Siddharth Malu

The Millimeter-Wave Bolometric Interferometer

We report on the design and tests of a prototype of the Millimeter-wave Bolometric Interferometer (MBI). MBI is designed to make sensitive measurements of the polarization of the cosmic microwave background (CMB). It combines the differencing capabilities of an interferometer with the high sensitivity of bolometers at millimeter wavelengths. The prototype, which we call MBI-4, views the sky directly through four corrugated horn antennas. MBI ultimately will have ~ 1000 antennas. These antennas have low sidelobes and nearly symmetric beam patterns, so spurious instrumental polarization from reflective optics is avoided. The MBI-4 optical band is defined by filters with a central frequency of 90 GHz. The set of baselines, determined by placement of the four antennas, results in sensitivity to CMB polarization fluctuations over the multipole range ℓ = 150 - 270. The signals are combined with a Fizeau beam combiner and interference fringes are detected by an array of spider-web bolometers. In order to separate the visibility signals from the total power detected by each bolometer, the phase of the signal from each antenna is modulated by a ferrite-based waveguide phase shifter. Initial tests and observations have been made at Pine Bluff Observatory (PBO) outside Madison, WI.

The millimeter-wave bolometric interferometer (MBI)

The Millimeter-Wave Bolometric Interferometer (MBI) is designed for sensitive measurements of the polarization of the cosmic microwave background (CMB). MBI combines the differencing capabilities of an interferometer with the high sensitivity of bolometers at millimeter wavelengths. It views the sky directly through corrugated horn antennas with low sidelobes and nearly symmetric beam patterns to avoid spurious instrumental polarization from reflective optics. The design of the first version of the instrument with four 7-degree-FOV corrugated horns (MBI-4) is discussed. The MBI-4 optical band is defined by filters with a central frequency of 90 GHz. The set of baselines determined by the antenna separation makes the instrument sensitive to CMB polarization fluctuations over the multipole range l=150-270. In MBI-4, the signals from antennas are combined with a Fizeau beam combiner and interference fringes are detected by an array of spider-web bolometers with NTD germanium thermistors. In order to separate the visibility signals from the total power detected by each bolometer, the phase of the signal from each antenna is modulated by a ferrite-based waveguide phase shifter. Observations are planned from the Pine Bluff Observatory outside Madison, WI.

Antenna-coupled transition-edge hot-electron microbolometers

We are developing a new type of detector for observational cosmology and astrophysical research. Incoming radiation from the sky is coupled to a superconducting microstrip transmission line that terminates in a thin film absorber. At sub-Kelvin temperature, the thermal isolation between the electrons and the lattice makes it possible for the electrons in the small absorber (100s of cubic micro-meter) and superconducting bilayer (Transition Edge Sensor) to heat up by the radiation absorbed by the electrons of the normal absorbing layer. We call this detector a Transition-edge Hot-electron Micro-bolometer (THM). THMs can be fabricated by photo lithography, so it is relatively easy to make matched detectors for a large focal plane array telescope. We report on the thermal properties of Mo/Au THMs with Bi/Au absorbers.

Modeling the quasi-optical performance of CMB astronomical interferometers

The Millimeter-Wave Bolometric Interferometer (MBI) is a ground-based instrument designed to measure the polarization anisotropies of the Cosmic Microwave Background (CMB) and contains a number of quasi-optical components, including a complex back-to-back system of corrugated feed-horn antennas. In this paper we use MBI as an example to demonstrate the existing modeling techniques and as a focus to develop extended modeling capabilities. The software we use to model this system targets the millimeter and sub-millimeter region of the electromagnetic spectrum and has been extended to efficiently model the performance of back-to-back corrugated horns embedded in larger optical systems. This allows the calculation of the coupling of radiation from the sky to the detector array through a back-to-back horn feed system.

Quasi-optical design and analysis of the millimeter-wave bolometric interferometer (MBI)

This paper presents work being done on the optical design and analysis of the beam combiner for the millimeter-wave bolometric interferometer (MBI), a ground based instrument designed to measure the polarization anisotropies of the cosmic microwave background at a central frequency of 90 GHz. Interferometry has never before been used to carry out such measurements at this frequency, nor using incoherent bolometers as detectors. MBI will therefore act as a prototype for this type of system and must be modeled in detail to ensure that its operation is fully understood.