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28 Mar 2024, 11:07 [ UTC - 5; DST ]


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 Post subject: Re: James Web Telescope
PostPosted: 24 Mar 2022, 11:28 
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Username Protected wrote:
So, then how is that not a problem when you are trying to observe distant objects with as much resolution as possible?

That depends on what you're trying to observe about those distant objects. You can still measure their size, brightness, spectrum, relative motion, doppler shift, and a bunch of things I don't remember. If you were trying to count the number of craters on a planet circling a star a few million light years away those image artifacts might make that impossible or at least more difficult but that's not what JW is about. It doesn't even have an imaging camera that can record "visible" light.

I believe that's what's tough for us mere mortals Lance. It's difficult not to think in terms of the visible. The James Webb telescope seems to embody the phrase, "there's more to it than meets the eye."
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 Post subject: Re: James Web Telescope
PostPosted: 24 Mar 2022, 20:03 
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 Post subject: Re: James Web Telescope
PostPosted: 29 Mar 2022, 12:43 
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More technical details here.

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 Post subject: Re: James Web Telescope
PostPosted: 20 Apr 2022, 16:20 
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Talk about cold

... reached its final operating temperature below 7 kelvins (minus 447 degrees Fahrenheit, or minus 266 degrees Celsius).

https://www.nasa.gov/feature/jpl/webb-t ... emperature

The things these engineers had to overcome is simply incredible. I'm an EE but I'd never heard of dark current.

Another reason Webb’s detectors need to be cold is to suppress something called dark current, or electric current created by the vibration of atoms in the detectors themselves. Dark current mimics a true signal in the detectors, giving the false impression that they have been hit by light from an external source. Those false signals can drown out the real signals astronomers want to find. Since temperature is a measurement of how fast the atoms in the detector are vibrating, reducing the temperature means less vibration, which in turn means less dark current.


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 Post subject: Re: James Web Telescope
PostPosted: 20 Apr 2022, 22:08 
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I'm an EE but I'd never heard of dark current.

I've seen it referred to before on optocoupler specifications.


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 Post subject: Re: James Web Telescope
PostPosted: 20 Apr 2022, 23:20 
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Username Protected wrote:
I'm an EE but I'd never heard of dark current.

I've seen it referred to before on optocoupler specifications.

Yep. Phototransistors (discrete or in a photocoupler) normally have a dark current spec which is essentially the collector leakage current. While that sounds like something related to "dark matter", it's not as far as we know. :D
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 Post subject: Re: James Web Telescope
PostPosted: 21 Apr 2022, 00:42 
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EEs who work in digital or high frequency systems usually don't care about dark current. For people doing low frequency analog, dark current is very similar to electronic offset and bias currents - which usually decrease with temperature.

I designed a lot of detector readouts in the past, so dark current is very familiar. On my first job I needed to get better noise out of a Ge detector, and ended up just pouring liquid nitrogen on it - worked beautifully.

These days I work with really cold detectors for CMB telescopes. Typically 0.1 Kelvin above absolute zero. They even have sub kelvin refrigeration systems on space telescopes - but much smaller ones than James Webb.

The quantum computing work is usually done at 0.01 degree above absolute zero

The atom cooling guys have this all beat - they cool atom clouds to less than a millionth of a degree above absolute zero.


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 Post subject: Re: James Web Telescope
PostPosted: 21 Apr 2022, 07:35 
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That's amazing stuff Josef. I'm very low tech. Much of what I do is related to medium voltage distribution, usually 5kV-33kV. We never have to worry about dark current.

Username Protected wrote:
EEs who work in digital or high frequency systems usually don't care about dark current. For people doing low frequency analog, dark current is very similar to electronic offset and bias currents - which usually decrease with temperature.

I designed a lot of detector readouts in the past, so dark current is very familiar. On my first job I needed to get better noise out of a Ge detector, and ended up just pouring liquid nitrogen on it - worked beautifully.

These days I work with really cold detectors for CMB telescopes. Typically 0.1 Kelvin above absolute zero. They even have sub kelvin refrigeration systems on space telescopes - but much smaller ones than James Webb.

The quantum computing work is usually done at 0.01 degree above absolute zero

The atom cooling guys have this all beat - they cool atom clouds to less than a millionth of a degree above absolute zero.


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 Post subject: Re: James Web Telescope
PostPosted: 21 Apr 2022, 12:04 
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OTOH, I don't have to worry about distributing megawatts, and the things I work on can't hurt anyone - unless you drop them on your foot.

The big accelerators here at SLAC need a lot of AC power distribution, but I've never been involved in any of that.

So many different fields of EE. The high speed digitial guys are yet another area - desicing >20 layer circuit boards with parts with >2000 pins.

Username Protected wrote:
EEs who work in digital or high frequency systems usually don't care about dark current. For people doing low frequency analog, dark current is very similar to electronic offset and bias currents - which usually decrease with temperature.

I designed a lot of detector readouts in the past, so dark current is very familiar. On my first job I needed to get better noise out of a Ge detector, and ended up just pouring liquid nitrogen on it - worked beautifully.

These days I work with really cold detectors for CMB telescopes. Typically 0.1 Kelvin above absolute zero. They even have sub kelvin refrigeration systems on space telescopes - but much smaller ones than James Webb.

The quantum computing work is usually done at 0.01 degree above absolute zero

The atom cooling guys have this all beat - they cool atom clouds to less than a millionth of a degree above absolute zero.


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 Post subject: Re: James Web Telescope
PostPosted: 21 Apr 2022, 13:32 
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When I graduated as a BSEE in 1982, there were only a few of us that went into power distribution. Everyone else, the smarter kids, went to electronics of some sort.


Username Protected wrote:
That's amazing stuff Josef. I'm very low tech. Much of what I do is related to medium voltage distribution, usually 5kV-33kV. We never have to worry about dark current.



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 Post subject: Re: James Web Telescope
PostPosted: 21 Apr 2022, 13:46 
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When I graduated as a BSEE in 1982, there were only a few of us that went into power distribution. Everyone else, the smarter kids, went to electronics of some sort.

From your perspective, how susceptible is our electric power distribution network to EMP?

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 Post subject: Re: James Web Telescope
PostPosted: 21 Apr 2022, 14:07 
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When I graduated as a BSEE in 1982, there were only a few of us that went into power distribution. Everyone else, the smarter kids, went to electronics of some sort.

From your perspective, how susceptible is our electric power distribution network to EMP?


Lance,
I'm not involved in anything dealing with that so I'm afraid I can't comment.
Dave

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 Post subject: Re: James Web Telescope
PostPosted: 21 Apr 2022, 14:16 
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Lance,
I'm not involved in anything dealing with that so I'm afraid I can't comment.
Dave

I hope SOMEONE is dealing with that.


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 Post subject: Re: James Web Telescope
PostPosted: 21 Apr 2022, 14:25 
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From your perspective, how susceptible is our electric power distribution network to EMP?

Probably no worse than squirrels:

https://en.wikipedia.org/wiki/Electrica ... _squirrels

"John C. Inglis, the former deputy director of the U.S. National Security Agency, said in 2015 that he judged the electrical grid was as likely to be paralyzed by a natural disaster as by a cyberattack and added: "[F]rankly, the No. 1 threat experienced to date by the U.S. electrical grid is squirrels."

Mike C.

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 Post subject: Re: James Web Telescope
PostPosted: 21 Apr 2022, 15:49 
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Username Protected wrote:
From your perspective, how susceptible is our electric power distribution network to EMP?

Probably no worse than squirrels:

https://en.wikipedia.org/wiki/Electrica ... _squirrels

"John C. Inglis, the former deputy director of the U.S. National Security Agency, said in 2015 that he judged the electrical grid was as likely to be paralyzed by a natural disaster as by a cyberattack and added: "[F]rankly, the No. 1 threat experienced to date by the U.S. electrical grid is squirrels."

Mike C.


I can't tell you how many times we've seen gear knocked out by squirrels and mice. There's usually something left of them. Trees and cars do a lot of damage too.

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