On Saturday, September 16, 2023 at 4:34:03 AM UTC-6, RichA wrote:
> https://phys.org/news/2023-09-large-space-telescopes.html

Up until now, space telescopes didn't use adaptive optics.

The article claims that if a space telescope were launched with a 6.5
metre mirror made from a single borosilicate glass blank, though, it
would be needed. Since there's no atmosphere to worry about, I suppose
the culprit is thermal expansion of the mirror itself.

John Savard

On 16/09/2023 12:59, Quadibloc wrote:
> On Saturday, September 16, 2023 at 4:34:03 AM UTC-6, RichA wrote:
>> https://phys.org/news/2023-09-large-space-telescopes.html
>
> Up until now, space telescopes didn't use adaptive optics.
>
> The article claims that if a space telescope were launched with a 6.5
> metre mirror made from a single borosilicate glass blank, though, it
> would be needed. Since there's no atmosphere to worry about, I suppose
> the culprit is thermal expansion of the mirror itself.

More likely differential thermal expansion of the support structure as
it goes in and out of the Earth' shadow. Pre HST launch much of the
knowledge about those thermal effects were highly classified.

--
Martin Brown

On Monday, September 18, 2023 at 12:08:54 AM UTC-6, RichA wrote:

> Changes to Beryllium (what the Webb has) with temperature are probably
> more than borosilicate glass. But the Webb has its heat shield.

Yes, but:

- The Webb space telescope is kept at a constant, and very low, temperature
to avoid it glowing at the very wavelengths it is to observe;

- It has a segmented mirror, not a monolithic one; and

- Beryllium, being a metal, conducts heat more effectively than borosilicate
glass, so temperatures within it are more uniform.

So there are multiple reasons why it would be less in need of adaptive optics -
which, in a sense, it has, since the individual mirrors were brought into position
to make it work - than a telescope with one large glass mirror.

John Savard