11 ms·

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, an

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 5y ago
Yes, one of the main drivers behind using the beryllium substrate was the thermal stability required to operate at cryogenic temperatures.
sbierwagen 5y ago
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_Infrared_Surveyor 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.
- max-ibel 5y ago
  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 5y ago
    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:Animation_of_Solar_and_Heliospheric_Observatory_trajectory_-_Polar_view.gif 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.

dr_orpheus 5y ago

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

sbierwagen 5y ago

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_Infrared_Surveyor 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.

max-ibel 5y ago
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 5y ago
  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:Animation_of_Solar_and_Heliospheric_Observatory_trajectory_-_Polar_view.gif 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.