APOD: HESS Telescopes Explore the Sky... (2019 Jan 08)

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APOD: HESS Telescopes Explore the Sky... (2019 Jan 08)

Post by APOD Robot » Tue Jan 08, 2019 5:14 am

Image HESS Telescopes Explore the High-Energy Sky

Explanation: They may look like modern mechanical dinosaurs but they are enormous swiveling eyes that watch the sky. The High Energy Stereoscopic System (H.E.S.S.) Observatory is composed of four 12-meter reflecting-mirror telescopes surrounding a larger telescope housing a 28-meter mirror. They are designed to detect strange flickers of blue light -- Cherenkov radiation --emitted when charged particles move slightly faster than the speed of light in air. This light is emitted when a gamma ray from a distant source strikes a molecule in Earth's atmosphere and starts a charged-particle shower. H.E.S.S. is sensitive to some of the highest energy photons (TeV) crossing the universe. Operating since 2003 in Namibia, H.E.S.S. has searched for dark matter and has discovered over 50 sources emitting high energy radiation including supernova remnants and the centers of galaxies that contain supermassive black holes. Pictured last September, H.E.S.S. telescopes swivel and stare in time-lapse sequences shot in front of our Milky Way Galaxy and the Magellanic Clouds -- as the occasional Earth-orbiting satellite zips by.

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Re: APOD: HESS Telescopes Explore the Sky... (2019 Jan 08)

Post by Boomer12k » Tue Jan 08, 2019 5:35 am

Awesome....love it... seeing the stars and even galaxies reflect off the mirror is incredible... WOW...

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Re: APOD: HESS Telescopes Explore the Sky... (2019 Jan 08)

Post by BDanielMayfield » Tue Jan 08, 2019 12:23 pm

Since blue Cherenkov radiation is in the visable part of the spectrum is it possible for a person to actually see one of these blue flashes in the night sky, or is the flash far too short (as I would assume)?

How long must a visual event be for us to notice it?
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Vitreous humour

Post by neufer » Tue Jan 08, 2019 1:27 pm

BDanielMayfield wrote:
Tue Jan 08, 2019 12:23 pm

Since blue Cherenkov radiation is in the visable part of the spectrum is it possible for a person to actually see one of these blue flashes in the night sky, or is the flash far too short (as I would assume)?
No. Cherenkov light produced by isolated individual cosmic rays in the upper atmosphere is "very dim" requiring dark skies, huge multiple mirrors and rapid electronics. However, astronauts on the ISS can see the Cherenkov radiation caused by isolated individual cosmic rays hitting their eyeballs: https://en.wikipedia.org/wiki/Cosmic_ra ... _phenomena
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Re: Vitreous humour

Post by BDanielMayfield » Tue Jan 08, 2019 1:47 pm

neufer wrote:
Tue Jan 08, 2019 1:27 pm
BDanielMayfield wrote:
Tue Jan 08, 2019 12:23 pm

Since blue Cherenkov radiation is in the visable part of the spectrum is it possible for a person to actually see one of these blue flashes in the night sky, or is the flash far too short (as I would assume)?
No. Cherenkov light produced by isolated individual cosmic rays in the upper atmosphere is "very dim" requiring dark skies, huge multiple mirrors and rapid electronics. However, astronauts on the ISS can see the Cherenkov radiation caused by isolated individual cosmic rays hitting their eyeballs: https://en.wikipedia.org/wiki/Cosmic_ra ... _phenomena
As I thought, thanks for the confirmation Art.

So to see cherenkov radiation in inner space one must travel to outer space.
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Re: APOD: HESS Telescopes Explore the Sky... (2019 Jan 08)

Post by daniel.reardon » Tue Jan 08, 2019 4:16 pm

Now I'm going to have Judas Priest singing 'Breaking The Law' in my head all day, because of this one sentence.
They are designed to detect strange flickers of blue light -- Cherenkov radiation --emitted when charged particles move slightly faster than the speed of light in air.
Breaking the Cosmic Speed Limit? Can someone shed some (pardon the pun) light on this?

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Re: APOD: HESS Telescopes Explore the Sky... (2019 Jan 08)

Post by Chris Peterson » Tue Jan 08, 2019 4:22 pm

daniel.reardon wrote:
Tue Jan 08, 2019 4:16 pm
Now I'm going to have Judas Priest singing 'Breaking The Law' in my head all day, because of this one sentence.
They are designed to detect strange flickers of blue light -- Cherenkov radiation --emitted when charged particles move slightly faster than the speed of light in air.
Breaking the Cosmic Speed Limit? Can someone shed some (pardon the pun) light on this?
The cosmic speed limit is c, the speed of light in a vacuum. In all other materials, light travels slower than c, but other things can still travel faster than light but slower than c. When they do, they emit Cherenkov radiation.
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Re: APOD: HESS Telescopes Explore the Sky... (2019 Jan 08)

Post by BDanielMayfield » Tue Jan 08, 2019 5:29 pm

Ignoring all the other reasons why it's impossible, consider the blinding blue flash that would be caused by a macroscopic object like, say the USS Enterprise, moving at FTL.
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Re: APOD: HESS Telescopes Explore the Sky... (2019 Jan 08)

Post by Chris Peterson » Tue Jan 08, 2019 5:37 pm

BDanielMayfield wrote:
Tue Jan 08, 2019 5:29 pm
Ignoring all the other reasons why it's impossible, consider the blinding blue flash that would be caused by a macroscopic object like, say the USS Enterprise, moving at FTL.
Technically, the Star Trek warp drive is generally described as a system that modifies the local properties of space, so you have a sort of bubble of spacetime moving between two points in less time than light could make the trip, but the Enterprise itself never exceeds the speed of light. Most modern science fiction starship drives utilize ideas along these lines to avoid the fact that it's impossible for a physical object in normal space to reach the speed of light.
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Re: APOD: HESS Telescopes Explore the Sky... (2019 Jan 08)

Post by Keith S » Tue Jan 08, 2019 6:21 pm

Is it possible to see what HESS sees?

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Re: APOD: HESS Telescopes Explore the Sky... (2019 Jan 08)

Post by neufer » Tue Jan 08, 2019 7:30 pm

Keith S wrote:
Tue Jan 08, 2019 6:21 pm

Is it possible to see what HESS sees?
http://asterisk.apod.com/viewtopic.php? ... 44#p288743
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Re: APOD: HESS Telescopes Explore the Sky... (2019 Jan 08)

Post by Chris Peterson » Tue Jan 08, 2019 7:48 pm

Keith S wrote:
Tue Jan 08, 2019 6:21 pm
Is it possible to see what HESS sees?
Possible? Yes. Under ideal conditions the eye can detect a single photon. Would you ever notice such an event against the background? Unlikely beyond reasonable doubt, even in the rare case of your retina actually responding to a photon or two.

There's a reason those mirrors are so large!
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Re: APOD: HESS Telescopes Explore the Sky... (2019 Jan 08)

Post by neufer » Tue Jan 08, 2019 9:40 pm

Chris Peterson wrote:
Tue Jan 08, 2019 7:48 pm
Keith S wrote:
Tue Jan 08, 2019 6:21 pm

Is it possible to see what HESS sees?
Possible? Yes. Under ideal conditions the eye can detect a single photon. Would you ever notice such an event against the background? Unlikely beyond reasonable doubt, even in the rare case of your retina actually responding to a photon or two.

There's a reason those mirrors are so large!
Two dilated eyes have an "effective detection area" of 1 cm2.

There is only a 1% chance of that a TeV energy gamma ray Cherenkov event will even deliver a single visible photon to your eyes.
https://www.mpi-hd.mpg.de/hfm/HESS/pages/about/telescopes/ wrote:
<<For a primary photon at TeV energy (1012 eV), only about 100 photons per m2 are seen on the ground. They arrive within a very short time interval, a few nanoseconds. A telescope located somewhere within the light pool will "see" the air shower, provided that its mirror area is large enough to collect enough photons.>>
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Re: APOD: HESS Telescopes Explore the Sky... (2019 Jan 08)

Post by Chris Peterson » Tue Jan 08, 2019 10:30 pm

neufer wrote:
Tue Jan 08, 2019 9:40 pm
Chris Peterson wrote:
Tue Jan 08, 2019 7:48 pm
Keith S wrote:
Tue Jan 08, 2019 6:21 pm

Is it possible to see what HESS sees?
Possible? Yes. Under ideal conditions the eye can detect a single photon. Would you ever notice such an event against the background? Unlikely beyond reasonable doubt, even in the rare case of your retina actually responding to a photon or two.

There's a reason those mirrors are so large!
Two dilated eyes have an "effective detection area" of 1 cm2.

There is only a 1% chance of that a TeV energy gamma ray Cherenkov event will even deliver a single visible photon to your eyes.
Exactly. Like I said, possible but extremely unlikely.
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Re: APOD: HESS Telescopes Explore the Sky... (2019 Jan 08)

Post by neufer » Tue Jan 08, 2019 11:17 pm

Chris Peterson wrote:
Tue Jan 08, 2019 10:30 pm
neufer wrote:
Tue Jan 08, 2019 9:40 pm

Two dilated eyes have an "effective detection area" of 1 cm2.

There is only a 1% chance of that a TeV energy gamma ray Cherenkov event will even deliver a single visible photon to your eyes.
Exactly. Like I said, possible but extremely unlikely.
I calculate that a magnitude 6 star sends ~10,000 photons/second into each dilated eye.
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Re: APOD: HESS Telescopes Explore the Sky... (2019 Jan 08)

Post by BDanielMayfield » Wed Jan 09, 2019 1:03 am

neufer wrote:
Tue Jan 08, 2019 11:17 pm
Chris Peterson wrote:
Tue Jan 08, 2019 10:30 pm
neufer wrote:
Tue Jan 08, 2019 9:40 pm

Two dilated eyes have an "effective detection area" of 1 cm2.

There is only a 1% chance of that a TeV energy gamma ray Cherenkov event will even deliver a single visible photon to your eyes.
Exactly. Like I said, possible but extremely unlikely.
I calculate that a magnitude 6 star sends ~10,000 photons/second into each dilated eye.
How many photons does it take for the human vision system (eye + brain) to notice something?
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Re: APOD: HESS Telescopes Explore the Sky... (2019 Jan 08)

Post by neufer » Wed Jan 09, 2019 1:49 am

BDanielMayfield wrote:
Wed Jan 09, 2019 1:03 am
neufer wrote:
Tue Jan 08, 2019 11:17 pm

I calculate that a magnitude 6 star sends ~10,000 photons/second into each dilated eye.
How many photons does it take for the human vision system (eye + brain) to notice something?
~10,000 photons/second?

Venus can be 10,000 time brighter and is very noticeable.
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Re: APOD: HESS Telescopes Explore the Sky... (2019 Jan 08)

Post by Chris Peterson » Wed Jan 09, 2019 2:10 am

BDanielMayfield wrote:
Wed Jan 09, 2019 1:03 am
neufer wrote:
Tue Jan 08, 2019 11:17 pm
Chris Peterson wrote:
Tue Jan 08, 2019 10:30 pm


Exactly. Like I said, possible but extremely unlikely.
I calculate that a magnitude 6 star sends ~10,000 photons/second into each dilated eye.
How many photons does it take for the human vision system (eye + brain) to notice something?
Under controlled conditions, a person can distinguish a ten photon event 50% of the time. The rate drops fast with photon count, but it is possible for a single photon to trigger one rod, which can produce a consciously detectable event. But that's pretty rare, and if you were outside looking at the sky, you would be in pretty uncontrolled circumstances.
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Re: APOD: HESS Telescopes Explore the Sky... (2019 Jan 08)

Post by MarkBour » Wed Jan 09, 2019 5:49 am

Fascinating how the reflection (if it is real) behaves at 1:00-1:04 in the video. Also at about 1:13-1:17. The surface is a paraboloid of revolution, right? Which famously will focus parallel light to a point. But it evidently has some surprising behaviors in this off-center viewing of a reflection.
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