Amazing James Webb Space Telescope

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Lou Jost
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Re: Amazing James Webb Space Telescope

Post by Lou Jost »

The acceleration due to dark energy is indeed very weird, but as presently understood it's not the primary reason for the redshift
That's right. Overall, the increment in redshift due to acceleration is currently almost undetectable, less than 1 part in a billion relative to the redshift one would expect under constant expansion.

The universal redshift, increasing linearly with distance from us, has been known since Hubbell's time (the person, not the telescope) and is what led to the conclusion that the universe is expanding. It has only recently become possible to measure extremely distant objects and detect a slight non-linearity.

MarkSturtevant
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Re: Amazing James Webb Space Telescope

Post by MarkSturtevant »

rjlittlefield wrote:
Mon Jan 24, 2022 2:35 pm

All that is correct, except the part about "Because of acceleration". That should read "Because of expansion of the universe".
--Rik
Agreed! Thanks to both you and Lou.
In other news, the amazing contraption has now inserted itself into its L2 orbit. Woo-hoo!
Mark Sturtevant
Dept. of Still Waters

Chris S.
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Re: Amazing James Webb Space Telescope

Post by Chris S. »

Pau wrote:
Mon Jan 24, 2022 12:47 pm
...but it says Wavelength µm. 5µm are 5000nm, well into the IR, not 500nm, maybe a typo .
This threw me off at first, too. Robert's particular link is to the Webb's "Mid-Infrared Instrument." However, try clicking on that Website's "home" button, and choose Near Infrared Camera, then look at "Table 1. NIRCam observing mode parameters." It includes the following:

Webb-2.jpg
So imaging will include light at wavelengths as short as 0.6 microns, or 600 nanometers--so orange and red portions of the visible spectrum, by my read.

Must confess I find it unnatural to discuss light wavelengths in microns, rather than nanometers--but for most of the wavelengths of interest to the Webb telescope, microns is the more convenient unit. (Unrelatedly, when every once in a while, someone discusses light in terms of angstroms, I think, "would it be easier to use tenths of nanometers?")

--Chris S.

Lou Jost
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Re: Amazing James Webb Space Telescope

Post by Lou Jost »

So imaging will include light at wavelengths as short as 0.6 microns, or 600 nanometers--so orange and red portions of the visible spectrum, by my read.
Chris, those numbers refer to center wavelength, and the spectral width of the passband for the visual filters is very wide. See the graph that started this thread. The telescope can see down to 500nm.

rjlittlefield
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Re: Amazing James Webb Space Telescope

Post by rjlittlefield »

Lou Jost wrote:
Mon Jan 24, 2022 6:00 pm
So imaging will include light at wavelengths as short as 0.6 microns, or 600 nanometers--so orange and red portions of the visible spectrum, by my read.
Chris, those numbers refer to center wavelength, and the spectral width of the passband for the visual filters is very wide. See the graph that started this thread. The telescope can see down to 500nm.
Lou, if you see something that says 500nm, I would like to know an exact pointer to that information.

The graph that I see at top of this thread is for the Mid-Infrared Instrument, for which the shortest wavelength filter is the F560W, which is described at https://jwst-docs.stsci.edu/jwst-mid-in ... ingfilters as λ0 (μm) 5.6 and Δλ (μm) 1.2, meaning that it covers the band from 5.0μm to 6.2μm, same as what is shown in the graph.

The instrument that gets into visual is the Near Infrared Camera, described at https://jwst-docs.stsci.edu/jwst-near-infrared-camera . That is described in text as "observes from 0.6 to 5.0 μm" and the shortest filter as shown at https://jwst-docs.stsci.edu/jwst-near-i ... ra-Filters is the F070W which covers roughly 0.6-0.8 microns as shown in the graph on that page (which is not the one shown at top of this thread).

Everything I read is the same as Chris S., that the scope only sees down to 600 nm. I also see only one band in the visual, for the F070W in the Near Infrared Camera. The next band would be the F090W, which is graphed as starting at 0.8 microns.

--Rik

Lou Jost
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Re: Amazing James Webb Space Telescope

Post by Lou Jost »

Yes, you and Chris are right, I made a units error when looking at the graph. Sorry. I was using the Hubble filter conventions.

Pau
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Re: Amazing James Webb Space Telescope

Post by Pau »

Chris, Rik, thanks for the clarification! No typo, just insufficient reading by my part #-o

The colours assigned to the wavelengths do show a curious "violet shift", the shorter wavelength is represented in violet despite corresponding to 600nm or to 5600nm in the respective graphs :lol:
(I know, IR has not colour, and colours look nice in graphs)
Pau

rjlittlefield
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Re: Amazing James Webb Space Telescope

Post by rjlittlefield »

I think the colors are just designed to be evocative of shorter versus longer wavelength.

For me this is a helpful and reasonable ordering, especially since even the shortest wavelengths shown in the graph are over 6X longer than anything that is visible.

--Ri

rjlittlefield
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Re: Amazing James Webb Space Telescope

Post by rjlittlefield »

Telescope watching telescope...

https://www.virtualtelescope.eu/2022/01 ... -jan-2022/
The James Webb Space Telescope has finally reached the L2 point and we imaged it.
<image omitted here>
The image above comes from a single 300-second exposure, unfiltered, remotely collected with the “Elena” (PlaneWave 17″+Paramount ME+SBIG STL-6303E) robotic unit available at the Virtual Telescope Project. Our robotic telescope tracked the apparent motion of the James Webb Space Telescope (JWST), which is marked by an arrow in the center.
--Rik

ModelZ
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Re: Amazing James Webb Space Telescope

Post by ModelZ »

Overview of the alignment, now on its way:
blogs.nasa.gov/webb/2022/02/03/photons-incoming-webb-team-begins-aligning-the-telescope/

rjlittlefield
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Re: Amazing James Webb Space Telescope

Post by rjlittlefield »

First image today, of the initial alignment mosaic: "Photons Received: Webb Sees Its First Star – 18 Times"

https://blogs.nasa.gov/webb/2022/02/11/photons-received-webb-sees-its-first-star-18-times/

--Rik

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Re: Amazing James Webb Space Telescope

Post by rjlittlefield »

Interesting video regarding the mirror positioning devices:
"JWST Mirror Actuators are AMAZING!" https://www.youtube.com/watch?v=5MxH1sfJLBQ

Short summary: each actuator contains one stepper motor plus some gears, one drive screw, a cam, and a flexure, giving linear positioning with a fine range of 10 microns by 7.7 nanometers, piggybacked on a coarse range of 21 mm by .058 microns.

Detailed paper at https://www.esmats.eu/amspapers/pastpap ... warden.pdf .

--Rik

ModelZ
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Re: Amazing James Webb Space Telescope

Post by ModelZ »

Nice video, thanks. Brilliant devices these hexapods. Yet I'm still dumbfounded, how/how many of these they (the design engineers) expected to survive the shock of the launch? I suspect there must be a fair amount of redundancy built in. Will they perhaps completely "decomission" a mirror segment if they cannot get aligned? Aim it completely off the secondary mirror. Or perhaps they can track the spurious image in full & just filter it off in post processing.
Another thing that I've wondered. Kourou is in rain forest. So a flight from very high humidity to deep subzero temperatures in minutes. A text book way to get all all kinds of electical and mechanical condensation problems! I think a tight enough payload fairing to keep the humidity out would be way too heavy. How did they deal with the humidity issues?
Amazing engineering, a pleasure to learn about these things.

rjlittlefield
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Re: Amazing James Webb Space Telescope

Post by rjlittlefield »

ModelZ wrote:
Sat Feb 12, 2022 7:56 am
Another thing that I've wondered. Kourou is in rain forest. So a flight from very high humidity to deep subzero temperatures in minutes. A text book way to get all all kinds of electical and mechanical condensation problems! I think a tight enough payload fairing to keep the humidity out would be way too heavy. How did they deal with the humidity issues?
I think the big issue with condensation would be if they were going the other way, taking cold equipment into high humidity.

Going the other way, during launch, I expect the humid air would vent well before the equipment had a chance to cool off.

Getting the mechanics to work in such a wide range of temperatures is still a problem. For that they used some fancy dry lubricant, as explained in the paper I linked.

It's a bit aside, but I was interested to note this section of the paper:
Gearmotor Life
The life requirement for the gearmotor was
estimated using expected operational cycles for the
life of the unit. A total of 1.7 million motor
revolutions were estimated. The dry film lubrication
in the gearmotor has been analyzed to last at least
3 million cycles.

The gearmotor has been noted as a life limited item
and motor revolutions must be recorded.
--Rik

RobertOToole
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Re: Amazing James Webb Space Telescope

Post by RobertOToole »

ModelZ wrote:
Sat Feb 12, 2022 7:56 am
......
Another thing that I've wondered. Kourou is in rain forest. So a flight from very high humidity to deep subzero temperatures in minutes. A text book way to get all all kinds of electical and mechanical condensation problems! I think a tight enough payload fairing to keep the humidity out would be way too heavy. How did they deal with the humidity issues?......
There is that issue and from what I've read the temp swings in space are brutal. With sun exposure the radiant energy will heat an object to +225 or +250 Deg. F. in seconds but block the radiant energy and it goes down to -200 deg F in no time at all. So +250 in the sun and -250 in the shade?

Speaking of humidity, didn't the humidity in Florida (of course) cause one of the Space-X launches to be cancelled or aborted once?

Best,

Robert

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