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