James Webb Telescope.

[Mr. Dazzle]

Well the one on the right is taken in IR, which we can't see, but there it is being all visible and stuff. So clearly something is going on

[Yambo]

Well the one on the right is taken in IR, which we can't see, but there it is being all visible and stuff. So clearly something is going on

I have an LED torch that I can put an IR LED in to replace a white or red LED. With a special scope fitted to my air rifle and the target area illuminated by the IR torch I can see the target area reasonably clearly. Not as good as with my own eyes in daylight but pretty good.

That's what's going on.

[Yambo]

The JWST is now fully deployed and around 82% of the way to L2 where it will be put into an orbit around the L2 point. It all looks as if the deployment has gone flawlessly.

https://webb.nasa.gov/content/webbLaunc ... sWebb.html

[Mr. Dazzle]

82% of the distance but only half the time. Poor old thing seems to be losing enthusiasm.

[Yambo]

It's going uphill and slowing down!

[Kneerly Down]

Whichever is used, they all seem to suffer the starburst effect like an early 1980s Top of the Pops!
Is it added or is it as a result of diffraction from the mountings of the mirror/instruments?

[Yambo]

Whichever is used, they all seem to suffer the starburst effect like an early 1980s Top of the Pops!
Is it added or is it as a result of diffraction from the mountings of the mirror/instruments?

It's diffraction from the secondary mirror supports - they're called diffraction spikes. They're a feature in Newtonian telescopes and maybe some other reflectors but you don't get them with refractor telescopes.

Some people like them, others don't. Personally I dislike them (and I only have refractors). Photo's of star clusters and the Pleiades for example look awful with them. They can be avoided when taking images apparently but doing that causes a loss of contrast.

[Mr. Dazzle]

They're also an artifact of the CCD (the camera chip). When pixels become saturated they bleed out into their neighbours. Really bright spots then end up with the 4 arns.

Thats why the arms are at exactly 90 to each other, that's the direction the rows and columns go in the CCD.

AIUI defraction spikes would tend to effect everything in the shot whereas CCD effects are limited to the brightest points.

[Yambo]

They're also an artifact of the CCD (the camera chip). When pixels become saturated they bleed out into their neighbours. Really bright spots then end up with the 4 arns.

Thats why the arms are at exactly 90 to each other, that's the direction the rows and columns go in the CCD.

AIUI defraction spikes would tend to effect everything in the shot whereas CCD effects are limited to the brightest points.

It's much less of a problem with CMOS sensors which are a lot more common in cameras nowadays. You can still buy astronomical cameras with CCDs but they're more expensive and as everything is expensive to start with . . .

[Mr. Dazzle]

Hubble is CCD though AFAIK. I think JWST is too, not sure.

[Yambo]

I think CCDs are the choice for professionals and scientific instruments and they've improved over the years. Shorter exposure times will limit the effect of saturated pixels.

My understanding is the JWST has IR detectors rather than CCD cameras. They're pretty special it seems https://webb.nasa.gov/content/about/inn ... rared.html

[Screwdriver]

I am somewhat obsessed by the sheer engineering perfection of this thing. So much so I have dedicated this year (so far) in creating a 3D printable model. Such things exist but imho, they'e all shit.

Here's my attempt so far:



It does fold up too:



But I am struggling to find a decent solution for folding up the sun screen. The original version is aluminium coated kapton tape and frankly, that's the thinnest material I could use for this 40:1 model! I will of course make a fully deployed version but having a folding screen is going to be tricky....

So far so good.



Waiting for some more PETG filament before I can print the main event. It will end up being a smidge over half a metre long.

[Screwdriver]

The IR debate above is missing the point entirely. Ordinary common or garden visible light has been stretched out by the expansion of spacetime over the vast oceans of time those photons have been travelling here. What was once good old fashioned visible white light has been red shifted and the galaxies at the very edge of the observable universe have been shifted into the deep infra red.

The crazy clever part is actually the cooling in the detectors if you ask me. Those clever boffins invented a new type of fridge which works on the principle of acoustic compression. Almost a perfect example of Maxwells demon in action. It shoves the hot compressed gasses out one end and keeps the cold rarefied gas at the other. Pretty damned smart and I think it's the very first example of such a thing??

Incidentally, when you look at a star, your eyes are detecting a million photons per second (ish). JWST will be lucky to capture ONE photon per second from the edge of time.

The engineering talent that has put this thing together makes me proud to be human. What an amazing achievent.

[Mr. Dazzle]

The real thing uses aluminum coated kapton too AFAIK, thinner than the stuff you can buy. So you're gonna struggle to get that bit to scale

Most satellites use loads of aluminised kapton. It looks like gold foil but it's actually browny orange kapton coated in silver aluminium on one side.

[Mr. Dazzle]

The IR debate above is missing the point entirely. Ordinary common or garden visible light has been stretched out by the expansion of spacetime over the vast oceans of time those photons have been travelling here.

Yes, exactly. It's not that big a leap to 'play jr back at double speed' and convert what JWST sees back into what it would have looked like when "new". In fact you pretty much have to do that (via spectroscopy) to figure out how much stuff has red shifted.

[MingtheMerciless]

I've just been reading the blog, they're unshipping the mirrors from their stowed launch position, a whole 12.5mm! This is at 1mm per day for all the segments on the mirror as they have to do its in steps to stop the actuators and motors putting too much heat into the mirrors. Apparently when they're focussing the segments the movements will be on the nanometer scale.

https://blogs.nasa.gov/webb/

[Mr. Dazzle]

I'm sure I heard the fine adjustment movements are at the same speed at which grass grows.

[Screwdriver]

Speaking of things progressing slowly, I have made some progress.



...and it folds correctly.



Some progress on the mid boom arms. I think it needs to be telescopic in the model and that is trickyfor all sorts of complicated reason, not only due to the tiny print scale even for a biggish model.

Looks a LOT better in real life and things will improve tremendously once I get some gold on there. Plus these are all quick prints at around 6 hours each for the bigger bits. Once I'm happy with the design, I'll stick it on "fine" and go watch some grass grow.

[Screwdriver]

[Yambo]

I'm going to be really disappointed if you don't coat those mirror segments with beryllium.