Nice #JWST piece by @MonicaGrady, including a welcome nod to the role played by Europe & Canada, alongside the US.

One minor nitpick though (forgive me, Monica 😬), but the last thing the primary mirror segments are is flat. Very smooth, but not flat 🙂

theguardian.com/commentisfree/…
I mean, in this case I’m sure it’s just a slight misphrasing, but it is interesting that many people don’t seem to realise that the primary mirror isn’t flat, but is very precisely curved in a concave parabolic shape to focus light towards the secondary mirror.
There are 18 segments in the primary & they’re not all identically curved, though. There are three different families with the same shape (A, B, C) depending on the distance of the segment from the centre, as you can see in this old chart made during polishing.
While they’re not flat, they are very precisely curved across their full size & very smooth, with an average roughness of 20 nanometres across the surface.

Scaled up, if Earth were that smooth, the tallest mountains would be less than a metre high.

Bit like The Netherlands 🙂
There’s a lot more here about how the primary mirror segments were made out of beryllium, shaped, light-weighted, polished, tested, & coated in this nice article 👇

webb.nasa.gov/content/observ…
And to finish for now, it’s worth noting that the secondary mirror, where that curved primary focuses all its light, is also not flat.

It’s not concave either, but convex 🙂

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More from @markmccaughrean

Jul 18
While semantically correct, this headline sensationalises reality.

Yes, the micrometeoroid hit on #JWST's C3 primary segment caused permanent damage.

But the *effect* of that damage is minimal & the telescope remains within its design specifications.

Very well within.
The article itself is a bit more nuanced, but still overplays the effect. If you go to page 23 of the observatory commissioning report, you'll get the balanced picture.

stsci.edu/files/live/sit… Image
We know that #JWST will be hit by micrometeoroids in its L2 orbit – it's inevitable.

We also know that we cannot protect the telescope from them with a tube around the primary as some believe – the telescope would not cool to 40K as required to fulfil its scientific mission.
Read 11 tweets
Jul 16
Who remembers my “adventures” with Avis at Frankfurt Airport?

They mixed my contract for someone else’s, someone who got caught speeding several times & I ended up with their police fines.

Took months & lawyers to sort out.

Well, let’s talk about Munich Airport today 🙄

1/
Of course, I didn’t rent with Avis this time – that’s never happening again, so I switched to Hertz.

I picked up a car from them at Munich Airport on Thursday, drove it to Alpbach in Austria, & returned it today.

2/
(Sorry for the break there – doing the security shuffle & walking across Munich Airport’s cavernous spaces.)

3/
Read 19 tweets
May 7
Finally caught a glimpse of some of the European bison that roam around the Zuid-Kennemerland national park just north of Zandvoort 🦬🙂

This pair were splashing around in a small pond, then wandered off into the dunes.

As seen during this evening’s 90km ride.

#cyclinglife 🚴‍♂️
The Kraansvlak herd of bison or “wisent” have free run of quite a large area of the dunes & are monitored as part of a rewilding research project. Despite the old idea that they’re predominantly forest animals, they do very well in this setting.

wisenten.nl/en
Some of the bison are fitted with GPS collars & you can check this map to see where they are. I did that when I reached Zandvoort, saw that some were close to one of the bike paths that go through the park, so I went looking.

wisenten.nl/en/where-are-t…
Read 7 tweets
Mar 30
Perhaps the most distant known star, detectable by Hubble thanks to gravitational lensing.

Or is it a star in our own Milky Way that just happens to lie on the lensing caustic?

The good news: #JWST will get a spectrum & tell us one way or the other 🤷‍♂️

bbc.com/news/science-e…
The authors have ruled out the latter by suggesting that a star in our own galaxy would likely have moved a bit in the 3.5 years since its discovery.

Perhaps, but in a semi-random distribution of motions, not all stars move tangentially; some will move mostly radially.
That’s the problem with the detection of a single object – statistical arguments about the possibility of finding such & such an object right at a given spot are not always going to be kind to you. It depends on how many places you’ve looked for such objects as well.
Read 11 tweets
Jan 24
Venus & Mercury, closer to the Sun, orbit more quickly, in 225 & 88 Earth days, respectively.

Mars is further out & takes 687 days to orbit; Jupiter 4332, Saturn 10759, Uranus 30685, & Neptune 60,189 days.

Ditto for non-planets like Pluto. Cough 😳

Image: Wikipedia/WP

18/
The relationship between the distance of a planet from the Sun & its orbital period was discovered by Johannes Kepler.

His third law says the square of the orbital period of a planet is proportional to the cube of its orbital semi-major axis.

19/

So, if your orbit is a bit further from the Sun than Earth's, you'll orbit a bit more slowly.

So, at 1.5 million km further out than Earth (i.e. 1% larger than Earth's orbit), you'd take 1.5% longer to go around the Sun, i.e. 370.74 days.

20/
Read 27 tweets
Jan 24
A few hours from now, #JWST will make its Mid-Course Correction 2 (MCC-2) burn, injecting it into its operational orbit around the Sun-Earth L2 point, ~1.5 million kilometres away.

What, why, how, when?!

A thread.

1/
First, a reminder: #JWST was launched on #Ariane5 #VA256 from Europe's spaceport in French Guiana on 25 December 2021. The #Ariane5 put it on a near-perfect trajectory towards L2 & two subsequent JWST Mid-Course Corrections have tweaked that.

2/
But why such a long journey to a place that's about four times further away from Earth than the Moon?

By contrast, the Hubble Space Telescope is in a low Earth orbit ~535km above the surface, making it accessible to several servicing missions over the past 31 years.

3/
Read 19 tweets

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