In this paper we discuss the latest developments of the STRIP instrument of the “Large Scale Polarization Explorer” (LSPE) experiment. LSPE is a novel project that combines ground-based (STRIP) and balloon-borne (SWIPE) polarization measurements of the microwave sky on large angular scales to attempt a detection of the “B-modes” of the Cosmic Microwave Background polarization. STRIP will observe approximately 25% of the Northern sky from the “Observatorio del Teide” in Tenerife, using an array of forty-nine coherent polarimeters at 43 GHz, coupled to a 1.5 m fully rotating crossed-Dragone telescope. A second frequency channel with six-elements at 95 GHz will be exploited as an atmospheric monitor. At present, most of the hardware of the STRIP instrument has been developed and tested at sub-system level. System-level characterization, starting in July 2018, will lead STRIP to be shipped and installed at the observation site within the end of the year. The on-site verification and calibration of the whole instrument will prepare STRIP for a 2-years campaign for the observation of the CMB polarization.
ECCOSORB<sup>TM</sup> CR/MF is a widely used absorber at radio and millimeter wavelengths. It is used both at room and at cryogenic temperature to realize loads and calibrators both for laboratory and for space-borne instruments. Data on its RF properties are available from the data sheet at room temperature. But it is also widely used outside the design wavelength range and at cryogenic temperature, where specific measurement of electromagnetic and thermal properties are needed. Scarce information is available in the literature and inconsistencies are frequent. We report here new RF data in Ka and W-band at room temperature obtained with waveguide measurements with different setups.
The LSPE is a balloon-borne mission aimed at measuring the polarization of the Cosmic Microwave Background (CMB)
at large angular scales, and in particular to constrain the curl component of CMB polarization (B-modes) produced by
tensor perturbations generated during cosmic inflation, in the very early universe. Its primary target is to improve the
limit on the ratio of tensor to scalar perturbations amplitudes down to r = 0.03, at 99.7% confidence. A second target is
to produce wide maps of foreground polarization generated in our Galaxy by synchrotron emission and interstellar dust
emission. These will be important to map Galactic magnetic fields and to study the properties of ionized gas and of
diffuse interstellar dust in our Galaxy. The mission is optimized for large angular scales, with coarse angular resolution
(around 1.5 degrees FWHM), and wide sky coverage (25% of the sky). The payload will fly in a circumpolar long
duration balloon mission during the polar night. Using the Earth as a giant solar shield, the instrument will spin in
azimuth, observing a large fraction of the northern sky. The payload will host two instruments. An array of coherent
polarimeters using cryogenic HEMT amplifiers will survey the sky at 43 and 90 GHz. An array of bolometric
polarimeters, using large throughput multi-mode bolometers and rotating Half Wave Plates (HWP), will survey the same
sky region in three bands at 95, 145 and 245 GHz. The wide frequency coverage will allow optimal control of the
polarized foregrounds, with comparable angular resolution at all frequencies.
We discuss the design and expected performance of STRIP (STRatospheric Italian Polarimeter), an array of coherent receivers designed to fly on board the LSPE (Large Scale Polarization Explorer) balloon experiment. The STRIP focal plane array comprises 49 elements in Q band and 7 elements in W-band using cryogenic HEMT low noise amplifiers and high performance waveguide components. In operation, the array will be cooled to 20 K and placed in the focal plane of a ~0.6 meter telescope providing an angular resolution of ~1.5 degrees. The LSPE experiment aims at large scale, high sensitivity measurements of CMB polarization, with multi-frequency deep measurements to optimize component separation. The STRIP Q-band channel is crucial to accurately measure and remove the synchrotron polarized component, while the W-band channel, together with a bolometric channel at the same frequency, provides a crucial cross-check for systematic effects.
In this paper the millimeter-wave passive components developed for the Ka-band Bar-SPOrt (Balloon-borne Radiometer for Sky Polarization Observatory) correlation radiometer are described. Comparison between numerical and experimental results are reported for all the building blocks of the radiometer: marker injector, polarizer, ortho-mode transducer, filtering sections and correlation unit. Due to the very low level of the polarized sky emission to be measured, all the components were designed and manufactured in order to achieve a very high level of sensitivity.
The noise of radioastronomy receivers is usually kept low by cooling the front-end into cryostats, where the internal and the external environments are interfaced by optical windows. Such dielectric windows can sistematically correlate the incoming unpolarized radiation and decorrelate its polarized component. Here, we present a study on the effects of dielectrics in high sensitivity microwave polarimetry, including a model of the induced spurious polarization, a selection of materials in term of their optical properties as well as measurements of their optical parameters.
The interesting result is that isotropic dielectrics can produce spurious polarization both when transmit anisotropic diffuse radiation or are not thermally uniform. Finally, such a model can be used to design a calibrator which generates very low polarized signal.
The measure of the faint polarized signal of the Cosmic Microwave Background (few percent of the CMB Anisotropy) requires instruments with very low contamination from systematic effects, high stability and high sensitivity. The BaR-SPOrt experiment, in sharing with the SPOrt project on ISS, is based on analog correlation receivers with components custom designed to match all of these requirements. Here we present the architecture, the design analysis and the status of the realization of the 32 GHz receiver.
SPOrt (Sky Polarization Observatory) is a space experiment to be flown on the International Space Station during Early Utilization Phase aimed at measuring the microwave polarized emission with FWHM = 7 deg, in the frequency range 22-90 GHz. The Galactic polarized emission can be observed at the lower frequencies and the polarization of Cosmic Microwave Background (CMB) at 90 GHz, where contaminants are expected to be less important. The extremely low level of the CMB Polarization signal calls for intrinsically stable radiometers. The SPOrt instrument is expressly devoted to CMB polarization measurements and the whole design has been optimized for minimizing instrumental polarization effects. In this contribution we present the receiver architecture based on correlation techniques, the analysis showing its intrinsic stability and the custom hardware development carried out to detect such a low signal.
BaR-SPOrt (Balloon-borne Radiometers for Sky Polarisation
Observations) is an experiment to measure the linearly polarized
emission of sky patches at 32 and 90 GHz with sub-degree angular
resolution. It is equipped with high sensitivity correlation
polarimeters for simultaneous detection of both the U and Q stokes
parameters of the incident radiation. On-axis telescope is used to
observe angular scales where the expected polarization of the
Cosmic Microwave Background (CMBP) peaks. This project shares most
of the know-how and sophisticated technology developed for the
SPOrt experiment onboard the International Space Station. The
payload is designed to flight onboard long duration stratospheric
balloons both in the Northern and Southern hemispheres where low
foreground emission sky patches are accessible. Due to the
weakness of the expected CMBP signal (in the range of microK),
much care has been spent to optimize the instrument design with
respect to the systematics generation, observing time efficiency
and long term stability. In this contribution we present the
instrument design, and first tests on some components of the 32