A Flaw the JWST Doesn't (Have to) Have?

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

On Jul 15, 3:58*pm, Quadibloc wrote:

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.

Unfortunately, apparently this won't work, as copper has about the
same emissivity as aluminum, instead of the lower emissivity of gold.

John Savard

On 15/07/2011 5:58 PM, Quadibloc wrote:
On Jul 15, 3:09 pm, Yousuf 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.

The original idea behind segmented mirrors was to make a reflector that
doesn't sag under its own weight, and therefor allows you to make bigger
telescopes. They found that it helped in compensating for atmospheric
blurring too.

A space telescope doesn't have to worry about blurring, nor does it
really need to worry about sagging, so it's just the bigger telescope
aspect that's relevant here. If they made the telescope large enough to
fit into a rocket nosecone, the telescope would still be the size of the
Hubble or the Herschel space telescopes.

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.

Most of the images coming from the Hubble are false color too. They come
into the scope as discrete bands of black'n'white images which are then
combined and colorized in computers on the ground. Hubble was able to
see ranges that our eyes couldn't see such as near-infrared and
ultraviolet, and they too were all false-colored into our range.

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.

I think it's been shown that visible light astronomy has nothing new
left to offer, and all of the really amazing stuff comes in other
wavelengths, especially Far Infrared, Gamma, and X-Ray. They all are
able to escape through dust clouds. Other wavelengths like Radio can be
done entirely from the ground. If we still need visible light images,
then it seems as if the modern ground telescopes are doing a better job
than Hubble.

These days its seems that something like a gamma-ray burst occurs, a
space telescope is the first to detect it, and then a network of optical
scopes on the dark side of the planet get herded together to take
after-images of something the space telescope discovered. Basically just
clean-up hitters.

Yousuf Khan