James Webb Telescope and L2 Orbit Question

[StringJunky]

9 hours ago, zapatos said:

Given the advances in technology I would assume much of the difference in 'power' between the two telescopes has to do with differences in the quality of the mirrors, computer software, sensors, reduction of dust and gas in front of the telescope, handling of diffraction, and the many other things I'm blithely unaware of regarding optics.

JWT has adaptive optics, which Hubble doesn't have  because it wasn't developed then. Here's a stackexchange post about it:

Quote

10

The primary and secondary mirrors on JWST can be adjusted:

Launching a mirror this large into space isn’t feasible. Instead, Webb engineers and scientists innovated a unique solution – building 18 mirrors that will act in unison as one large mirror. These mirrors are packaged together into three sections that fold up - much easier to fit inside a rocket. Each mirror is made from beryllium and weighs approximately 20 kilograms (46 pounds). Once in space, getting these mirrors to focus correctly on faraway galaxies is another challenge entirely. Actuators, or tiny mechanical motors, provide the answer to achieving a single perfect focus.

The primary and secondary mirror segments are both moved by six actuators that are attached to the back of the mirrors. The primary segment has an additional actuator at the center of the mirror that adjusts its curvature. The third mirror segment remains stationary.

Lee Feinberg, Webb Optical Telescope Element Manager at NASA's Goddard Space Flight Center in Greenbelt, Md. explained "Aligning the primary mirror segments as though they are a single large mirror means each mirror is aligned to 1/10,000th the thickness of a human hair. This alignment has to be done at 50 degrees above absolute zero! What's even more amazing is that the engineers and scientists working on the Webb telescope literally had to invent how to do this."

Each mirror can be adjusted in only 7 spots: each corner of the hexagon and the middle. This seems to be too few spots to really do adaptive optics. In adaptive optics, you adjust the shape of the mirror itself in multiple locations to compensate for changes in the light wavefront as it's distorted.

Those 7 spots are adjustable for one reason: to align the 18 segments of the primary mirror to each other, and to adjust the primary and secondary mirrors to the rest of the optical path after launch.

https://space.stackexchange.com/questions/18938/why-would-the-james-webb-space-telescope-need-adaptive-optics-outside-the-atmosp

Searching  'telescope adaptive optics' will give you more general info about it. It was originally developed to counter atmospheric effects affecting  Earth-based telescopes. It appears to have been adapted for the JWT in a first, for reasons stated in the quote.

Edited January 28, 2022 by StringJunky

[geordief]

20 minutes ago, StringJunky said:

Each mirror can be adjusted in only 7 spots: each corner of the hexagon and the middle. This seems to be too few spots to really do adaptive optics. In adaptive optics, you adjust the shape of the mirror itself in multiple locations to compensate for changes in the light wavefront as it's distorted.

The exchange you referenced above suggests that 7 spots seems too few to do adaptive optics

I wonder how they got around  that or are they saying that it only seemed too few but was adequate  in reality?

Are those mirrors super flexible (and strong) so that the local distortions are spread evenly through the material?

Edited January 28, 2022 by geordief

[StringJunky]

45 minutes ago, geordief said:

The exchange you referenced above suggests that 7 spots seems too few to do adaptive optics

I wonder how they got around  that or are they saying that it only seemed too few but was adequate  in reality?

Are those mirrors super flexible (and strong) so that the local distortions are spread evenly through the material?

It's not adapting for atmospheric effects, since there is none in space. It is for making remote on-site assembly corrections. It's that technique that allows the mirror to be so big. You couldn't fold a large sectional mirror, lauch it, and expect it to be aligned automatically on unfolding to the thickness much less than a human hair without it. Compared to the Earth-based ones, which adapt continuously in real-time at many points, it's relatively crude, but it's made such a big space-based mirror possible.

Edited January 28, 2022 by StringJunky

[Airbrush]

Everything you need to know about L2.

 

Edited February 2, 2022 by Airbrush

[exchemist]

38 minutes ago, Airbrush said:

Everything you need to know about L2.

 

I found this very informative indeed. It answers the original question I posed perfectly. 

[Airbrush]

Is L2 a good place to have one or more telescopes dedicated to scanning for threatening NEOs and bolides?  Or orbiting the moon?

Edited February 10, 2022 by Airbrush