[HN Gopher] Depth of Field
___________________________________________________________________
Depth of Field
Author : moultano
Score : 80 points
Date : 2022-03-27 11:51 UTC (11 hours ago)
(HTM) web link (moultano.wordpress.com)
(TXT) w3m dump (moultano.wordpress.com)
| bobcostas55 wrote:
| What causes that particular starburst pattern in the James Webb
| telescope image?
| zasdffaa wrote:
| Search 'diffraction spikes' IIRC
| Aengeuad wrote:
| The struts of the secondary mirror cause diffraction spikes on
| all 18 segments which combine to give the final pattern seen,
| the horizontal spikes are caused specifically by the top strut
| in particular. This video mostly covers how JWST was focused
| but from 01:15-04:00 it has an excellent of how the pattern is
| formed.
|
| https://www.youtube.com/watch?v=cWXTy_GeCis
| andybak wrote:
| If anyone missed it, the last paragraph is of interest in its own
| right.
| gumby wrote:
| > The oldest of the photons that ended their life in the
| electronics of the Hubble had traveled from their birth star
| across the universe for 13.2 billion years. These photons had
| already completed half of their journey when the earth coalesced.
|
| Somehow this made me think of playing a video game in which
| you're trying to steer between a bunch of obstacles and one
| magically grows right in your path.
| amelius wrote:
| What does "understanding" mean in this context?
| InCityDreams wrote:
| Being able to visualise, and thereby.....comprehend?
| amelius wrote:
| But "comprehend" and "understand" mean the same thing.
| dylan604 wrote:
| eh, maybe? I understand that a quadrillion dollars is a
| large number, but I don't comprehend exactly how large of a
| pile of cash that would be (assuming Benjamins and not
| Jacksons).
| macintux wrote:
| I think, at least in common vernacular, they are indeed
| different when placed side by side, much like "listen" vs
| "hear".
| dylan604 wrote:
| or "hear" and "comprehend" being the same.
|
| Take the phrase "they don't hear that well". Does that
| mean they cannot actualy hear based on volumes and noise
| or some sort of hearing problem? Or does that mean they
| can "hear" just fine but don't comprehend what they are
| being told? Or that they do comprehend it just fine, but
| just don't like it being told to them?
|
| Now we're getting into idioms territory and less about
| definition of words and why have two words with the same,
| but different, meanings.
| daniel-cussen wrote:
| When people actually talk about quadrillion dollars, I no
| longer listen, I only hear.
| dylan604 wrote:
| I hear that's common in the world of the peasants. Those
| of use in the 5 Commas club enjoy those reminders, as we
| try to comprehend what it would be like to have so
| little. It's beyond our understanding though
| InCityDreams wrote:
| And that was my point. I understand those numbers better as
| i have a better visualisation of them. We are going to a
| beach tomorrow. There are trees behind us. My kid, and his
| friends, have a better idea of the stars than ever before -
| they'll be even more impressed at the beach.
| dylan604 wrote:
| I always liked the idea of "Look at all of the land you
| can see around you. That's just a fraction of the land.
| Of all of the land on Earth, it is just a fraction of the
| surface. The rest is water." From a kid's perspective,
| water just got huge.
| omnicognate wrote:
| It means distinguishing intuitively between these particular
| large numbers and other large numbers.
|
| Once you get into the trillions, quadrillions, etc., it can be
| tricky to get a sense of the scale vs other large numbers that
| may sound similarly impressive but differ by orders of
| magnitude.
|
| These examples show that a quadrillion is _smaller_ than I
| would have imagined without thinking about it, and I found they
| helped to get an intuitive sense of the scale.
| thriftwy wrote:
| Just a century ago, people thought there's infinitely many stars.
| Any real number of stars pales in comparison. It's not hard to
| think of a criteria where only a handful of stars in observable
| universe will fit. Or even none at all (but non-zero chance of).
| whoisburbansky wrote:
| Pretty nifty how the example images of things like the sand
| grains (but make it Deep-Field-y) were made. I was hoping for an
| explanation, and the author didn't disappoint.
| moultano wrote:
| If you're interested in AI art, I've made a few more things
| built out of a series of generated images around a theme.
|
| https://moultano.wordpress.com/2021/08/23/doorways/
|
| https://moultano.wordpress.com/2021/07/20/tour-of-the-sacred...
| copperx wrote:
| I need this in my life; it would be great to create engaging
| lectures. Is there anything like MidJourney but public?
| moultano wrote:
| Nothing super user friendly. Most of the things people make
| right now are via various Google Colab notebooks.
|
| https://twitter.com/rivershavewings/status/1427580354651586
| 5...
|
| https://colab.research.google.com/drive/1go6YwMFe5MX6XM9tv-
| c...
| refulgentis wrote:
| GP, the links above are far out of date, you want
| reddit.com/r/discodiffusion and the Disco Diffusion v4.1
| (note: I'm specifically saying 4.1 instead of 5.0,
| focused more on animation and trades off quality for
| speed of frame generation)
|
| They're very, very, easy to use if you have any
| familiarity with coding. The hard part is patience.
|
| Midjourney is (hand-waving) a Discord UI on top of that.
| Again, hand-waving, there's some secret sauce Midjourney
| does that makes it more likely to recognize your prompt.
| I assume they swapped out a model somewhere with one they
| trained on a wider variety of images
| alecbz wrote:
| What exactly makes the light from these distant galaxies so
| "weak" that we need to point the telescope there for months to
| see it?
|
| Like, isn't there a constant stream of photons? How does just
| waiting longer for them make them show up if they wouldn't have
| otherwise? Are things blocking most of the photons? Things from
| within the galaxies they're coming from, or things in between the
| galaxy and the telescope?
|
| edit: Ah, I suppose the further an object is the fewer photons
| from it are making it to the telescope, and so we need to wait
| longer for enough photons to arrive to be able to distinguish it
| from the random background noise on our end?
| hexatin wrote:
| The strength of light over a distance obeys an inverse square
| law[1], causing it to effectively lose power over long
| distances, since the same amount of energy is being spread over
| larger and larger spheres as it radiates out. For photons, I
| think this manifests as there being a lower rate of photons
| occurring in each section of the sphere since they're "spread
| out".
|
| [1] https://en.wikipedia.org/wiki/Inverse-square_law
| site-packages1 wrote:
| Doesn't this fail because we're looking at lots of stars
| spread out evenly, and so the farther you get away, the more
| stars fill the space?
| jmopp wrote:
| This is Olbers' Paradox[1] and is one piece of evidence
| that the universe isn't static.
|
| 1 https://en.wikipedia.org/wiki/Olbers%27_paradox
| hypertele-Xii wrote:
| Space is mostly empty.
| mananaysiempre wrote:
| As a sibling comment to yours says, Olbers' paradox is
| that it doesn't matter.
|
| _If_ we assume that the universe is homogeneous,
| infinite, and eternal and the density of stars (possibly
| averaged over a large but constant scale) is constant,
| then in any direction on the sky there is some (possibly
| very faraway) star, occupying a finite (possibly
| minuscule) solid angle in our field of view. The 1 /r2
| falloff (aka conservation of energy) means that the
| energy received per unit of solid angle is independent of
| distance from the emitter, equal for example to that on
| its surface, so every piece of the sky containing a star
| means we should see stellar-surface amounts of energy
| shining upon us from _every_ direction.
|
| Assuming some sort of absorbing dust would obscure the
| stars doesn't help: if the universe truly is eternal,
| every dust cloud, being unable to store arbitrarily large
| amounts of energy, will eventually heat up to the point
| that it radiates as much as it absorbs and so is as hot
| as the stars which it obscures (this is the insight
| behind Kirchhoff's law and the existence of blackbody
| radiation).
|
| What does help is either implementing "no point in space
| is special" through a device other than simply a constant
| stellar density ( _e.g._ having stars distributed on a
| self-similar fractal of Hausdorff dimension < 3, our
| falloff argument having included what amounts to a
| definition of Hausdorff dimension) or abandoning "no
| point in _time_ is special" (giving the universe a finite
| age or at least having no stars in the far past).
| Observations show the second possibility (named the "Big
| Bang" by its critics in what was meant to show its
| ridiculousness) to be true.
|
| (Modern cosmology has much more direct arguments for a
| finite age of the universe, but they also require more
| advanced physics and/or observational technology, so the
| directness is in the eye of the beholder.)
| setr wrote:
| It still depends on their contribution right? If you said
| all lights are equal in strength, then being on top of one
| gives a strength of 1. Being x units away from it gives a
| some value less than 1, based on the sqrt of distance
|
| Being close & between two lights would take the sum of
| their strengths, say .75 each so 1.5, giving a value
| greater than 1 (more light than either individually
| produces)
|
| Being far from the two lights, each contributes .25
| strength, so .5 total -- half the light of standing on top
| of one
|
| And of course if you get really far, a ridiculously large
| number of light sources are contributing a ridiculously
| small amount of light, which may still sum to something
| fairly small
|
| I'm imagining it works like an influence map:
| https://www.gamedev.net/tutorials/programming/artificial-
| int...
| photoGrant wrote:
| Yes, it's called NOISE!
| alecbz wrote:
| But what specifically is it? Just random stuff in deep space
| between galaxies? I'd have thought most of the space between
| galaxies is mostly truly empty.
| site-packages1 wrote:
| Isn't the paradox that if the universe is static in size or
| shrinking, eventually all the light would fill the sky into
| a blinding display, rather than dark. So it's a function of
| the universe expanding.
|
| https://en.wikipedia.org/wiki/Olbers%27_paradox
| nprecup wrote:
| The noise is inherent to our sensor technology... Very
| sensitive telescopes are cryogenically cooled to reduce the
| temperature of the focal plane, reducing the noise. As in
| an above comment, the light from distant objects is more
| spread out and appears weaker - fewer photos are arriving
| each second and hitting the focal plane than if the
| telescope were closer. At a certain point, the signal gets
| buried in the noise.
|
| I worked on the BICEP Array telescope at the south Pole -
| the light we are observing is actually extremely low energy
| and we can't see it unless our detectors are colder than
| the light source we are looking at (which is about 2.7K).
| We cool our detectors to 0.3K!
| daniel-cussen wrote:
| Does the natural cold at the South Pole make it much
| easier to cool the instruments down that low? I know it's
| around 200K at its lowest, how does that make it easier
| and cheaper?
| nprecup wrote:
| Yes, there are several factors that make that location
| particularly good, not only temperature. The cold and
| absence of sunlight for months at a time causes a
| relative absence of water vapor in the atmosphere during
| winter, which is extremely important for CMB
| observations. The high elevation (~10k ft) means there is
| less atmosphere to look through as well. During summers
| the sky heats up, contains more water vapor, and reflects
| so much more light that our telescopes become
| substantially less effective. Another advantage to the
| South Pole is the ability to continuously observe the
| same part of the sky, as the telescope is located on the
| earths axis there is no rising and setting of sky
| overhead. There is also a particularly 'dark' patch of
| sky in the southern hemisphere called the 'southern
| hole'. This is located up and away from the plane of our
| galaxy and contains very few objects standing in the
| 'path' of the CMB which also helps obtain better quality
| observations of the CMB.
|
| Edit - it is as good as it gets on earth and way cheaper
| than a satellite! Other benefits are rapid upgrades and
| repairs with newer technology. A satellite, by the time
| it is deployed, is already pretty old! But the South Pole
| still isn't cheap!
| pjc50 wrote:
| The noise is coming from inside the sensor:
| https://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise
|
| Cryogenically cooling helps, but since you can't hit
| absolute zero you can't entirely eliminate it.
| alecbz wrote:
| Right, what I was missing was why how far the source of
| the light is effects how much the noise matters, but the
| inverse square photon concentration explains that.
| passivate wrote:
| There is read noise from downstream electronics
| converting photons to electrons, but the "shot noise" is
| a property of the photon stream and will be present
| regardless of how much we improve the electronics.
| Cooling the detector won't help there.
|
| https://en.wikipedia.org/wiki/Shot_noise#Optics
| pgorczak wrote:
| As a sibling already mentioned, thermal noise occurs in the
| sensors. And there is also a range of cosmic background
| radiation coming in, including the cosmic microwave
| background which is just everywhere and is almost as old as
| the universe.
| frereubu wrote:
| I often think about these kinds of numbers when talking with my
| father-in-law, who's a marine geologist, and who regularly talks
| about time in millions of years. I wonder if he has a real grasp
| of the scale of what that means, in the same way that I might
| understand hours / days / months / years / decades. The only way
| he can explain it to me is by metaphor, which is what this post
| does. It's sort-of-effective, but what it mainly does it prove to
| me how little I comprehend numbers like that. I find even
| centuries are a little difficult to wrap my head around.
| hilbert42 wrote:
| I don't think we should try, when scale is this large we should
| wonder and marvel in the numinous of this existence.
| omnicognate wrote:
| Why not both?
| mywacaday wrote:
| Deep field was pointed at an "empty" piece of space, what would
| the number be in the most populous piece of space?
| issa wrote:
| Space at the scale being discussed here is uniform, so no one
| direction should be "more populous" than another. The "empty"
| here designates an area without stars easily visible from
| Earth. Our local area is denser in our galactic plane, in the
| direction of our supercluster, etc. But 13 billion lightyears
| in ANY direction should look about the same.
| supernova87a wrote:
| One of the key findings of astronomy / cosmology of the modern
| age is that the universe _is_ exactly so uniform in all
| directions, at the largest scales. In a way this is support for
| the notion of the big bang itself. There are density
| differences at some distance scales which have to do with
| clumping of matter (galaxies) as it began forming, but those
| differences are not a notion of "the universe being different
| depending on where you look".
|
| Funny how every subdivision / suburb looks kind of the same in
| the shape of the roads, density of houses. It is the nature of
| the laws that govern how these things get built.
|
| Btw, you could see a lot more _stars_ (not galaxies) if you
| pointed the telescope in certain directions, but likely that
| would be because you 're just looking at the direction of our
| own galaxy (which we're at the relative edge of), instead of
| looking out of our galaxy to see the rest of the universe,
| which is made up of the kinds of places shown in these images.
|
| These images are chosen to be taken in a direction far from our
| galaxy's disc (thing of us as sitting on the edge of a
| frisbee), because if you aimed at the disc, your image would be
| full of the giant overwhelming circles with spikes rather than
| tiny grains of sand, each of which is a galaxy far far away.
___________________________________________________________________
(page generated 2022-03-27 23:01 UTC)