China's Space Program News Thread
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China's large aperture space telescope has entered formal engineering stage. Deployment may begin as early as 2025. The telescope will be assembled in space.
They can use this as a blue print for how to assemble spaceships in orbit.
The team behind the space station robotic arm.
Amazing work by this team.
Cannot wait for this to be a reality.China's large aperture space telescope has entered formal engineering stage. Deployment may begin as early as 2025. The telescope will be assembled in space.
Brain and beauty in one package!
and the vitality of YOUTH too.
Indeed, if completed this will be an extraordinary example of space construction.
Having done some amateur astronomy with a small Newtonian telescope let me explain why it's hard: that big mirror is not spherical but rather a section of a parabola. The parabolic shape allows incoming light (which are travelling parallel since it comes from distance measured in light years) to be focused to the detector at the focal point. Using a spherical mirror instead of a parabolic mirror will cause a major optical defect in focusing called "spherical aberration" which will make this telescope useless.
Because the primary mirror is a section of a parabola it means all the mirror segments are actually different and not interchangeable. Not only that when you assemble a segmented primary mirror each segment have to be aligned accurately to a small fraction of the the wavelength of the light you're measure (in other words, the mirror has to look "smooth" on the scale of visible light photons), so for visible light we're talking accuracy of around 10 nanometers. If a segmented mirror is not correctly aligned it's as if a single big glass mirror has lots of bumps in it which will not be acceptable.
For that sort of accuracy it means both the overall mirror segments have to be aligned to quite good accuracy, then each mirror segment have to be further finely aligned using machinery built into the back of them called "active optics" which basically means lining up the back of the mirror with lots of tiny actuators that can adjust them.
JWST for example uses this technology and has seven actuators per mirror segments (one per corner and one in the middle, with 18 segments all together). This big Chinese telescope will need at least 7 per segment as well. Note that these nanometer accurate actuators can only move very tiny distances, so the overall mirror assembly still has to be done to very high degree of accuracy first.
You might hear the world "adoptive optics" when dealing with reflecting telescopes. This is a similar technology but involves much more actuators per mirror segments. These actuators are not only responsible for tilting the segments to align them, but also flexing the mirror by tiny amount (and adjusting the flex in real time) to compensate for atmosphere seeing. Since space telescopes are above the atmosphere they do not require this much more elaborate system.
Official response regarding photos from Martian surface:
On the day of the landing the lander will be talking to earth via direct communication. In this mode the bandwidth is 16bps, so 2 bytes per second. At this speed the comms is only sufficient for judging spacecraft status and not enough for transmission of photos. A 100KB uncompressed low resolution photo will take 14 hours to transfer at this rate, and this is not taking into account Zhurong's own bandwidth requirements. 3 Sols after landing (Monday night, 17/05/2021) the rover will attempt first UHF communication with Tianwen-1 in orbit, and each transmission attempt will last 8-10 minutes. Once established each day it will support 20Mbit (2.5MB) of data. Then after this every 3 days the rover will attempt to establish X band communication, this channel will support 50Mbit (6.25MB) of data per connection. Thus tonight (night of 17/05/2021) first data pack of 2.5MB should arrive, hopefully this 2.5MB data pack will have some post landing photos. If not worry not, more data will come in the coming days, sooner or later you will see first photo from the Martian surface.
People probably aren't going to be impressed with low resolution photos and trying to cram a high resolution photo in the 2.5mb data isn't really a good use of that bandwidth since there are lots of other things needing to be sent back to earth I'm sure.
Many people are asking how come there still aren't any photos from Zhurong yet. It's been three days since landing and still no photo back, is there a problem blablabla. Don't be so hasty, we will analyse the reason behind lack of photo.
On the day of the landing the lander will be talking to earth via direct communication. In this mode the bandwidth is 16bps, so 2 bytes per second. At this speed the comms is only sufficient for judging spacecraft status and not enough for transmission of photos. A 100KB uncompressed low resolution photo will take 14 hours to transfer at this rate, and this is not taking into account Zhurong's own bandwidth requirements. 3 Sols after landing (Monday night, 17/05/2021) the rover will attempt first UHF communication with Tianwen-1 in orbit, and each transmission attempt will last 8-10 minutes. Once established each day it will support 20Mbit (2.5MB) of data. Then after this every 3 days the rover will attempt to establish X band communication, this channel will support 50Mbit (6.25MB) of data per connection. Thus tonight (night of 17/05/2021) first data pack of 2.5MB should arrive, hopefully this 2.5MB data pack will have some post landing photos. If not worry not, more data will come in the coming days, sooner or later you will see first photo from the Martian surface.
People probably aren't going to be impressed with low resolution photos and trying to cram a high resolution photo in the 2.5mb data isn't really a good use of that bandwidth since there are lots of other things needing to be sent back to earth I'm sure.
I recall China has some interest in space based solar power, and the designed called for two big mirror arrays focusing sunlight to a centre collecting point which will convert it into microwave and beam it to the ground.
I can't help but think the techniques that will be developed for assembling the 10m space telescope will be useful for the orbital solar power plant too.
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