A deployable, annular, 30m telescope, space-based observatory [pdf] | Hacker News Reader

A deployable, annular, 30m telescope, space-based observatory [pdf]

northropgrumman.com

60 pointsmarcodiego22 comments

Looks over at an old copy of Ringworld on the shelf and wonders what kind of imaging one could do of neighboring star systems if you put mirrors around the edge of the ring.

We sort of do that with radiotelescopes, we place them all over the world and combine the signals, analogously to them being part of one primary mirror.

We also take images when the Earth is at opposite ends of its orbit and combine those.

Good reason to check your fly is zipped before you walk outside.

Use the sun as a lens and you won't need a megastructure.

It’s gravity is too weak. It only deflects stars optically near the edge of the corona by a very small amount.

Funny, all the simulations point to the wrong direction (ground view vs space view). I assume this design is intended for surveillance, not astronomy.

> wrong direction

Why wrong? This is a project proposed by Northrop Grumman, a defense company. It would be wrong for them to look at the stars, rather than down to Earth.

The JWST mirror was made of solid beryllium isogrid substrate, with gold sputtered on to the front. Each mirror had to be individually polished to a very precise shape before coating.

This proposed mirror is a foil of zirconium copper that is sputtered onto a glass mandrel and then epoxied to a silicon carbide Isogrid. The advantage is you only have to accurately polish the one glass mandrel, the substrate need not be super accurate.

Anyone know if this method is really better than using solid beryllium, or is this just Northrop Grumman marketing?

Not sure how relevant that is, but JWST will operate in quite extreme setting. The mirror has to be cooled to 44K IIRC because it is trying to observe in IR, and this is the main reason why JWST is so complex in terms of engineering.

The abstract at least doesn’t mention IR observation, in eg visible spectrum JWST would surely be built differently (more cheaply).

EDIT in fact they do reference JWST but I guess only as a point of comparison, as it does have a large mirror. Their parameters table shows observations would be in visible spectrum (450-750nm).

A sunshield big enough to shade a 30m by 10m space telescope would have to be darn large. Radiation pressure would certainly want to push it out of orbit. Space telescopes with big mirrors seem to be sited at L2, like LUVOIR. https://en.wikipedia.org/wiki/Large_Ultraviolet_Optical_Infr... I assume some property of halo orbits tolerate hanging off a solar sail better.

Hubble is an unshaded warm-mirror telescope that can observe down to 1700nm, though, so I wonder why they wouldn't use that as comparison.

I'm nor sure - does the big mirror have to be cooled all the way, or just a bit, and what needs to be cooled are the more compact fiddly bits in the middle ?

Also, I wonder since the L2 spot is theoretically (partly) shaded by the earth, how much Watts/m^2 do you avoid ?

An object in a halo orbit spends most of its time out of Earth's shadow: https://en.wikipedia.org/wiki/Halo_orbit#/media/File:Animati...

Hubble used cooled instruments, warm mirror. If you only partially cool a mirror then thermal expansion effects will mess up the focus. In the paper they propose heating segments that are shadowed, to keep the whole mirror at the same temp.

Yes, one of the main drivers behind using the beryllium substrate was the thermal stability required to operate at cryogenic temperatures.

A properties comparison between Be and the paper's SiC ceramic is here [1].

Be is lower density, which would matter for payload mass. The SiC material has a much lower thermal expansion coefficient and change of CTE with temperature, both of which should reduce thermal deformation.

[1] https://www.northropgrumman.com/wp-content/uploads/CERAFORM_...

The paper contrasts the deployable annular telescope to JWST. However, all of the performance discussions are in relation to Earth imaging from a geo-sync orbit.

Not surprised because it’s clearly derivative of the sort of huge antenna trickery they have to use on the massive sigint satellites which tend to be Geostationary adjacent to either just let them loiter over a hemisphere, or conveniently drift in front or behind important geostationary assets of other countries in order to Hoover up stray RF signals with their massive dishes in order to perform as much analysis on it as possible since unless you broadcast 24/7 white noise mixed with encrypted traffic up and down at a constant power level, they are going to be able to work out … something… from the RF which will make it worth their while.

These are huge beasts with dishes of 100 meters in the previous generation of MAGNUM satellites and over 100 meters in the latest MENTOR satellites… of course these numbers are based on the usual educated rumour mill sources necessary with any spy satellite information that has yet to be declassified, and the entire SIGINT family of satellites going right back to almost the very first is as black as pitch, it’s code name only educated guesswork and a couple of rare leaks of surprisingly inconsequential information. But the dish sizes are one of the more consistent bits of information so it’s a fairly safe bet these lower bound numbers are accurate.

If you make your telescope big enough, you can use a collection of optically flat mirrors.

Maybe put them on independently orbiting satellites that station-keep with ion rockets. Or, in a space frame formed of nanowires, with the nodes individually charged electrically to repel one another, maintaining the wires in tension.

Oh, Proceedings of SPIE. I wasted more time in college rifling through those glorious yellow tomes in the solace of the engineering library than I did getting drunk.

Let's just stop deploying them. It'll be easier to assemble them from multiple launches soon.

Starlink is basically a 6000 km big radiotelescope.

But pointing the wrong direction? I guess it's unlikely any of these satellite constellations will be put to use as telescopes since there isn't much business case for astronomy...

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