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

One thing about the JWST that disturbed me was the fact that its
mirrors are coated with gold.

While at the present time, the infrared is a very exciting area for
astronomy, the fact that gold is strongly colored instead of
reflecting all colors equally seemed to me to be precluding the use of
the telescope for astronomy at other wavelengths.

Looking up information about mirror materials, I found out that
aluminum and silver are also highly reflective in the infrared; they
don't have any limitation in reflectivity there that would prevent
them from being used for infrared observations. This seems to make
things worse!

However, the same source that gave me that information also revealed
the *real* reason gold is needed for the JWST. The JWST mirror is to
be cooled to very low temperatures to prevent its own emissions of
infrared radiation from interfering with observations in the far
infrared. Gold has *half* the emissivity of aluminum.

Silver wouldn't tarnish in the vacuum of space, but its reflectance
has a notch around 310 nanometers where it decreases to 5%.

Aluminum, although noticeably less reflective than silver in visible
light, is generally 90% reflective all the way out to 220 nanometers
in the ultraviolet - which is not true of other materials.

While gold, with its yellowish color, has its reflectivity decline
within the visible at 540 nanometers or so, the good news is that its
reflectivity only declines to about 40% - and stays there right out to
260 nanometers or so. (Copper, with its reddish color, experiences an
earlier decline, but retains 40% reflectivity right out to 230
nanometers.)

Of course, it isn't just the primary mirror in the JWST that is coated
with gold. Also, since the JWST has a segmented mirror, another issue
arises - on Earth, observatories can use adaptive optics to compensate
for Earth's atmosphere, but with current technology, this works much
better in the infrared than in visible light.

But since a gold-coated primary, _in itself_, doesn't preclude using
the JWST to obtain images in blue, green, and ultraviolet light - the
brightness of which can be adjusted to compensate for the lower
reflectivity of gold at those wavelengths - if it *can* be designed to
avoid being limited to working at wavelengths longer than a limit such
as 720 nm or 560 nm, I would think it is worth the effort.

John Savard

On Jul 15, 6:29*pm, Quadibloc wrote:

But since a gold-coated primary, _in itself_, doesn't preclude using
the JWST to obtain images in blue, green, and ultraviolet light - the
brightness of which can be adjusted to compensate for the lower
reflectivity of gold at those wavelengths - if it *can* be designed to
avoid being limited to working at wavelengths longer than a limit such
as 720 nm or 560 nm, I would think it is worth the effort.

John Savard

http://www.platinummetalsreview.com/...se&record=1219

Let's give a nod to our forefathers.
Make the mirrors out of speculum.

On 2011-07-15, Quadibloc wrote:
One thing about the JWST that disturbed me was the fact that its
mirrors are coated with gold.

While at the present time, the infrared is a very exciting area for
astronomy, the fact that gold is strongly colored instead of
reflecting all colors equally seemed to me to be precluding the use of
the telescope for astronomy at other wavelengths.

Because the telescope is not serviceable, the instrument package will do
a good enough job of restricting the observations to the infrared.

Bud

In article ,
wrote:
On 2011-07-15, Quadibloc wrote:
One thing about the JWST that disturbed me was the fact that its
mirrors are coated with gold.

While at the present time, the infrared is a very exciting area for
astronomy, the fact that gold is strongly colored instead of
reflecting all colors equally seemed to me to be precluding the use of
the telescope for astronomy at other wavelengths.

Because the telescope is not serviceable, the instrument package will do
a good enough job of restricting the observations to the infrared.

I imagine that it's also extravagently more difficult to get a
deployable mirror to work nicely at 300nm than at 600nm; the
shortest-wavelength filter on NIRCAM cuts off at about 600nm (that is,
JWST cannot work in the ultra-orange). I think the HgCdTe array is
sensitive to shorter-wavelength light, but as designed the instrument
does not have a 'clear' filter.

Tom

On 15/07/2011 12:29 PM, Quadibloc wrote:
One thing about the JWST that disturbed me was the fact that its
mirrors are coated with gold.

While at the present time, the infrared is a very exciting area for
astronomy, the fact that gold is strongly colored instead of
reflecting all colors equally seemed to me to be precluding the use of
the telescope for astronomy at other wavelengths.

If it was to be used in other wavelengths, then they would've put
cameras sensitive to other wavelengths onboard. They have only put
infrared cameras onboard, and once it's launched they won't be able to
put any other kind of camera on it.

Of course, it isn't just the primary mirror in the JWST that is coated
with gold. Also, since the JWST has a segmented mirror, another issue
arises - on Earth, observatories can use adaptive optics to compensate
for Earth's atmosphere, but with current technology, this works much
better in the infrared than in visible light.

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.

But since a gold-coated primary, _in itself_, doesn't preclude using
the JWST to obtain images in blue, green, and ultraviolet light - the
brightness of which can be adjusted to compensate for the lower
reflectivity of gold at those wavelengths - if it *can* be designed to
avoid being limited to working at wavelengths longer than a limit such
as 720 nm or 560 nm, I would think it is worth the effort.

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

Yousuf Khan

In article ,
Thomas Womack wrote:

I think the HgCdTe array is
sensitive to shorter-wavelength light, but as designed the instrument
does not have a 'clear' filter.

This turns out to be wrong; the HgCdTe array is manufactured on a base
of CdZnTe which is opaque to the shorter-wavelength light.

Tom

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 Fri, 15 Jul 2011 09:29:07 -0700 (PDT), Quadibloc
wrote:

One thing about the JWST that disturbed me was the fact that its
mirrors are coated with gold...

It's an IR telescope. By design. IR isn't just interesting at the
moment, it is key to the scientific intent of the instrument. Trying
to make the JWST do too many things is just asking for problems. And
for IR out to 27um, gold is better than aluminum (and in near IR, it
is a lot better).

Also, the optical system does incorporate adaptive optics, which as
you note is much easier to do with long wavelengths. Making a super
light weight segmented mirror that was diffraction limited in visible
wavelengths would have been much more difficult and expensive.