Starship Asterisk*

Milky Way over Easter Island

Explanation: Why were the statues on Easter Island built? No one is sure. What is sure is that over 900 large stone statues called moais exist there. The Rapa Nui (Easter Island) moais stand, on average, over twice as tall as a person and have over 200 times as much mass. It is thought that the unusual statues were created about 600 years ago in the images of local leaders of a vibrant and ancient civilization. Rapa Nui has been declared by UNESCO to a World Heritage Site. Pictured here, some of the stone giants were imaged last month under the central band of our Milky Way galaxy. Previously unknown moais are still being discovered.

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Forgive me for disregarding Easter Island and just looking at the Milky Way instead!


Here is some of the stuff we can see in the APOD:


Here is a closeup of the Rho Ophiuchi nebular complex:


Baade's Window is the relatively clear and dust-free region in Sagittarius where we can see deep into the bulge of the Milky Way, quite close to the galactic center. Baade's Window is yellow from the light of millions (and possibly billions) of mostly small yellow and red stars.


The Scutum Star Cloud is another relatively dust-free region where we see a part of the bulge of the Milky Way:


There is no ongoing star formation in the Scutum Star Cloud, but rich open cluster M11 is located near one of its dust lanes:


M24 is the only extended region in the Milky Way that is bright from young stars:


The most famous nebula in the Milky Way is almost certainly the Orion Nebula. The Carina Nebula, which is much larger, is also very famous. But the Lagoon Nebula, which can actually be seen in today's APOD, must certainly be one of the top three:


Back to the APOD. There are two things I'd like you to notice about the appearance of the Milky Way. First, that our galaxy appears to be "smoking", as smoke seems to rise from it and spread in all directions. This may be a real phenomenon, although I can't recall seeing it before.


The second thing I want you to notice is the entire right part of the thick broad central dust lane, where absolutely no star formation appears to be taking place. Indeed, star formation in the Milky Way is winding down, and we are a "green valley" galaxy, transitioning from a richly starforming "blue" galaxy into a "red and dead" one, where no (or extremely little) star formation is taking place. Between the "Blue Cloud" galaxies and the "Red Sequence" galaxies is the Green valley. We are in the valley!


Well, being in a green valley is not so bad. There are worse things.

Ann

The statues should be looking up! Impressive image...

Moai created as units of value ?
No one knows by whom, or where, or how built, or how they were used in exchange for food, or fot portable units of trade,
Or if there are more to be found. Or where, or how they were moved and emplaced. Just like Bitcoin!

"Still being discovered" eh? Meow.

Perhaps it is only my connection to APOD that gives a photograph of a cat when the link to "Still being discovered" at the bottom is accessed.

Jim Armstrong wrote: ↑Wed Nov 06, 2024 4:58 pm Perhaps it is only my connection to APOD that gives a photograph of a cat when the link to "Still being discovered" at the bottom is accessed.

It would not be an APOD without at least one caption link to a cat image.

Ann wrote: ↑Tue Nov 05, 2024 8:02 am
The second thing I want you to notice is the entire right part of the thick broad central dust lane, where absolutely no star formation appears to be taking place. Indeed, star formation in the Milky Way is winding down, and we are a "green valley" galaxy, transitioning from a richly starforming "blue" galaxy into a "red and dead" one, where no (or extremely little) star formation is taking place. Between the "Blue Cloud" galaxies and the "Red Sequence" galaxies is the Green valley. We are in the valley!



Well, being in a green valley is not so bad. There are worse things.

Ann

That sounds really sad! I didn't know our Milky Way was that inactive. I wonder if the Andromeda galaxy also is a dead galaxy just like ours.

It was an experience being there. Actually met the photographer at Easter Island when we were there the same week.

beryllium732 wrote: ↑Thu Nov 07, 2024 1:36 am

Ann wrote: ↑Tue Nov 05, 2024 8:02 am
The second thing I want you to notice is the entire right part of the thick broad central dust lane, where absolutely no star formation appears to be taking place. Indeed, star formation in the Milky Way is winding down, and we are a "green valley" galaxy, transitioning from a richly starforming "blue" galaxy into a "red and dead" one, where no (or extremely little) star formation is taking place. Between the "Blue Cloud" galaxies and the "Red Sequence" galaxies is the Green valley. We are in the valley!



Well, being in a green valley is not so bad. There are worse things.

Ann

That sounds really sad! I didn't know our Milky Way was that inactive. I wonder if the Andromeda galaxy also is a dead galaxy just like ours.

Neither Andromeda nor the Milky Way are "dead" galaxies, because both are indeed forming new stars. Our galaxy is forming about one new star per year. Andromeda, however, is forming stars at a slower rate than that.


I deliberately picked an Andromeda image where the young blue stars are almost invisible. But if you look at the far left part of Andromeda, you can just make out the brightest region of star formation in Andromeda, NGC 206.


Ann

Ann wrote: ↑Thu Nov 07, 2024 4:11 am

beryllium732 wrote: ↑Thu Nov 07, 2024 1:36 am

Ann wrote: ↑Tue Nov 05, 2024 8:02 am
The second thing I want you to notice is the entire right part of the thick broad central dust lane, where absolutely no star formation appears to be taking place. Indeed, star formation in the Milky Way is winding down, and we are a "green valley" galaxy, transitioning from a richly starforming "blue" galaxy into a "red and dead" one, where no (or extremely little) star formation is taking place. Between the "Blue Cloud" galaxies and the "Red Sequence" galaxies is the Green valley. We are in the valley!



Well, being in a green valley is not so bad. There are worse things.

Ann

That sounds really sad! I didn't know our Milky Way was that inactive. I wonder if the Andromeda galaxy also is a dead galaxy just like ours.

Neither Andromeda nor the Milky Way are "dead" galaxies, because both are indeed forming new stars. Our galaxy is forming about one new star per year. Andromeda, however, is forming stars at a slower rate than that.

Andromeda galaxy. Credit: Torben Hansen.

---

I deliberately picked an Andromeda image where the young blue stars are almost invisible. But if you look at the far left part of Andromeda, you can just make out the brightest region of star formation in Andromeda, NGC 206.

NGC 206 in Andromeda Torben Hansen wiki.png

NGC 206 in Andromeda.

Ann

That looks so surreal! You can barely see it. Is that what it looks like in a binocular and what our eyes can see? Would our galaxy look like a standard white lightbulb to our eyes? What spectrum would a starburst galaxy look in our eyes if it were a neighbour like Andromeda?

beryllium732 wrote: ↑Fri Nov 08, 2024 2:46 am

Ann wrote: ↑Thu Nov 07, 2024 4:11 am

beryllium732 wrote: ↑Thu Nov 07, 2024 1:36 am

That sounds really sad! I didn't know our Milky Way was that inactive. I wonder if the Andromeda galaxy also is a dead galaxy just like ours.

Neither Andromeda nor the Milky Way are "dead" galaxies, because both are indeed forming new stars. Our galaxy is forming about one new star per year. Andromeda, however, is forming stars at a slower rate than that.

Andromeda galaxy. Credit: Torben Hansen.

---

I deliberately picked an Andromeda image where the young blue stars are almost invisible. But if you look at the far left part of Andromeda, you can just make out the brightest region of star formation in Andromeda, NGC 206.

NGC 206 in Andromeda Torben Hansen wiki.png

NGC 206 in Andromeda.

Ann

That looks so surreal! You can barely see it. Is that what it looks like in a binocular and what our eyes can see? Would our galaxy look like a standard white lightbulb to our eyes? What spectrum would a starburst galaxy look in our eyes if it were a neighbour like Andromeda?

Indeed, that's what Andromeda would look like in binoculars and to our eyes, if we can see the disk at all, which is not certain. That depends on the darkness and the clarity of the sky and on the person's eyesight.

Chris Peterson discussed this phenomenon in this post, although he was discussing galaxies in general and not Andromeda or the Milky Way.

Chris posted two images of famous spiral galaxy M51, the Whirlpool galaxy. In today's pictures, the arms of M51 typically look very bright, but in reality, they are dim.


The faintness of the spiral arms of spiral galaxies is why it took a relatively long time for astronomers to discover spiral arms. It was Lord Rosse who had a tremendously huge (for its time) telescope, the Leviathan, built at Birr Castle in Ireland.


With his huge telescope, Lord Rosse was the first one to discover the spiral arms of M51, in 1845. This is Lord Rosse's drawing of M51:


So, given that the first telescopes were invented in 1608, it took more than 200 years for astronomers to discover the spiral arms of spiral galaxies. That should give you an idea of how faint the arms typically are.

Why are the spiral arms so faint? Well, take a look at the picture below which shows you both NGC 206 in Andromeda, and Andromeda's small and compact satellite galaxy, M32. We may note that NGC 206 and M32 are at very similar distances from us, and their diameters are very similar, so in many ways they are really comparable. Which object looks brighter to you, NGC 206 or M32?


Here's the deal. M32 contains only old stars, probably 95% of them are yellow or red, and the vast majority of them are faint. But, yes, a few of them - 1%? 0.1%? 0.01%? - are bright red giants, say, 10-300 times the luminosity of the Sun.


So, how many stars are there in M32? According to Hubblesite, there are roughly 400 million stars within a diameter of only 1,000 light-years in M32! That 's 400,000,000,000 stars. Therefore, if only one out of 400,000 stars is a red giant, there will still be 100,000 red giants within a volume with a diameter of only 1,000 light-years in M32. And a thousand light-years is roughly the distance from here to the bright stars in Orion. Imagine having 100,000 red giant stars between us and Orion. Imagine having 100,000 stars as bright as Arcturus (some 100 times brighter than the Sun) between us and Orion.


What about NGC 206? According to Wikipedia, there are more than 300 stars in NGC 206 that are brighter than M_b= —3.6. An absolute magnitude of —3.6 means a brightness similar to Polaris, whose absolute luminosity is some 2,400 solar luminosities. So there are more than 300 stars in NGC 206 that are brighter than Polaris.


I'm sure that there can't be more than a handful of stars in M32 that come close to the brilliance of Polaris. Actually, there may not be a single star in M32 that can rival Polaris. The brightness of the brightest stars in NGC 206 is no match for M32.

Yes, but how many stars are there in NGC 206? I haven't been able to find out. My own guess is that there may be as many as 100,000 stars there, the vast majority of them quite faint. Yes, but if so, 100,000 stars is no match for 400 million stars. Remember that in M32, there might be 100,000 bright stars, and you can bet your boots that there are not 100,000 bright stars in NGC 206.

And that's why centers of galaxies are bright and spiral arms are faint.

Ann

Ann wrote:So, how many stars are there in M32? According to Hubblesite, there are roughly 400 million stars within a diameter of only 1,000 light-years in M32! That 's 400,000,000,000 stars. Therefore, if only one out of 400,000 stars is a red giant, there will still be 100,000 1000 red giants within a volume with a diameter of only 1,000 light-years in M32. And a thousand light-years is roughly the distance from here to the bright stars in Orion. Imagine having 100,000 1000 red giant stars between us and Orion. Imagine having 100,000 1000 stars as bright as Arcturus (some 100 times brighter than the Sun) between us and Orion.

You're off by 1000 or so.

johnnydeep wrote: ↑Fri Nov 08, 2024 1:22 pm

Ann wrote:So, how many stars are there in M32? According to Hubblesite, there are roughly 400 million stars within a diameter of only 1,000 light-years in M32! That 's 400,000,000,000 stars. Therefore, if only one out of 400,000 stars is a red giant, there will still be 100,000 1000 red giants within a volume with a diameter of only 1,000 light-years in M32. And a thousand light-years is roughly the distance from here to the bright stars in Orion. Imagine having 100,000 1000 red giant stars between us and Orion. Imagine having 100,000 1000 stars as bright as Arcturus (some 100 times brighter than the Sun) between us and Orion.

You're off by 1000 or so.

Oops! Yes, it looks like it! Thanks, Johnny!

Ann

Ann wrote: ↑Fri Nov 08, 2024 8:06 am So, given that the first telescopes were invented in 1608, it took more than 200 years for astronomers to discover the spiral arms of spiral galaxies. That should give you an idea of how faint the arms typically are.

It's worth mentioning (again) that telescopes don't make anything brighter, only bigger. A perfect telescope produces an image on the back of the eye with the same brightness as the eye alone receives. In actual life, telescopes aren't perfect, and the image is always somewhat less bright. But we don't necessarily need brighter, what we need is more information, and if you can make an image bigger on the retina, our brains have more to work with, and we may see more, even without anything being brighter. But you can only increase the magnification of a telescope to a certain point before the image becomes dimmer, and that point is determined by the aperture of the telescope. Large apertures demanded technological changes that took time after telescopes were first invented. Technology in making large accurate mirrors, and technology in grinding lenses bigger than just a few centimeters. That's what allowed for high magnifications without a reduction in brightness, and the first visual observations of subtle detail in astronomical objects.

Chris Peterson wrote: ↑Fri Nov 08, 2024 4:20 pm

Ann wrote: ↑Fri Nov 08, 2024 8:06 am So, given that the first telescopes were invented in 1608, it took more than 200 years for astronomers to discover the spiral arms of spiral galaxies. That should give you an idea of how faint the arms typically are.

It's worth mentioning (again) that telescopes don't make anything brighter, only bigger. A perfect telescope produces an image on the back of the eye with the same brightness as the eye alone receives. In actual life, telescopes aren't perfect, and the image is always somewhat less bright. But we don't necessarily need brighter, what we need is more information, and if you can make an image bigger on the retina, our brains have more to work with, and we may see more, even without anything being brighter. But you can only increase the magnification of a telescope to a certain point before the image becomes dimmer, and that point is determined by the aperture of the telescope. Large apertures demanded technological changes that took time after telescopes were first invented. Technology in making large accurate mirrors, and technology in grinding lenses bigger than just a few centimeters. That's what allowed for high magnifications without a reduction in brightness, and the first visual observations of subtle detail in astronomical objects.

I fear I will never understand this point that you're always making.

johnnydeep wrote: ↑Fri Nov 08, 2024 5:26 pm

Chris Peterson wrote: ↑Fri Nov 08, 2024 4:20 pm

Ann wrote: ↑Fri Nov 08, 2024 8:06 am So, given that the first telescopes were invented in 1608, it took more than 200 years for astronomers to discover the spiral arms of spiral galaxies. That should give you an idea of how faint the arms typically are.

It's worth mentioning (again) that telescopes don't make anything brighter, only bigger. A perfect telescope produces an image on the back of the eye with the same brightness as the eye alone receives. In actual life, telescopes aren't perfect, and the image is always somewhat less bright. But we don't necessarily need brighter, what we need is more information, and if you can make an image bigger on the retina, our brains have more to work with, and we may see more, even without anything being brighter. But you can only increase the magnification of a telescope to a certain point before the image becomes dimmer, and that point is determined by the aperture of the telescope. Large apertures demanded technological changes that took time after telescopes were first invented. Technology in making large accurate mirrors, and technology in grinding lenses bigger than just a few centimeters. That's what allowed for high magnifications without a reduction in brightness, and the first visual observations of subtle detail in astronomical objects.

I fear I will never understand this point that you're always making.

Maybe it would help if you considered a telescope with a one meter aperture, and an objective and an eyepiece with the same focal length. That is, a one meter aperture and a magnification of one. Can you see why that won't produce an image brighter than your eye alone?

Chris Peterson wrote: ↑Fri Nov 08, 2024 5:28 pm

johnnydeep wrote: ↑Fri Nov 08, 2024 5:26 pm

Chris Peterson wrote: ↑Fri Nov 08, 2024 4:20 pm

It's worth mentioning (again) that telescopes don't make anything brighter, only bigger. A perfect telescope produces an image on the back of the eye with the same brightness as the eye alone receives. In actual life, telescopes aren't perfect, and the image is always somewhat less bright. But we don't necessarily need brighter, what we need is more information, and if you can make an image bigger on the retina, our brains have more to work with, and we may see more, even without anything being brighter. But you can only increase the magnification of a telescope to a certain point before the image becomes dimmer, and that point is determined by the aperture of the telescope. Large apertures demanded technological changes that took time after telescopes were first invented. Technology in making large accurate mirrors, and technology in grinding lenses bigger than just a few centimeters. That's what allowed for high magnifications without a reduction in brightness, and the first visual observations of subtle detail in astronomical objects.

I fear I will never understand this point that you're always making.

Maybe it would help if you considered a telescope with a one meter aperture, and an objective and an eyepiece with the same focal length. That is, a one meter aperture and a magnification of one. Can you see why that won't produce an image brighter than your eye alone?

Well, yes, I suppose if you put your face in the path of the eyepiece your eyes would get the same number of photons per unit area as if there were no telescope at all. But I think of telescopes with magnification > 1 as concentrating the photons into a smaller area, thus increasing brightness. Is that not correct?

johnnydeep wrote: ↑Fri Nov 08, 2024 5:46 pm

Chris Peterson wrote: ↑Fri Nov 08, 2024 5:28 pm

johnnydeep wrote: ↑Fri Nov 08, 2024 5:26 pm

I fear I will never understand this point that you're always making.

Maybe it would help if you considered a telescope with a one meter aperture, and an objective and an eyepiece with the same focal length. That is, a one meter aperture and a magnification of one. Can you see why that won't produce an image brighter than your eye alone?

Well, yes, I suppose if you put your face in the path of the eyepiece your eyes would get the same number of photons per unit area as if there were no telescope at all. But I think of telescopes with magnification > 1 as concentrating the photons into a smaller area, thus increasing brightness. Is that not correct?

Ok, now make your 1-m telescope have a magnification of two. Propagate your eye's pupil backwards through that system, and you'll see that the only light entering your eye is from a central disk on the objective that is twice the diameter of your pupil. Does that make sense? So you are collecting four times more light (woo hoo!) and distributing it across four times the area on your retina (oops!). The size is doubled, but the surface brightness is exactly the same. The photon flux encountered by a rod cell in your retina is exactly the same.

Chris Peterson wrote: ↑Fri Nov 08, 2024 5:52 pm

johnnydeep wrote: ↑Fri Nov 08, 2024 5:46 pm

Chris Peterson wrote: ↑Fri Nov 08, 2024 5:28 pm

Maybe it would help if you considered a telescope with a one meter aperture, and an objective and an eyepiece with the same focal length. That is, a one meter aperture and a magnification of one. Can you see why that won't produce an image brighter than your eye alone?

Well, yes, I suppose if you put your face in the path of the eyepiece your eyes would get the same number of photons per unit area as if there were no telescope at all. But I think of telescopes with magnification > 1 as concentrating the photons into a smaller area, thus increasing brightness. Is that not correct?

Ok, now make your 1-m telescope have a magnification of two. Propagate your eye's pupil backwards through that system, and you'll see that the only light entering your eye is from a central disk on the objective that is twice the diameter of your pupil. Does that make sense? So you are collecting four times more light (woo hoo!) and distributing it across four times the area on your retina (oops!). The size is doubled, but the surface brightness is exactly the same. The photon flux encountered by a rod cell in your retina is exactly the same.

How does that pertain to this image, which sure looks like it is concentrating more photons onto a lesser area:

johnnydeep wrote: ↑Fri Nov 08, 2024 7:05 pm

Chris Peterson wrote: ↑Fri Nov 08, 2024 5:52 pm

johnnydeep wrote: ↑Fri Nov 08, 2024 5:46 pm

Well, yes, I suppose if you put your face in the path of the eyepiece your eyes would get the same number of photons per unit area as if there were no telescope at all. But I think of telescopes with magnification > 1 as concentrating the photons into a smaller area, thus increasing brightness. Is that not correct?

Ok, now make your 1-m telescope have a magnification of two. Propagate your eye's pupil backwards through that system, and you'll see that the only light entering your eye is from a central disk on the objective that is twice the diameter of your pupil. Does that make sense? So you are collecting four times more light (woo hoo!) and distributing it across four times the area on your retina (oops!). The size is doubled, but the surface brightness is exactly the same. The photon flux encountered by a rod cell in your retina is exactly the same.

How does that pertain to this image, which sure looks like it is concentrating more photons onto a lesser area:

Click to view full size image

Yes... but how much of that light is making it into the eye? All of the light that is outside the diameter of your pupil is lost. That's why you have to project the size of your pupil backwards through the system. Doing that you'll see that the effective aperture is not the physical aperture, but the magnification times your pupil size. Which can be much smaller. And more light spread out over more area doesn't mean anything got brighter.

There's a reason that you can look at the Moon through a giant telescope and not incur damage. It's because that telescope doesn't increase the brightness of the Moon at all. It can't.

Chris Peterson wrote: ↑Fri Nov 08, 2024 7:12 pm

johnnydeep wrote: ↑Fri Nov 08, 2024 7:05 pm

Chris Peterson wrote: ↑Fri Nov 08, 2024 5:52 pm

Ok, now make your 1-m telescope have a magnification of two. Propagate your eye's pupil backwards through that system, and you'll see that the only light entering your eye is from a central disk on the objective that is twice the diameter of your pupil. Does that make sense? So you are collecting four times more light (woo hoo!) and distributing it across four times the area on your retina (oops!). The size is doubled, but the surface brightness is exactly the same. The photon flux encountered by a rod cell in your retina is exactly the same.

How does that pertain to this image, which sure looks like it is concentrating more photons onto a lesser area:

Click to view full size image

Yes... but how much of that light is making it into the eye? All of the light that is outside the diameter of your pupil is lost. That's why you have to project the size of your pupil backwards through the system. Doing that you'll see that the effective aperture is not the physical aperture, but the magnification times your pupil size. Which can be much smaller. And more light spread out over more area doesn't mean anything got brighter.

There's a reason that you can look at the Moon through a giant telescope and not incur damage. It's because that telescope doesn't increase the brightness of the Moon at all. It can't.

Well, what if the eyepiece diameter is the same size as the pupil? Isn't it receiving more photons per area than it would without the optics in place?

johnnydeep wrote: ↑Fri Nov 08, 2024 7:40 pm

Chris Peterson wrote: ↑Fri Nov 08, 2024 7:12 pm

johnnydeep wrote: ↑Fri Nov 08, 2024 7:05 pm

How does that pertain to this image, which sure looks like it is concentrating more photons onto a lesser area:

Click to view full size image

Yes... but how much of that light is making it into the eye? All of the light that is outside the diameter of your pupil is lost. That's why you have to project the size of your pupil backwards through the system. Doing that you'll see that the effective aperture is not the physical aperture, but the magnification times your pupil size. Which can be much smaller. And more light spread out over more area doesn't mean anything got brighter.

There's a reason that you can look at the Moon through a giant telescope and not incur damage. It's because that telescope doesn't increase the brightness of the Moon at all. It can't.

Well, what if the eyepiece diameter is the same size as the pupil? Isn't it receiving more photons per area than it would without the optics in place?


telescope and eye.jpg

You certainly collect more photons. But you distribute them over a larger area. That's why there's no change in brightness.

Say you have a dark adapted pupil size of 6mm. With a scope operating at 100X, you'd need an objective of 600mm to achieve the same surface brightness on your retina that your eye would have without the scope. 10,000 times more photons, but spread out over 10,000 time more area. Same photon flux per unit area on your retina. Anything less than that aperture and the brightness will be less bright. Anything more won't change the brightness beyond that of the 600mm objective because the light at the edges will be outside your pupil.

Chris Peterson wrote: ↑Fri Nov 08, 2024 7:49 pm

johnnydeep wrote: ↑Fri Nov 08, 2024 7:40 pm

Chris Peterson wrote: ↑Fri Nov 08, 2024 7:12 pm
Yes... but how much of that light is making it into the eye? All of the light that is outside the diameter of your pupil is lost. That's why you have to project the size of your pupil backwards through the system. Doing that you'll see that the effective aperture is not the physical aperture, but the magnification times your pupil size. Which can be much smaller. And more light spread out over more area doesn't mean anything got brighter.

There's a reason that you can look at the Moon through a giant telescope and not incur damage. It's because that telescope doesn't increase the brightness of the Moon at all. It can't.

Well, what if the eyepiece diameter is the same size as the pupil? Isn't it receiving more photons per area than it would without the optics in place?


telescope and eye.jpg

You certainly collect more photons. But you distribute them over a larger area. That's why there's no change in brightness.

Say you have a dark adapted pupil size of 6mm. With a scope operating at 100X, you'd need an objective of 600mm to achieve the same surface brightness on your retina that your eye would have without the scope. 10,000 times more photons, but spread out over 10,000 time more area. Same photon flux per unit area on your retina. Anything less than that aperture and the brightness will be less bright. Anything more won't change the brightness beyond that of the 600mm objective because the light at the edges will be outside your pupil.

Still making no sense to me. If I put my pupil at the focal point of the objective lens in the picture, and we're pointed at the Sun, my eye would be severely burned (as would a piece of paper placed there). Right? So why isn't that intensity merely a bit reduced by the expansion of the rays on the way from that focal point to the eyepiece any my eye?

johnnydeep wrote: ↑Fri Nov 08, 2024 9:30 pm

Chris Peterson wrote: ↑Fri Nov 08, 2024 7:49 pm

johnnydeep wrote: ↑Fri Nov 08, 2024 7:40 pm

Well, what if the eyepiece diameter is the same size as the pupil? Isn't it receiving more photons per area than it would without the optics in place?


telescope and eye.jpg

You certainly collect more photons. But you distribute them over a larger area. That's why there's no change in brightness.

Say you have a dark adapted pupil size of 6mm. With a scope operating at 100X, you'd need an objective of 600mm to achieve the same surface brightness on your retina that your eye would have without the scope. 10,000 times more photons, but spread out over 10,000 time more area. Same photon flux per unit area on your retina. Anything less than that aperture and the brightness will be less bright. Anything more won't change the brightness beyond that of the 600mm objective because the light at the edges will be outside your pupil.

Still making no sense to me. If I put my pupil at the focal point of the objective lens in the picture, and we're pointed at the Sun, my eye would be severely burned (as would a piece of paper placed there). Right? So why isn't that intensity merely a bit reduced by the expansion of the rays on the way from that focal point to the eyepiece any my eye?

An objective lens by itself is not a telescope. A single objective is a focal optical system. A telescope is an afocal optical system. The only reason a telescope forms an image is because the lens of your eye is a focal optical system.

Again, work backwards from your retina. Can you figure out any way that you can get more photons per unit area on your retina with a telescope?