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termsofsurrender t1_iu3crm8 wrote

This is the sort of national competition that's net positive. The Chinese developed new engineering techniques to make this 4.03 SiC mirror. Their new radio telescope has the largest aperture in the world.

Ah, if only the US would rise to meet the telescope gap. But no, we can't even scrape up a few million to rebuild Arecibo.


dreadpiratewombat t1_iu3hct4 wrote

> if only the US would rise to meet the telescope gap. But no, we can't even scrape up a few million to rebuild Arecibo.

Pardon my ignorance but isn't the reason because the utility of terrestrial telescopes inferior to space based ones and so investment is being optimised?


SpartanJack17 t1_iu3iyl9 wrote

No. Massive radio telescopes like FAST and Arecibo can't feasibly be built in space, and outside some exceptions they'd actually perform worse orbiting earth than on it, since they'd be exposed to more of our radio noise. If we could build over on the far side of the moon it'd be a lot better, but we're not capable of that right now.

And outside radio telescopes ground based visible light telescopes are still extremely important. Back when Hubble was launched atmospheric distortion was a problem for telescopes, but it isn't anymore. Adaptive optics technology allows it to be almost entirely corrected, and with no launch vehicle restrictions ground based telescopes can be built far bigger. Hubble and the James Webb Space Telescope are not the most powerful telescopes ever built, not by a decent margin. Hubble has a 2.4 metre mirror and JWST's is 6.5m, while the largest terrestrial trlescope currently in use is 10m.

And in the near future that'll go a lot further, with the ESO's Extremely Large Telescope coming online in the 2020s with a 39 metre mirror. Back when Hubble was launched space telescopes could get clearer images than ground based ones, but that's not the case anymore. These days the utility of space telescopes is in wavelengths of light that don't penetrate earth's atmosphere, primarily infrared. That's why Hubble was retrofitted for more infrared capabilities during it's servicing missions, and JWST is an entirely infrared telescope.


dreadpiratewombat t1_iu3lxye wrote

Thank you for the detailed, and really informative, reply. I learned a lot from you.


c4chokes t1_iu3qfmk wrote

2 different spectrum Buddy.. visible light and radio waves..


SpartanJack17 t1_iu3uxde wrote

Yes, I know. I addressed radio telescopes specifically in the first paragraph. But the comment I replied to was talking about telescopes in general, and that's what I was responding to.


Ard-War t1_iu3iw5a wrote

For radio telescope there's minimal incentive for space based telescope (beyond ludicrous baseline interferometry or very specific mission that is) since there's mostly negligible amount of atmospheric distortion.


alvinofdiaspar t1_iu4odi0 wrote

Not to diminish this particular achievement but that’s kind of a weird take - considering JWST. Also SiC was used for the 3.5m mirror on ESA’s Herschel (Far IR) telescope.


termsofsurrender t1_iu4sz3h wrote

You're right. JWST is an amazing achievement. Herschel's mirror was comprised of smaller mirrors, while the PhysOrg article says that the Chinese made a record-breaking 3m blank.


mfb- t1_iu3eer0 wrote

> silicon carbide

That's SiC.

Did they just made this as demonstration object? I don't see a specific telescope being mentioned.


tommos OP t1_iu3emtf wrote

I think it's for the Xuntian space telescope they're launching at the end of 2023.


mfb- t1_iu3ezc6 wrote

They made a 4 meter mirror with a hole in the center while the telescope will have a 2 meter mirror with an off-axis design (no hole in the center). But the material is the same in both cases.


Arcosim t1_iuaey39 wrote

Xuntian has a 2m mirror, and Earth 2.0 (the space telescope China is launching in 2026 into L2) is an array of six smaller telescopes sharing the same superstructure. So if this mirror is used it'll be used in another not currently announced space telescope.

Which makes sense, because Xuntian is a survey telescope (with a modest mirror but huge 2.4 gigapixel sensor) and Earth 2.0 is an exoplanet finder (5 IR telescopes plus a microlensing telescope). Which means there's a gap right in the middle for a depth of field telescope (think Hubble). This mirror would be perfect for it.


ThickTarget t1_iu3y0k7 wrote

The article states that it was delivered to a customer in 2019, but nothing further. Not heard anything about a 4 meter before. I looked around and all I can find is one SPIE paper presenting an instrument for an upcoming Chinese 4 meter, but it doesn't actually name the telescope.