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19 May 2022, 17:09 [ UTC - 5; DST ]


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 Post subject: Re: James Web Telescope
PostPosted: 13 Jan 2022, 01:03 
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Username Protected wrote:

So, put the earth back in - it's adding some additional gravitational attraction - that if you're in line with the sun and earth - simply adds the two gravitational attractions (+/- some relativistic amounts) to find our new gravitation component of the orbital period. Unfortunately, this is variable since earth will have very little effect if you're far from earth, and a lot more if you're closer. But, you can solve these two equations - earth + sun gravity and orbital period based on the radius and that gravity - to arrive at the "LaGrange point." Where you are a larger radius, with a slightly higher gravitational pull (earth + sun) and end up with the same orbital period - a point that stays behind the earth as it orbits the sun.

L1 is the same, but earth is reducing the effective suns gravity, hence slowing the orbit requiring a smaller orbital radius to have the same orbital period around the sun as the earth.

Clear as mud.


Andrew, so from your description combined with a smidgeon of possible common sense...the aim seems to be to keep the telescope largely out of the sun by keeping it behind the earth most of the time for better viewing of the "night sky." Hence the use of the Lagrange point. It gets sufficient sun for electrical power but sufficient darkness for better viewing. Is that correct?

Since there is no atmosphere in space to ruin viewing by atmosphericly reflected light maybe my "common sense" is in error.

In any case this certainly helps me understand Lagrange points. Now to understand why we use them.

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 Post subject: Re: James Web Telescope
PostPosted: 13 Jan 2022, 01:13 
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Username Protected wrote:
Andrew, so from your description combined with a smidgeon of possible common sense...the aim seems to be to keep the telescope largely out of the sun by keeping it behind the earth most of the time for better viewing of the "night sky."

No, that would deny the telescope of power. It will operate basically in constant sunlight, never being in the dark. In theory, it wouldn't need a battery since it has 100% sunlight, but there is one for redundancy and for the launch and commissioning process.

The telescope maintains the darkness of the optical side through the complex sun shield that was deployed.

The telescope is designed to orbit around the L2 point, thus never being exactly behind the Earth.

Mike C.

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 Post subject: Re: James Web Telescope
PostPosted: 13 Jan 2022, 04:44 
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Username Protected wrote:

The telescope is designed to orbit around the L2 point, thus never being exactly behind the Earth.

Mike C.


So is the purpose of a Lagrange orbit to keep the telescope at the same relative position to the earth? This, unlike an earth orbit, would allow it to look at the same point in the universe as we look at with seasonal, rather than constant changes.

I guess more importantly, it would be easier to keep it pointed away from the sun.

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 Post subject: Re: James Web Telescope
PostPosted: 13 Jan 2022, 08:27 
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So is the purpose of a Lagrange orbit to keep the telescope at the same relative position to the earth? This, unlike an earth orbit, would allow it to look at the same point in the universe as we look at with seasonal, rather than constant changes.

I guess more importantly, it would be easier to keep it pointed away from the sun.

The point is to give it a continuous, unobstructed view of the sky opposite the sun, and at the same time give it a clear view to transmit signals back to Earth. This could be done in other ways, but using a Lagrange point minimizes the boosts and fuel required to maintain its relative position. Orientation relative to the sun can be done using inertia wheels, which is how it is pointed anyway. The overall “hot side back, cold side away” orientation is a function of its rotation, the same way a spacecraft orbiting the Earth rotates at a rate that matches its orbital period so it doesn’t do flips relative to the surface as it orbits.

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 Post subject: Re: James Web Telescope
PostPosted: 13 Jan 2022, 14:52 
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Momentum wheels are more precise and a lot less disruptive (smoother) at maintaining orientation than firing thrusters. At the IR wavelengths they are using, I expect that the “shutter” will be open for a very lengthy period … and tracking corrections are required while imaging … thrusters are a poor solution for perch, track & stare.

Paging Dr. Apt. Dr. Apt to the white courtesy phone please.

Hubble is a derivative of NRO spy satellite tech. But NRO was not especially helpful and Team Hubble had to re-invented a bunch of tech.

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 Post subject: Re: James Web Telescope
PostPosted: 13 Jan 2022, 18:52 
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My highly educated scientific {sniff} comment after watching the video: This thing is really cool, unbelievably complicated, and strikingly pretty with the gold-plated reflectors and the hex designs.

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 Post subject: Re: James Web Telescope
PostPosted: 19 Jan 2022, 20:38 
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All of the primary mirror segments have been deployed from their launch positions. See the latest update at the project blog. The telescope is now about 95% of the way along its journey to L2.

Quote:
“Today, the James Webb Space Telescope team completed the mirror segment deployments. As part of this effort, the motors made over a million revolutions this week, controlled through 20 cryogenic electronics boxes on the telescope. The mirror deployment team incrementally moved all 132 actuators located on the back of the primary mirror segments and secondary mirror. The primary mirror segments were driven 12.5 millimeters away from the telescope structure. Using six motors that deploy each segment approximately half the length of a paper clip, these actuators clear the mirrors from their launch restraints and give each segment enough space to later be adjusted in other directions to the optical starting position for the upcoming wavefront alignment process. The 18 radius of curvature (ROC) actuators were moved from their launch position as well. Even against beryllium’s strength, which is six times greater than that of steel, these ROC actuators individually shape the curvature of each mirror segment to set the initial parabolic shape of the primary mirror.

“Next up in the wavefront process, we will be moving mirrors in the micron and nanometer ranges to reach the final optical positions for an aligned telescope. The process of telescope alignment will take approximately three months.”

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 Post subject: Re: James Web Telescope
PostPosted: 22 Jan 2022, 13:45 
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When it’s fuel supply is nearly all used up, will they use the last bit to blip it out of the L2 zone to keep that zone clear dead craft? Or do “we” think of that area as home builders though of the space in the wall below medicine cabinets back in the early 20th Century? If you’ve renovated an old bathroom, you know exactly what I mean.


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 Post subject: Re: James Web Telescope
PostPosted: 22 Jan 2022, 13:55 
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The telescope will get a final gentle push into its orbit at L2 on Monday. Details at the project blog.

Fun fact: "Webb’s orbit around L2 is larger in size than the Moon’s orbit around Earth!"

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 Post subject: Re: James Web Telescope
PostPosted: 22 Jan 2022, 14:37 
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Username Protected wrote:
Fun fact: "Webb’s orbit around L2 is larger in size than the Moon’s orbit around Earth!"

I think that's by design. It takes an orbit that large to remain out of the Moon's shadow at all times.


Thus there will be plenty of room left around L2 for other objects when Webb reaches it's end of life. IOW your bathroom wall will probably reach it's limit for razor blades long before there's not enough space around the Earth/Sun L2 for spacecraft.

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 Post subject: Re: James Web Telescope
PostPosted: 22 Jan 2022, 20:46 
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When it’s fuel supply is nearly all used up, will they use the last bit to blip it out of the L2 zone to keep that zone clear dead craft? Or do “we” think of that area as home builders though of the space in the wall below medicine cabinets back in the early 20th Century? If you’ve renovated an old bathroom, you know exactly what I mean.


Given that it's orbit is not stable in the L2 position (requiring fuel to stay there), when they run out of fuel it will fall towards Earth / sun. Also, as a post below yours indicates, the size of the orbit out there is huge, it's not like it is occupying a small zone.


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 Post subject: Re: James Web Telescope
PostPosted: 24 Jan 2022, 10:34 
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FYI it has been designed to be refueled if we want to.

https://www.lockheedmartin.com/en-us/ne ... space.html


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 Post subject: Re: James Web Telescope
PostPosted: 24 Jan 2022, 13:00 
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FYI it has been designed to be refueled if we want to.

https://www.lockheedmartin.com/en-us/ne ... space.html

I think Hubble could use something like this. I recall that it is nearly (or totally) out of fuel for positioning changes.

Dan


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 Post subject: Re: James Web Telescope
PostPosted: 24 Jan 2022, 13:03 
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FYI it has been designed to be refueled if we want to.

https://www.lockheedmartin.com/en-us/ne ... space.html

I think Hubble could use something like this. I recall that it is nearly (or totally) out of fuel for positioning changes.

I think Hubble is done. It's gradually failing components, and there is no way to service it. Previous repairs and upkeep were done via EVA from the Shuttle. We no longer have that capability.
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 Post subject: Re: James Web Telescope
PostPosted: 24 Jan 2022, 14:09 
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Username Protected wrote:
I think Hubble could use something like this. I recall that it is nearly (or totally) out of fuel for positioning changes.

I think Hubble is done. It's gradually failing components, and there is no way to service it. Previous repairs and upkeep were done via EVA from the Shuttle. We no longer have that capability.

Maybe Musk could combine a CyberTruk and Dragon for satellite repair.
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