In addition to the work of
Meinhold and Lubin (1991)
mentioned above, one other group, led by de Bernardis
(1988,
1989,
1990)
has carried out a number of experiments sensitive to these
angular scales with a one-meter class telescope on a balloon. Their
instrument consists of a 1.2 meter primary mirror and a wobbling
secondary mirror which is used to beam switch with an amplitude of
1° on the sky. They used two bolometer detectors cooled to
0.3K; one operating in the band 126 GHz-165 GHz and the other very
broad band and operating below 840 GHz. The beams on the sky of the
telescope-feed combinations were 60' FWHM and 25' FWHM,
respectively. The low-frequency channel was subject to internal RF
interference so that this group has only reported on their
high-frequency results thus far. During the balloon flight they
performed ~ constant azimuth drift scans at an elevation of
45°. They report results from a high Galactic latitude
(-50° < b < - 43°) region observed covering a 20°
strip in right ascension at =
6°.61-7°.89.
The single switching procedure produced difference measurements for 87
independent fields. Signals were clearly detected which the authors
ascribe to Galactic dust emission. In addition they derive upper
limits to the intrinsic anisotropy which are given in
Table 3.
In a recent balloon experiment
Alsop et al (1992)
made observations with
a 1-meter telescope, having a beamwidth of 0°.5 and switched
sinusoidally in azimuth with an amplitude of 1°.3. Observations
were made simultaneously at 180 GHz, 270 GHz, and 360 GHz, using a
3He-cooled bolometer receiver. Variance in excess of that
expected from the noise alone was detected at the level of T/T = 5 x
10-5. This signal could not be interpreted as uniquely
cosmological in origin because of concerns about possible systematic
errors. An upper limit on the anisotropy derived from the observations
is given in Table 3.