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Mark McCaughrean Profile picture
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8 Jan, 4 tweets, 3 min read
And … as one third of you got right, the correct answer is “Glass” 🙂

Yes, even though the beryllium mirrors of #JWST are coated with a highly infrared reflective 100nm layer of gold, that in turn is coated with a thin layer of SiO2 (aka silica) to protect it from dings.

1/
Gold is soft & easily scratched, hence the overcoat. Silica is used in many applications, but in this form, it’s reasonable to refer to it as glass.

Of course, almost all photons hitting #JWST pass straight through before hitting & getting reflected by the gold, but still.

2/
A little more information here

jwst.nasa.gov/content/observ…

and here

laserfocusworld.com/test-measureme…

and thanks again to @apolitosb for posing the question. What we haven’t found out yet is how thick the SiO2 layer is – probably similar to the 100nm of gold, but more exactly … 🤷‍♂️

3/3
Coda: I’m assuming that everyone in the 35% that got it right did know it’s just a coating, rather than thinking the mirrors are made of glass 😬

That said, large (2m), thin (2mm), flexible, glass mirrors *were* tested for NGST/JWST way back when 😉

loft.optics.arizona.edu/documents/jour…

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More from @markmccaughrean

Mark McCaughrean Profile picture
8 Jan
As the start of the last major #JWST deployment approaches, the starboard primary mirror wing, it's time for a thread about what that helps enable – excellent spatial resolution.

It's #SharpnessSaturday (yes, the hashtag symbol also denotes a "sharp" in music 🙂)
So what do we mean by "spatial resolution"?

It's a way of quantifying the sharpness of an image scene, the amount of detail visible at small scales, or at some rather fundamental level, how close two things can be in a scene & still be separated.

2/ Image
For astronomers, that's often simplified to saying "how close can two stars of equal brightness be on the sky & still separable or resolvable?"

That's not to say the stars need actually be close in space, but just how they appear on the sky.

3/
Read 26 tweets
Mark McCaughrean Profile picture
7 Jan
Today’s the day – #JWST starts spreading the wings around its eyes 👀

And yes, I know this joke would make more sense if this was a pit viper rather than a cobra, but they don’t have a deployable hood 🐍🤷‍♂️

1/
That is, cobras are members of the elapid family of snakes, whereas pit vipers, including rattlesnakes, are from the crotaline family.

And what pit vipers share with boas & pythons is an ability to sense the infrared, which cobras lack.

2/
Not with their eyes, but via specialised “pit organs” near their snout. These hold a thin membrane with many nerve endings & blood vessels: the former sense infrared light (aka heat) between 5 & 30 microns, & the latter cool the membrane to refresh it.

en.m.wikipedia.org/wiki/Infrared_…
Read 14 tweets
Mark McCaughrean Profile picture
6 Jan
Morning. As we near the end of #JWST’s deployments (& how mad is that?! 😱), the big focus (😉) will naturally be on the primary mirror wings swinging into place 🔭

But don’t forget the aft deployable radiator, key to the instruments keeping their cool as they do science 😎

1/
Unfortunately I don’t have time today for a megathread on the physics of the radiator, as I’m going to be driving my son back to university in Groningen.

But there’s one equation to keep in mind with the ADR: sigma.A.T^4

2/
That’s the amount of power radiated by a black body in Watts & can be used to determine how big that radiator & others on the roof of the Integrated Science Instrument Module (ISIM) need to be to shed the heat generated by the instruments.

3/
Read 12 tweets
Mark McCaughrean Profile picture
2 Jan
As we wait for the #JWST sunshield tensioning to begin after a day of well-deserved down time for the mission team, let’s talk in a bit more detail about how having a big, cold telescope helps us detect faint things.

Yes, folks, it’s Signal-To-Noise Sunday 😬

1/
Now, a health & safety warning – this could get a bit technical, maybe mathematical, & possibly even quantum mechanical 😱

I don’t know – I haven’t planned this thread at all, so it’ll just come out as a stream of consciousness. But hopefully a semi-intelligible one 🤷‍♂️🙂

2/
In the optical & infrared parts of the electromagnetic spectrum (i.e. light), as well as at higher energies like the ultraviolet, X-rays, & gamma rays, we typically thing of it comprising photons, individual packets or quanta, like little bullets of energy.

3/
Read 38 tweets
Mark McCaughrean Profile picture
1 Jan
With the huge #JWST sunshield now successfully pulled out (I can’t believe I’m actually typing that after so many sleepless nights 🤯), the next step is to tension it into its proper shape.

That’s crucial too, so *our* tension is by no means over yet 😨

1/
As my long thread yesterday described, the sunshield plays the key role in establishing a temperature difference of ~300°C between the sun & space-facing sides of the observatory.

Only when the cold side reaches 40K or -233°C do we have the infrared performance we desire.

2/
So far though, the five wafer-thin metallised Kapton layers are in a relatively floppy state & touching each other. That means they can conduct heat & thus the cold side can’t, well, get very cold.

Thus the layers have to be separated & held apart from each other.

3/
Read 19 tweets
Mark McCaughrean Profile picture
31 Dec 21
Huge day ahead for the NASA/ESA/CSA #JWST, with the scheduled deployment of the so-called midbooms, which extend out to the side of the spacecraft.

These will pull out the five-layer, tennis court-sized sunshield, critical to the cooling of the observatory.

1/
By the way, the clips I’m posting each day of the deployment come from this full video by NASA:



2/
Until now, the sunshield has been carefully folded up in a zig-zag fashion, held down under the sunshield covers that were rolled back yesterday & in the pallets that were folded down away from the telescope earlier in the week.

3/
Read 21 tweets

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