SAFIR will bridge the critical gap between the two fundamentally different types of
astronomical facilities. On the short wavelength side, we will have the
JWST
observing in the near-IR and using the same basic principles as the Hubble Space
Telescope. The sensitivity of the JWST, however will be limited for wavelengths
beyond about 20 microns (0.02 mm), because the telescope is passively cooled to
only 45 K.
On the long-wavlength side of SAFIR's range, the ground-based Atacama Large Millimeter Array
(
ALMA)
will be astronomers' primary tool for wavelengths beyond about 1 mm. ALMA will be a
very powerful array of 64 telescopes, and will use the heterodyne receiver techniques
of radio astronomy. As such, its sensitivity at wavelengths less than about 1 mm will
be limited by the relative roughness of its mirror surfaces, performance of the receivers,
and atmospheric transmission.
In SAFIR's spectral regime of 20 microns to 1 mm, the fundamental limit imposed by the
astrophysical background allows more than 1000 times better sensitivity than the currently
planned US and European missions as shown in the figure below.
Unlike in the optical, there is a huge sensitivity gain possible with a cold telescope.
And unlike at millimeter wavelengths, a single large mirror provides sufficient light
collecting area for observations at great distances, into the early universe. Together,
the resulting increase in astronomical capability is tremendous, especially when compared
with the relatively small incremental technology costs after the JWST and
TPF
are built.
To put SAFIR's sensitivity into perspective, suppose you turned on a small flashlight, powered
by two AA batteries, sealed it inside a basketball, and launched it into the frigid depths
of space. The little light bulb would raise the temperature of the ball's surface to 115
Kelvin (115 degrees Celsius above absolute zero, or around -253° F), and this tiny amount
of heat would radiate at far-infrared wavelengths. SAFIR would be able to detect it at a
distance of nearly four million miles, or 15 times the distance to the moon.
While other systems such as the JWST might be capable of detecting the visible light from a
small flashlight at 4 million miles, they could not detect the energy from the flashlight
if it were shrouded inside a basketball, because those telescopes are not sensitive to the
far-IR wavelengths at which the cold basketball would radiate.
In the same way, SAFIR's long-wavelength sensitivity is critical for observations of
dust-enshrouded formation of stars, planets, and entire galaxies.
