On Jul 15, 3:09*pm, Yousuf Khan wrote:

The reason for having adaptive optics on ground telescopes is to deal
with atmospheric blurring. In space, they won't have any of those
problems, obviously.

Yes, but it has a *segmented mirror*, and keeping the segments aligned
presents problems which are analogous to those of adaptive optics, so
there might be a similar limitation.

Don't worry, they'll make the images available for you to see in
wavelengths you recognize.

Oh, of course the images will be available in false-color.

But because the Hubble will eventually be retired, the limitation of
the JWST to wavelengths of 600 nm and longer will mean that data at
shorter wavelengths will not be obtainable. I think that's a lack.

Unfortunately, given the design of the JWST as a three-mirror
anastigmat, just adding an instrument package sensitive to visible
light and UV won't be enough.

The primary mirror being gold, as I noted, still allows 40%
reflectance at off-design wavelengths.

A primary and a secondary - 40% of 40% is 16%, and that might still be
livable. But three mirrors?

Of course, making the secondary and the tertiary aluminized - and just
cooling them more to make up for the increased emissivity - is a
possibility. But increasing the amount of cooling means more
consumables. Better to use that budget for a longer lifespan. And a
swappable secondary and tertiary is something that doesn't bear
thinking about - that would add too much cost and complexity.

Perhaps if only the tertiary were swappable...

Also, if, say, the six mirrors at the corner of the primary were
coated with copper instead of gold - assuming copper also has low
infrared emissivity like gold - that would involve a gain because
while copper falls from near 100% to 40% reflectance somewhat earlier
than gold, it stays around 40% right down to 220 nm, while gold
becomes less reflective at 260 nm.

Since it looks like the JWST will be all alone up there, if it even
gets up there, not having some capability in all the visible spectrum
and the UV, even though it is optimized for UV, is a problem in my
opinion. But there probably isn't a way to get around it at a
reasonable cost.

John Savard