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.
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 is from NASA)