For a comprehensive Dyson Sphere search it is
useful to have a
whole sky survey. This rules out point and shoot satellite instruments
like the Spitzer Space
Telescope and NICMOS
(the Near Infrared Camera and Multi-object
Spectrograph on Hubble). In addition, the search instrument needs to be
sensitive to temperatures ranging from roughly 100 to 600 ºK. The
wavelengths
associated with this interval span 10 to 100 μm. Ground-based infrared
telescopes are ruled out for this regime because of the high sky
background.
This eliminates the sensitive, whole sky, ground-based 2MASS survey as
the
principle search tool because it only goes out to 2.17 μm. Finally good
angular
resolution is useful to rule out associations with nearby stars.
The IRAS database
is the best existing
resource available to
address these three
requirements [see, for
example, C. Beichman, Ann. Rev. Ast 25, 521-63 (1987)]. The IRAS satellite
flew in 1983. It
identified 250,000 infrared
point sources and scanned 98% of the sky. These sources were measured
with four
filters centered at 12, 25, 60, and 100 μm. This is almost ideal for a
Dyson
Sphere search. One of the principle motivations of the IRAS program was
an
investigation of cosmic dust. Partly as a result of that the lens of
the
telescope had a diameter of only 60 cm so that the angular extent of a
“point
source” is O(1 minute). The positional reconstruction error is
quoted as about 2"
to
6" in-scan and about 8" to 16" cross-scan. The sensitivity
was O(0.5 Jy)
for the 12, 25, and 60 μm bands and 1 Jy for the 100 μm band.
The 2MASS survey with nearly
500 M sources is both much more sensitive and precise in the
near infrared. Each of the two 2MASS telescopes had a mirror diameter
of 1.3 m.
The positional reconstruction error is 0.5". However a blackbody source
centered on the 12 μm IRAS band must be at least 10 Jy to register in
the 2MASS
2.17 μm band. This is a factor of order ten times higher than the
minimum IRAS
sensitivity. If the two sources can be correlated one can take
advantage of the
better 2MASS angular resolution and pointing accuracy.
(The photo of IRAS on the right is from NASA)
