4 ms·

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 precis

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 5y ago
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.
ridgeguy 5y ago
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_SiC_DataSheet.pdf https://www.northropgrumman.com/wp-content/uploads/CERAFORM_...

rich_sasha 5y ago

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.

ridgeguy 5y ago

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_SiC_DataSheet.pdf https://www.northropgrumman.com/wp-content/uploads/CERAFORM_...