So clearly the photons/sec won't change depending on the aperture of the objective. That I understand. But, there are other subtleties in your explanation that aren't totally clear to me...yet.

Surface brightness is flux per unit area. What you're describing is just the flux. A larger aperture captures more photons. But as you increase the magnification, it delivers those photons to a larger area. So while more photons hit your retina, the number in any given area doesn't change. This is the same thing that happens when you get closer to an extended object, as well.

[johnnydeep]

As you increase the aperture, you can operate at higher magnifications, which generally let us see more detail even though the surface brightness is the same (or usually a little less) than we would have with our eyes alone. And for any given magnification, there is a point where increasing the aperture makes no difference at all.

Ok, let's see. Surface brightness would be the photons per second coming from every point of something being observed, whether extended or point (though I think you've said in the past that there are no true point sources). So clearly the photons/sec won't change depending on the aperture of the objective. That I understand. But, there are other subtleties in your explanation that aren't totally clear to me...yet.

Surface brightness is flux per unit area. What you're describing is just the flux. A larger aperture captures more photons. But as you increase the magnification, it delivers those photons to a larger area. So while more photons hit your retina, the number in any given area doesn't change. This is the same thing that happens when you get closer to an extended object, as well.

Which would seem to imply that a larger aperture using the same magnification would deliver more photons to your retina than a smaller aperture would, thereby making the image appear brighter, right? I must still be missing something.

[Chris Peterson]

Ok, let's see. Surface brightness would be the photons per second coming from every point of something being observed, whether extended or point (though I think you've said in the past that there are no true point sources). So clearly the photons/sec won't change depending on the aperture of the objective. That I understand. But, there are other subtleties in your explanation that aren't totally clear to me...yet.

Surface brightness is flux per unit area. What you're describing is just the flux. A larger aperture captures more photons. But as you increase the magnification, it delivers those photons to a larger area. So while more photons hit your retina, the number in any given area doesn't change. This is the same thing that happens when you get closer to an extended object, as well.

Which would seem to imply that a larger aperture using the same magnification would deliver more photons to your retina than a smaller aperture would, thereby making the image appear brighter, right? I must still be missing something.

More photons to the retina, yes. But not more photons to any fixed area of the retina (like a single rod, for instance). More photons spread over a larger area doesn't result in greater brightness.

We might consider the case of a 100mm aperture and operating at 20 power. Lets take a nice middle-aged pupil size of 5mm. So the area of the aperture is 400 times larger than the native pupil, so collects 400 times more light. But it's spreading the image over 400 times more retina, so the surface brightness is unchanged. The same as naked eye. What if we reduce the aperture to 50mm? Now we've reduced the surface brightness by a factor of four. The image in the scope is four times less bright than the naked eye view. And if we increase the aperture to 200mm? It doesn't get any brighter at all, because now the exit pupil is 10mm, and all that extra light is missing our eye's pupil completely. Trace it backwards, and the column of light entering our 5mm pupil is all coming from the central 100mm of that 200mm objective!

[johnnydeep]

Surface brightness is flux per unit area. What you're describing is just the flux. A larger aperture captures more photons. But as you increase the magnification, it delivers those photons to a larger area. So while more photons hit your retina, the number in any given area doesn't change. This is the same thing that happens when you get closer to an extended object, as well.

Which would seem to imply that a larger aperture using the same magnification would deliver more photons to your retina than a smaller aperture would, thereby making the image appear brighter, right? I must still be missing something.

More photons to the retina, yes. But not more photons to any fixed area of the retina (like a single rod, for instance). More photons spread over a larger area doesn't result in greater brightness.

We might consider the case of a 100mm aperture and operating at 20 power. Lets take a nice middle-aged pupil size of 5mm. So the area of the aperture is 400 times larger than the native pupil, so collects 400 times more light. But it's spreading the image over 400 times more retina, so the surface brightness is unchanged. The same as naked eye. What if we reduce the aperture to 50mm? Now we've reduced the surface brightness by a factor of four. The image in the scope is four times less bright than the naked eye view. And if we increase the aperture to 200mm? It doesn't get any brighter at all, because now the exit pupil is 10mm, and all that extra light is missing our eye's pupil completely. Trace it backwards, and the column of light entering our 5mm pupil is all coming from the central 100mm of that 200mm objective!

Thank you for your patience! Now I have to read that a few more times.

[JimB]

I'm saying that every (extended) object that is bright enough to see visually through a telescope eyepiece is bright enough to see with the naked eye. All a telescope can do is make it bigger, not brighter. Bigger can make it more visible because more of the retina is involved... but that is exactly what happens when you get closer, as well.

I really have to query this. The bigger the telescope aperture, the fainter the objects that you will be able to observe. An 8 inch diameter mirror will have almost twice the light gathering power as a 6 inch mirror, so if the same magnification is used on both then the image will be brighter with the 8 inch telescope.

Question: why not make a 1-meter diameter telescope with a 1-meter focal length, and use it with an eyepiece having a 1-meter focal length. That would have a magnification of one. It should let us view the sky as with our eyes, but hundreds of times brighter, right?

In theory, yes. But there are practical limitations. As the focal length is reduced, there are serious problems with focusing the light to avoid distortions. There are also limits on the ratio between the focal length of the eyepiece and the telescope mirror. Typical telescopes for viewing faint objects would have f/5, so a 1 meter telescope would have a 5 meter focal length and typical magnification of 10 times - so still much brighter that with the naked eye. If you are tempted to go to a higher magnification to get a closer look, then the brightness of the image will be reduced.

[Chris Peterson]

I really have to query this. The bigger the telescope aperture, the fainter the objects that you will be able to observe. An 8 inch diameter mirror will have almost twice the light gathering power as a 6 inch mirror, so if the same magnification is used on both then the image will be brighter with the 8 inch telescope.

Question: why not make a 1-meter diameter telescope with a 1-meter focal length, and use it with an eyepiece having a 1-meter focal length. That would have a magnification of one. It should let us view the sky as with our eyes, but hundreds of times brighter, right?

In theory, yes. But there are practical limitations. As the focal length is reduced, there are serious problems with focusing the light to avoid distortions. There are also limits on the ratio between the focal length of the eyepiece and the telescope mirror. Typical telescopes for viewing faint objects would have f/5, so a 1 meter telescope would have a 5 meter focal length and typical magnification of 10 times - so still much brighter that with the naked eye. If you are tempted to go to a higher magnification to get a closer look, then the brightness of the image will be reduced.

No. In theory no. It's not a practical problem, it's pure physics. You can never increase the brightness more than what you can see with the unaided eye. The limiting pupil in the system is the entrance pupil of your eye.