Degree Angular Scale Interferometer

The CMB was created when the universe was about 380,000 years old, when the opaque plasma fog which existed after the Big Bang began to recombine into hydrogen atoms and allowed photons to travel freely through space. This radiation has since been redshifted by the expansion of the universe and can be seen faintly in the microwave part of the electromagnetic spectrum. In 1992 it was observed that there are very slight anisotropies in the effective temperature of the CMB, and the DASI telescope was outfitted to precisely measure these temperature anisotropies and then to detect polarization; the first to look at the polarization of the CMB. The ΛCDM (dark energy and cold dark matter) theory, regarded as the standard model of cosmology, predicted polarization of the CMB due to the scattering effects of the first atoms in the universe. Project leader John Carlstrom said that if polarization had not been found, astrophysicists would have to reject all their interpretations of recent data. ==Construction==

Location The DASI telescope was set up over the 1999–2000 austral summer, on the inner of the two towers of the Martin A.Pomerantz Observatory at the Amundsen–Scott South Pole Station 0.7 km from the geographic south pole. The southern polar site is an exceptional location for this sort of telescope because of the extremely favourable atmospheric conditions. The high altitude and extreme dryness means the air is thinner and contains much less water vapour than other climates, which is important as water is a good absorber of microwave radiation and blocks a portion of the CMB signal. Also, the sun is only visible for six months of the year negating the need to shield the telescope from it during that time. See the Amundsen–Scott South Pole Station article for a list of astrophysics experiments at the site. Design The DASI was a compact interferometer made up of 13 small telescope elements in a pattern with three lines of symmetry. Each of the 13 telescopes had a lensed corrugated horn, with a corrugated shroud to reduce crosstalk between the antennae. Each of the telescopes had a high-electron-mobility transistor (HEMT) amplifier working at the frequencies 26–36 GHz (Ka band), cryogenically cooled to about 10 K. The receiver noise temperatures ranged from 15K–26 K at band centre, and 30 K at the edges. The overall system temperature reached about 26 K. The telescope mount was of an altitude-azimuth (altazimuth) design, with a counterbalanced gear and pinion elevation drive, that gave great stability when tracking and pointing. The mount had a heavy box steel construction, which was necessary to ensure stability of the mass of the telescope. The design of the DASI was very similar to that of the VSA and the CBI. The electronic equipment behind the CBI and DASI were actually identical as were the 26–36 GHz frequencies operated at. However the CBI was designed for smaller angular scales, so it had a greater resolution over a smaller area of the sky, and so had 0.9 m dishes instead of DASI's 20 cm horns. The VSA was also similar in concept and operated at the same frequency, but operated at smaller angular resolutions of 0.2–3°. Funding The project was funded by the National Science Foundation (NSF) office of polar programs, first through the Centre for Astrophysical Research in Antarctica and then directly through the office. It was additionally supported by the Centre for Cosmological Physics in Chicago. ==Results==

DASI made its first observations over 97 days during the 2000 austral winter, measuring temperature anisotropies of the CMB in 32 roughly adjacent circular areas of the sky, each 3.4° in diameter. These observations were highly sensitive, typically at an rms precision of 10 μK. After another set of measurements in 2001 the team reported the results the first measurements of the 2nd and 3rd acoustic peaks in the CMB's power spectrum, the 1st had been discovered by earlier experiments MAT/TOCO, BOOMERanG and MAXIMA. The cuts of DASI's data were done with a strict "jackknife" test which removed data which was gathered when certain parameters were exceeded, for example if the sun rose more than 5° over the horizon, or there was a large offset or excessive noise in the data suggesting a hardware glitch. This is a standard practise which sacrifices some accuracy for improved reliability. ==References==