[HN Gopher] A deployable, annular, 30m telescope, space-based ob...
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A deployable, annular, 30m telescope, space-based observatory [pdf]
Author : marcodiego
Score : 55 points
Date : 2022-01-03 15:56 UTC (7 hours ago)
(HTM) web link (www.northropgrumman.com)
(TXT) w3m dump (www.northropgrumman.com)
| rahen wrote:
| Funny, all the simulations point to the wrong direction (ground
| view vs space view). I assume this design is intended for
| surveillance, not astronomy.
| credit_guy wrote:
| > 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.
| dr_orpheus wrote:
| 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.
| techdragon wrote:
| 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.
| ajuc wrote:
| Starlink is basically a 6000 km big radiotelescope.
| blacksmith_tb wrote:
| 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...
| opwieurposiu wrote:
| 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?
| rich_sasha wrote:
| 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).
| dr_orpheus wrote:
| Yes, one of the main drivers behind using the beryllium
| substrate was the thermal stability required to operate at
| cryogenic temperatures.
| sbierwagen wrote:
| 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.w
| ikipedia.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.
| max-ibel wrote:
| 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 ?
| sbierwagen wrote:
| 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 instrument, 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.
| ridgeguy wrote:
| 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_...
| yodon wrote:
| 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.
| ScaleneTriangle wrote:
| Use the sun as a lens and you won't need a megastructure.
| simonh wrote:
| It's gravity is too weak. It only deflects stars optically
| near the edge of the corona by a very small amount.
| noduerme wrote:
| Good reason to check your fly is zipped before you walk
| outside.
| rich_sasha wrote:
| 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.
| simonh wrote:
| We also take images when the Earth is at opposite ends of its
| orbit and combine those.
| DrFell wrote:
| 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.
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