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Author Topic: Webb Space Telescope  (Read 3966 times)

Offline Mark wood

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Re: Webb Space Telescope
« Reply #50 on: December 28, 2021, 02:30:15 PM »
     My point was that to really explain how space-time expands in 4D, you have to use general relativity. I was conceding that people understood the Doppler effect, whether or not they understood Special Relativity and relativistic Doppler effect.

       Brett

I was initially giving you crap for not being precise. A little pay back in kind actually. ;) But then it turned into an interesting discussion on the semantics of relativity. You know, how does the engineer state Newtons second law? Newtons second law states Force is equal to the mass times acceleration. While correct it isn't accurate as the physicist points out Newton actually stated that Force is equal to the time rate of change of momentum. One cannot directly derive rocket thrust from the first version nor the force of accretion of mass on a conveyer belt while both are part of the second version. With your knowledge of orbital mechanics, I sorted that out previously some how, I pretty much figured you knew. It is fun for me to walk down memory lane. I haven't really done much more than watch since I had Modern physics and both relativity classes in college. It wasn't easy for me.
 
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Offline Mark wood

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Re: Webb Space Telescope
« Reply #51 on: December 28, 2021, 02:30:46 PM »
My mind just cannot grasp infinity.  I defer to God.

Mike

Me too.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline phil c

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Re: Webb Space Telescope
« Reply #52 on: December 28, 2021, 02:53:09 PM »
Yes indeed. It does trouble the mind trying to grasp how grown, educated, people can believe such nonsense.  That, and their companion idea that gravity isn’t real, things just have weight……. LL~ LL~ LL~

Gary

Alot of those people have it right.  When you get to the edge  it warps around, so tight you're can't sense it, and you're on the other side!!
Holy cow!  We were using that back when I got my first job out of college!  Those were the days when they'd bring your tray of cards back with ones sticking up side-ways: those were the ones that got chewed by the card reader or got spit out with some error code.  Or sometimes they'd drop the tray and you then had the joy of putting them all back in order.

People still use FORTRAN because it works, plain and simple.  There are also literally millions of FORTRAN programs that one might be a program or a piece of one that does exactly what you want.  I bet that NASA still uses it because it is proven and ready for use- something like mapping the course of a satellite and confirming that it is on the right path- both from the satellite and from the ground but using different equipment.

Also, FORTRAN users probably have a good understanding where there might be pitfalls.

I always was interested in programming, but have run into my own personal wall.  I took the first class in programming at the college I went to.  The prof waved a big, thick paperback book and said " you probably will need to pick this up.  We forgot to put it in the syllabus.  You WILL need it to do the homework".
The next year the first course was a semester course in FORTRAN.
phil Cartier

Online Ken Culbertson

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Re: Webb Space Telescope
« Reply #53 on: December 28, 2021, 03:00:07 PM »
Me too.
God sold the rights to Nissan.

Infinity is one of those things that is best not contemplated.  I once asked my grandmother what "World without end" meant since they were making me memorize it in Bible school.  She told me it meant infinity.  That bothered me until I was in high school and found out that it was simply division by -0-. 

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Offline Avaiojet

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Re: Webb Space Telescope
« Reply #54 on: December 28, 2021, 03:29:23 PM »
Maybe this new Webb Telescope will finally answer the question,

"Do black holes really exist?"  LL~
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Offline phil c

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Re: Webb Space Telescope
« Reply #55 on: December 28, 2021, 04:05:14 PM »
My mind just cannot grasp infinity.  I defer to God.

Mike

A religious type, technically educated, might say "God holds the universe in himself."  Grasping infinity takes more than our minds can do.
phil Cartier

Online Brett Buck

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Re: Webb Space Telescope
« Reply #56 on: December 28, 2021, 05:16:19 PM »
Maybe this new Webb Telescope will finally answer the question,

"Do black holes really exist?"  LL~

https://en.wikipedia.org/wiki/Sagittarius_A*

Online Brett Buck

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Re: Webb Space Telescope
« Reply #57 on: December 28, 2021, 08:20:17 PM »

   By the way, on the topic of why it is so far out there...

   For the rest of us, you might at first blush think that they would put it in low earth orbit like Hubble, where they can get to it. Several problems with that idea  - even in low earth orbit, with the shuttle retired, we have, right now, no way of getting to it with any significant repair capability. Soyuz could get to it in really low orbit like the space station, but has no real capability for capturing something like NGST/Webb or carrying repair parts, grappling arms, etc.

   But the big reasons are that in low earth orbit, you have A LOT of thermal input from the earth. This is called albedo heating, heating from reflected and radiated heat from the earth albedo.

   The earth is ~70 degrees, and the background of space is -450 degrees. In low earth orbit, having half the sphere of the sky filled up with a nice warm 70 degree earth imparts tremendous heat to the telescope. Since the telescope sees in infrared frequencies (heat), this is a huge noise input in the band they are looking that has to be dealt with. Of course, it also blocks half the potential targets , and may leave only 45 minutes of uninterrupted observation time if the target is in not perpendicular to the orbit plane. Additionally, if the telescope is pointing in one inertial direction, the heat input comes from different directions at pretty high frequency, causing thermal distortion of the telescope mirror that has to be dealt with.

    Putting it out at the L2 Lagrange point, a million miles away, the earth is just a tiny speck, so no real heat input and no varying directional heat  inputs. This makes it much more stable. The earth is not consequentially interfereing with the telescope field of view, so it can stare in one direction without having to consider where in the orbit it is. But mostly, it is just cold - which means, with the extraordinary effort put into the heat shields to block the sun, they can "run the telescope without any active cooling system". It stays cold enough even for an infrared mission without having to carry cryogenic coolants, that inevitably run out. This telescope's predecessor , SIRTF (built by my Denver colleagues), ran for 6 years, when it ran out of liquid helium coolant, did a reduced mission for a while, and then was deactivated. NGST/Webb does not require that, so it can last indefinitely (presumably until the stationkeeping propellant runs out).

    Putting at a Lagrange point means that there is no net secular acceleration. This happens because the various gravitational forces and centrifugal force happen to balance each other at these points. There are 5 of them:

https://en.wikipedia.org/wiki/Lagrange_point

   NGST/Webb is going to point 2, that it, it will stay out beyond earth's orbit around the sun, and you will be able to draw a straight line from the sun to the earth to the point. It won't stay there without some small effort to push it around and so will require some on-board propulsion to stop it once it gets there and then to nudge it around to stay, but it does not have to push against some force, because the net force is zero.

    As noted above, these guys have really thought all this out and definitely know what they are doing. I forget what our proposal for this mission was, I am sure it was credible. But Northrop-Grumman are highly competent and have a lot of experience with the sort of deployable structures dating back the deployable, moderately-large mesh antennas on the FLTSAT program dating to the early 70's. I have worked with various TRW/NG engineers as our subcontractors and over the years and on the engineering level, they are wonderful to work with, peers of equal capability.

   Having said all that, a mission like this is generally designed to have a probability of success around 70% - because trying to get 80% is disproportionately expensive. The 70% then flows down to thousands of elements, all of which must have much greater than 70% chance of making it for the whole mission. Strings of these 99.7% or so probabilities are multiplied together to get the 70%. 

    There is usually a lot of hidden conservatism, but be prepared for and aware of the chance that something *could* go wrong. That wouldn't make the people involved incompetent or lackadaisical, it means that doing these missions is very difficult and complex, and very unforgiving of even a single mistake.

       Brett




Offline Howard Rush

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Re: Webb Space Telescope
« Reply #58 on: December 28, 2021, 08:30:40 PM »
Great explanation, Brett.  Thanks.
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Offline Mark wood

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Re: Webb Space Telescope
« Reply #59 on: December 28, 2021, 08:34:51 PM »
Great explanation, Brett.  Thanks.

Second that motion.
Life is good AMA 1488
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Offline Dave Hull

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Re: Webb Space Telescope
« Reply #60 on: December 28, 2021, 10:17:02 PM »
Five nines.... or you're just making Mattel....

Online Brett Buck

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Re: Webb Space Telescope
« Reply #61 on: December 28, 2021, 10:39:06 PM »
Five nines.... or you're just making Mattel....

   The highest component spec I have seen in 40-ish years is 3-sigma, that is 99.7, and the highest overall mission reliability including all sources is 75%.  99.7% on a component usually means you need a redundant unit.

   Reality, of course, is that most of these systems have nowhere near enough flights to get reasonable statistics. Agena (all types), STS, and Soyuz are the only systems I think have sufficient N to evaluate the correctness of the analysis, and all are far greater than the required reliability. Everybody has their conservatism and the reliability people sometimes underestimate/misunderstand how the probabilities should be combined, so usually it beats the mission, and sometimes by an order of magnitude.

    This includes some of the most critical systems currently used.

     Brett

Offline Mike Callas

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Re: Webb Space Telescope
« Reply #62 on: December 29, 2021, 02:43:01 AM »
I see they successfully got it launched and on its way. Seems like it was one of the most glitch free launches I've seen recently. No last minute delays or hiccups. Hope it lives up to all the hype. It will take it a month to reach its destination orbit of 1 million miles. Should be free from orbiting space debris at that distance. (Yes, I know that's not the reason for the chosen  orbiting spot.)

The launch was relatively glitch free. However, the program had several delays due to its complexity. My company built a couple of Ka Filters for it back in 2006 or so. Back then it was supposed to launch around 2011. Glad it finally made it off the ground.

Offline Avaiojet

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Re: Webb Space Telescope
« Reply #63 on: December 29, 2021, 07:05:25 AM »
9 Billion dollars can get a bunch of our Vets off the street.

Is any of this investment tax dollars?

If it was, we probably wouldn't know it anyway.

For what? The more "they see," the less they know. I never understood this type of spending?

Charles

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Online Bill Morell

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Re: Webb Space Telescope
« Reply #64 on: December 29, 2021, 08:18:56 AM »
9 Billion dollars can get a bunch of our Vets off the street.

Is any of this investment tax dollars?

If it was, we probably wouldn't know it anyway.

For what? The more "they see," the less they know. I never understood this type of spending?

Charles

I've never understood why you are here?
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Offline Mark wood

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Re: Webb Space Telescope
« Reply #65 on: December 29, 2021, 08:25:17 AM »
9 Billion dollars can get a bunch of our Vets off the street.

Is any of this investment tax dollars?

If it was, we probably wouldn't know it anyway.

For what? The more "they see," the less they know. I never understood this type of spending?

Charles

The interesting thing is that the technology you used to type these words are direct offshoot of the technology that was developed for the space program. Without having invested in the space program, you wouldn't be able to bitch about investing in the space program on these pages.  Without making such investments no progress is made. Yes, there are a lot of Vets on the street. Yes, we should funnel more money towards helping them but we shouldn't stop doing research.

Being able to build and launch a spacecraft a million miles in to space may seem wasteful but consider what the challenges of doing so. It requires many new technologies, new techniques, new processes and new materials. When those are developed everyone benefits, including those Vets on the street.
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Online Ken Culbertson

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Re: Webb Space Telescope
« Reply #66 on: December 29, 2021, 09:24:18 AM »
I never understood this type of spending?

Charles
I have never understood why there is so little of it.  9 billion is not going to get the Vets off of the street.  The only thing that will get vets off of the street is to stop having wars. 

Ken
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Online Dwayne Donnelly

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Re: Webb Space Telescope
« Reply #67 on: December 29, 2021, 09:35:32 AM »
The interesting thing is that the technology you used to type these words are direct offshoot of the technology that was developed for the space program. Without having invested in the space program, you wouldn't be able to bitch about investing in the space program on these pages.  Without making such investments no progress is made. Yes, there are a lot of Vets on the street. Yes, we should funnel more money towards helping them but we shouldn't stop doing research.

Being able to build and launch a spacecraft a million miles in to space may seem wasteful but consider what the challenges of doing so. It requires many new technologies, new techniques, new processes and new materials. When those are developed everyone benefits, including those Vets on the street.

Not to hijack this post but I've had this conversation before. I once hitch hiked across Canada and then down the west coast, along the way I slept in all kinds of places most of us wouldn't including Sally Ann hostels and met all kinds of people, one thing I learned is, not all, but a lot of homeless are homeless by choice, they are choosing to live on the streets to give a big middle finger to society, I will not conform, it's so easy to look at the space program and say what about the homeless, well sorry a lot of those people are there by choice, hard to believe but it's true. The hard part is helping those who want the help but won't ask.
« Last Edit: December 30, 2021, 10:48:07 AM by Dwayne Donnelly »
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Offline Dan McEntee

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Re: Webb Space Telescope
« Reply #68 on: December 29, 2021, 09:56:20 AM »
Not to hijack this post but I've had this conversation before. I once hitch hiked across Canada and then down the west coat, along the way I slept in all kinds of places most of us wouldn't including Sally Ann hostels and met all kinds of people, one thing I learned is, not all, but a lot of homeless are homeless by choice, they are choosing to live on the streets to give a big middle finger to society, I will not conform, it's so easy to look at the space program and say what about the homeless, well sorry a lot of those people are there by choice, hard to believe but it's true. The hard part is helping those who want the help but won't ask.

    This is so, so so true! I live in a suburb if St. Louis County. Not the best, toniest part of town but not a stereotypical are where you see a lot of homeless people. There are three home less guys that I see all over town. Lots of good hearted people buy them food, sometimes pay for a few nights in local hotel for them, give them gift cards and all sorts of stuff. They even have cell phones! And that is exactly why the hand around here. If offered a job or a permanent place to stay , they refuse it because they do not want it. One guy even has family living here, and they are thankful to people trying to help this guy out, but he is what he is and he won't change. It's a free country, (for now!!) and they aren't breaking any laws, so live and let live.
    Type at you later,
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Offline Mark wood

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Re: Webb Space Telescope
« Reply #69 on: December 29, 2021, 10:04:00 AM »
Not to hijack this post but I've had this conversation before. I once hitch hiked across Canada and then down the west coat, along the way I slept in all kinds of places most of us wouldn't including Sally Ann hostels and met all kinds of people, one thing I learned is, not all, but a lot of homeless are homeless by choice, they are choosing to live on the streets to give a big middle finger to society, I will not conform, it's so easy to look at the space program and say what about the homeless, well sorry a lot of those people are there by choice, hard to believe but it's true. The hard part is helping those who want the help but won't ask.

I was about to say this as well but chose not to. My experience leans in the same direction. Many of the panhandlers actually produce a fairly decent wage. There are those whom just don't fit in society and wander aimlessly.
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Offline Avaiojet

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Re: Webb Space Telescope
« Reply #70 on: December 29, 2021, 10:52:09 AM »
People do have the right to view or look at things differently, don't we?

I'm sure you do agree, I cannot see how you wouldn't.

I view the "Space Industry" broad brush please, the same way I view the "Turbine Windmill Industry."

Unfortunately, you do have to have an understand the Turbine Windmill Industry, to have an inkling as to what I'm offering in this comparison.

Really similar industries but not all that obvious. "They" fix it so it's never obvious.

Charles

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Online Brett Buck

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Re: Webb Space Telescope
« Reply #71 on: December 29, 2021, 11:51:28 AM »
Just ignore tha *sshole. As predicted, he got caught gassing on about something he had absolutely no clue about, panicked for a few days when he realized he had gotten caught, and then changed the subject. He knows the "wind turbine industry" about as well as he knows quantum mechanics, he had to pick something that he might get backing on.

     He also, obviously, knowns nothing about the space industry, either, and seems to think it is run/dominated by NASA - which it certainly is not. NASA has a few important civilian programs (like this 1/2 billion a year or so effort over 20 years), does have a monopoly on manned space flight, but is a *tiny* player in the space business - which is dominated by military and other defense-related agencies.

   That's why breaking out the US Space Force made perfect sense, but why everyone just tried to mock it - the DoD, mostly directed the USAF/USSF, has numerous huge programs upon which the defense of the United States (and by extension, the free world) depends, and have for more than half-a-century. They are the big players, not NASA and certainly not commercial space. No one seems to really grasp that. Acquiring and operating space assets is a fundamentally different proposition from acquiring and operating aircraft, it makes absolutely perfect sense to have broken it out as it's own command - and that move had been in the works for 25+ years.

     Brett

Offline Avaiojet

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Re: Webb Space Telescope
« Reply #72 on: December 29, 2021, 12:46:37 PM »
Just ignore tha *sshole. As predicted, he got caught gassing on about something he had absolutely no clue about, panicked for a few days when he realized he had gotten caught, and then changed the subject. He knows the "wind turbine industry" about as well as he knows quantum mechanics, he had to pick something that he might get backing on.

     He also, obviously, knowns nothing about the space industry, either, and seems to think it is run/dominated by NASA - which it certainly is not. NASA has a few important civilian programs (like this 1/2 billion a year or so effort over 20 years), does have a monopoly on manned space flight, but is a *tiny* player in the space business - which is dominated by military and other defense-related agencies.

   That's why breaking out the US Space Force made perfect sense, but why everyone just tried to mock it - the DoD, mostly directed the USAF/USSF, has numerous huge programs upon which the defense of the United States (and by extension, the free world) depends, and have for more than half-a-century. They are the big players, not NASA and certainly not commercial space. No one seems to really grasp that. Acquiring and operating space assets is a fundamentally different proposition from acquiring and operating aircraft, it makes absolutely perfect sense to have broken it out as it's own command - and that move had been in the works for 25+ years.

     Brett

You have absolutely no idea what I know and don't know.

We do have different backgrounds. Can't loose sight of this.

I also stand by my comments and I am not attacking yours.

So, exactly what am I saying that disturbs you?

Charles
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If you're Trolled, you know you're doing something right.  Alpha Mike Foxtrot. "No one has ever made a difference by being like everyone else."  Marcus Cordeiro, The "Mark of Excellence," you will not be forgotten. "No amount of evidence will ever persuade an idiot."- Mark Twain. I look at the Forum as a place to contribute and make friends, some view it as a Realm where they could be King.   Proverb 11.9  "With his mouth the Godless destroys his neighbor..."  "Perhaps the greatest challenge in modeling is to build a competitive control line stunter that looks like a real airplane." David McCellan, 1980.

Offline Dave Hull

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Re: Webb Space Telescope
« Reply #73 on: December 29, 2021, 08:42:05 PM »
You can track progress easily here:

https://webb.nasa.gov/content/webbLaunch/whereIsWebb.html

Dave

Online Brett Buck

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Re: Webb Space Telescope
« Reply #74 on: December 30, 2021, 12:57:24 PM »
You can track progress easily here:

https://webb.nasa.gov/content/webbLaunch/whereIsWebb.html

Dave
   

     Good nuggets on that page, including live telemetry of some temperatures (one of which is -229F  - as mentioned, it wants to be cold) and videos of the deployment sequence.

      Brett
« Last Edit: December 30, 2021, 10:33:59 PM by Brett Buck »

Online Mark Gerber

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Re: Webb Space Telescope
« Reply #75 on: December 30, 2021, 01:02:08 PM »
Thanks for posting the JWST website.  I worked on it in 2007 at Ball Aerospace and I did my master's thesis at MIT on L2.

Mark Gerber

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Re: Webb Space Telescope
« Reply #76 on: December 30, 2021, 01:12:39 PM »
Thanks for posting the JWST website.  I worked on it in 2007 at Ball Aerospace and I did my master's thesis at MIT on L2.

Mark Gerber

  So, Mark - L2 is not naturally stable, but how fast does the force build up as you move away? Is a typical stationkeeping cycle more driven by the divergent force, or does the acceleration from other bodies (like the moon) matter more?  I presume that other earth orbit effects like oblateness, triaxiality, etc, are negligible at that altitude.

     Brett

Online Mark Gerber

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Re: Webb Space Telescope
« Reply #77 on: December 30, 2021, 04:06:57 PM »
Brett,

I don't know which forces dominate, but the perturbing forces must be very small -- especially a million miles from Earth.  The propellant expended to keep JWST in the halo orbit around L2 is minimal.  The 14,300-pound spacecraft has only 400 lbs. of fuel which is expected to last 5-10 years.

The spacecraft will not actually be at L2 but in a large, slow orbit around it.  The diameter of the JWST halo orbit around L2 is roughly the same diameter as the moon's orbit.  The halo orbit is slanted in relation to the Earth's orbit, so the spacecraft is never in the Earth's shadow.  The period is about 6 months.  Someone referred to this as "controlled drifting."  So, the perturbing forces must not change much as you move off L2.

Mark Gerber

Offline Doug Moon

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Re: Webb Space Telescope
« Reply #78 on: December 30, 2021, 06:53:47 PM »
Brett,

I don't know which forces dominate, but the perturbing forces must be very small -- especially a million miles from Earth.  The propellant expended to keep JWST in the halo orbit around L2 is minimal.  The 14,300-pound spacecraft has only 400 lbs. of fuel which is expected to last 5-10 years.

The spacecraft will not actually be at L2 but in a large, slow orbit around it.  The diameter of the JWST halo orbit around L2 is roughly the same diameter as the moon's orbit.  The halo orbit is slanted in relation to the Earth's orbit, so the spacecraft is never in the Earth's shadow.  The period is about 6 months.  Someone referred to this as "controlled drifting."  So, the perturbing forces must not change much as you move off L2.

Mark Gerber

The orbit of this thing kind of blows my mind a little.

https://youtu.be/6cUe4oMk69E?list=TLGG8tIphgpDAHkzMTEyMjAyMQ

It orbits around L2. Does Earth's gravity keep it in the slanted orbit around L2? If not how does it stay in the L2 orbit? If so that is amazing that the earths gravity is so strong it can hold an object 1.5M/KM away in a specific orbit like that but at the same time it is not so strong that we can go about our normal day without being crushed to the surface of our planet. This stuff is truly amazing.

I find this whole thing incredibly fascinating. I constantly watch the stars at night with or without my telescope.
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Offline Dave Hull

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Re: Webb Space Telescope
« Reply #79 on: December 30, 2021, 09:53:35 PM »
As noted, temperature monitoring is crucial to the performance of the entire optics set, especially the Cassegrain, including the secondary support structure and the primary mirror reaction structure. Even a quarter of a degree would likely significantly defocus the optics since the primary secondary combo are both beryllium. Unless you have a focus drive control loop closed around the temperature map, you would always be doing calibration shots on a pinpoint source (distant star) to refocus. That wastes a huge amount of imaging time on a limited asset. My understanding is that they are counting on long-period temperature stabilization.

Dave

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Re: Webb Space Telescope
« Reply #80 on: January 06, 2022, 06:54:50 PM »
URL of the ESA video of the JWST deployment.  The sun shade s successfully deployed.




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Re: Webb Space Telescope
« Reply #81 on: January 06, 2022, 08:05:16 PM »
Even better, the secondary mirror deployment was a success. The majority of the single-point deployment failures are behind us now....

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Re: Webb Space Telescope
« Reply #82 on: January 07, 2022, 08:12:04 AM »
Glad to hear its deployment is going so smoothly. From what I understand the telescope will have to wait a couple of months for the temperatures to stabilize once it's in position before they start doing observations.

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Re: Webb Space Telescope
« Reply #83 on: January 07, 2022, 08:33:02 AM »
I was amazed at how complex this telescope is.  Even able to take pictures of itself deploying.  Mark, did you put one of your little boom cameras on it when they weren't looking? LL~

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Re: Webb Space Telescope
« Reply #84 on: January 07, 2022, 11:39:58 AM »
I was amazed at how complex this telescope is.  Even able to take pictures of itself deploying.  Mark, did you put one of your little boom cameras on it when they weren't looking? LL~

Ken

Actually that video was taken from the booster of the observatory as it was released. The observatory itself doesn't have any cameras on it as there wouldn't be much to see without illumination which could cause problems with the instrumentation packs and is not mission necessary. I'd love to see the videos though but I was satisfied to watch the animations real time.

Thanks for the thoughts though...  <=
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Re: Webb Space Telescope
« Reply #85 on: January 07, 2022, 01:58:52 PM »
Question: what happens to the Ariane rocket that did the final burn? The one that was where the Webb was attached and which showed the separation in the video. Isn't it at the same velocity and trajectory as the Webb? Will it end up at L2 as well?
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Online Brett Buck

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Re: Webb Space Telescope
« Reply #86 on: January 07, 2022, 04:48:56 PM »
Question: what happens to the Ariane rocket that did the final burn? The one that was where the Webb was attached and which showed the separation in the video. Isn't it at the same velocity and trajectory as the Webb? Will it end up at L2 as well?

    I am not that familiar with Ariane upper stage behavior, but in many other cases, shortly after separation, they do a collision-avoidance maneuver  at some radical angle relatively to the line of flight, to avoid exactly that. Of course, the separation rate is about 1 foot/second or so, just from the push-off springs, so after  30 days, there would be a huge separation distance in the hundred or thousands of miles.

    At any rate, the telescope has to do a delta-v maneuver to go into the halo orbit at L2. Even if was trailing along behind it, the booster would just keep on going and probably fall back towards the sun (since it has insufficient velocity to stay there).

    Brett

p.s. From the ESA website about the Webb launch:

Quote
After separation, the upper stage underwent a delicate series of contamination and collision avoidance maneuvers, making sure that its thruster plumes did not impinge on Webb and its precious optics.

Finally, an end-of-life maneuver was performed to avoid potential long term collision risks with Webb.
« Last Edit: January 07, 2022, 06:08:58 PM by Brett Buck »

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Re: Webb Space Telescope
« Reply #87 on: January 07, 2022, 06:32:18 PM »
    I am not that familiar with Ariane upper stage behavior, but in many other cases, shortly after separation, they do a collision-avoidance maneuver  at some radical angle relatively to the line of flight, to avoid exactly that. Of course, the separation rate is about 1 foot/second or so, just from the push-off springs, so after  30 days, there would be a huge separation distance in the hundred or thousands of miles.

    At any rate, the telescope has to do a delta-v maneuver to go into the halo orbit at L2. Even if was trailing along behind it, the booster would just keep on going and probably fall back towards the sun (since it has insufficient velocity to stay there).

    Brett

p.s. From the ESA website about the Webb launch:

Way cool. I was wondering the same thing myself.
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Online John Hammonds

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Re: Webb Space Telescope
« Reply #89 on: January 24, 2022, 12:16:22 PM »
Well. It's reached L2. Now I guess we wait a little longer and then hopefully start to see what this thing is really capable of.  #^


Nasa are doing a Science Live podcast shortly.



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Offline Elwyn Aud

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Re: Webb Space Telescope
« Reply #90 on: January 28, 2022, 04:29:17 AM »
If I read correctly all of the tedious tweaking to get the mirrors in alignment involves movements  less than the width of a human hair.

Online Brett Buck

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Re: Webb Space Telescope
« Reply #91 on: January 28, 2022, 07:58:34 AM »
If I read correctly all of the tedious tweaking to get the mirrors in alignment involves movements  less than the width of a human hair.

     Yes. Part of it is aligning the mirrors segments in total, there is also a part that involves very slightly bending each one so it is curved just tight.

     Brett

Offline Dave Hull

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Re: Webb Space Telescope
« Reply #92 on: January 28, 2022, 03:17:18 PM »
The easy way to begin to understand the Cassegrain alignment is to think of it as 18 separate telescopes. Each one has to be correctly aligned, and they concurrently must be co-aligned. There is a very specific sequence to this optimization else you are just chasing your tail. (Kind of like trimming a stunt ship? I know less about that....)

The first order term to fix is also the simplest to imagine--focus. You have to translate the entire primary mirror segment along the optical axis without tilting it. Each mirror segment I understand is hexapod mounted with "'whiffle tree" supports to the back of the mirror. That lets you do both axial translations but also tip and tilt. I don't think they can do or want to do axis rotations--that likely was taken care of in the tolerancing of the mirror design, and it is probably the least sensitive of all the aberrations.

If they look at a good point source (lots of them in space!) you can look at the blur circle of each sub-aperture (individual primary mirror segment) and diagnose focus, astigmatism, etc. The last alignment you likely will see is the co-boresight adjustments. In other words, image a good star, and get 18 separate, equally spaced images of the star. Then tighten up the focus so that all 18 are as clear as possible, then nudge them until they all overlap. The gain will go up as they overlap, but the combined image will be as fuzzy as the worst of the individual images.

The good news for the designers is that the wavelengths they are looking at (based on detector sensitivity) are longer, which means that the alignment tolerances are ever so slightly larger to get the same quality image.

This is a very, very cool instrument!

Dave


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