The universe is very transparent at millimeter wavelengths, allowing the measurement of radiation travelling for billion light years, and emitted in early phases of the evolution. Cosmological redshift shifts visible-IR radiation present in the primeval plasma into the mm-range, thus providing a marvellous cosmological probe: the Cosmic Microwave Background (CMB). In addition, dust emission in early galaxies is shifted into the mm range as well. For all these reasons, mm-wavelengths are very important for observational cosmology. Our group is active in the development of techniques, experiments, analysis methods for the CMB and mm-wave astronomy. In this research project we plan to develop new specific devices and experimental and analysis methods, in the tradition of our research group. While we already had support from other agencies (ASI, PNRA, INFN) for the HW contribution to missions like Planck, OLIMPO, LSPE, QUBIC, here we request support for innovative devices (in particular Kinetic Inductance Detector Arrays) to optimize the measurement of the observable, and the development of innovative models and methods aimed at reducing the contamination of the measurements due to noise and local foreground emission. These developments will be important for both the current missions cited above and the forthcoming next-generation missions for the CMB.
WP1: LARGE-SCALE POLARIZATION EXPLORER. The modulation of the faint polarized CMB is one of the key elements of the experiment. It is obtained by means of a cryogenic rotating Half Wave Plate (HWP). This configuration has important advantages:
- It moves the signal from the frequency range induced by sky scan (fractions of Hz) to 4 times the rotation frequency (10 Hz or so), so that the effects of 1/f noise from the residual atmosphere and the instrument are mitigated
- There is more freedom in optimizing the sky coverage (which in LSPE is obtained by spinning the payload in azimuth and changing the elevation)
Note that LSPE is the first instrument using the HWP as the first optical element. In this configuration the polarization properties of the subsequent telescope and detectors are almost irrelevat. But the HWP has a diameter of 50 cm to cover the full beam of the experiment [see Pisano et al. arXiv:1610.00582, 2016], and needs to be cooled at cryogenic temperatures to avoid introducing important offsets. We have designed a cryogenic rotator (figure 3) based on a large supercoducting bearing to achieve continuous rotation at a few K with minimal dissipation and high stability. In this WP we'll optimize the rotation frequency of the HWP and the sky scan by means of
- Accurate modelling of HWP-induced systematics
- Development of a detailed instrument simulator accounting for these systematic effects
- End-to-end simulations of the LSPE survey, for different rotation frequencies, scan speeds, elevation steps
In view of further development of the LSPE after its first flight, we will analyze the performance of a new focal plane using an array of 9000 single-mode detectors replacing the 300 multi-mode detectors present in the current configuration. Their distribution in the observation bands will be optimized, the feasibility of KIDs arrays to this purpose will be assessed, as well as the required readout electronics, and the improvement in angular resolution and its impact on science will be studied.
WP2: QUBIC. The QUBIC experiment will go through the demonstration phase this year and will be deployed to the observation site (Alto Chorrillo, Argentina) in 2018. Our group is producing the main cryogenic system, consisting of a large cryostat with two puse-tube refrigerators, and the polarization modulator. Our presence will be needed during the operation of the demonstrator, at the APC Paris, and during the intstrument installation and commissioning at the high altitude site. In addition, we plan to develop a parallel data analysis pipeline, modifying the LSPE pipeline to account for the peculiar beam of the interferometer and its frequency dependance.
WP3: OLIMPO. The instrument features a differential Fourier-Transform spectrometer to produce spectra of the Sunyaev-Zeldovich effect in clusters of galaxies [see Schillaci et al., A&A, 565, A125 (2014)]. We plan to carry out extensive campaigns of instrumental characterization during the last trimester of 2017 and the first trimester of 2018, before instrument commissioning at the launch site. These measurements are needed to assess the (very low) level of common-mode residuals in the instrument outputs [see D'Alessandro et al. Applied Optics, 54, 9269-9276 (2015)], the balancing of the two interferometers, the best polarizers orientation and in general the best (efficiency-wise) alignment between the instrument and the telescope. A detailed mapping of the performance versus misalignment will be needed, as well as a strategy development for in-flight re-alignment. In addition, we need to implement the spectral analysis modules in the quick-look and data analysis software.
WP4: KINETIC INDUCTANCE DETECTORS. We propose to study detector arrays able to detect both polarizations of the incoming radiation separately. The configuration consists of a bi-array structure (as shown in fig.2) in which two arrays are stacked and separated by a polarizing grid acting as a backshort for the top array. The first array will be sensitive to one polarization and the second to the orthogonal one. We would like to investigate this approach for detectors working from the ground in the optical bands at 90, 150, and 220 GHz. Very preliminary simulations on a 150 GHz pixel are promising, but we need to carry out extensive simulations in order to optimize the geometry of the polarizing grid, the geometry of the bottom absorber, and to extend the optimization for the other optical bands (centered at 90 and 220 GHz). Here we ask support for these simulation activities and for the production and test of samples. The activity will be carried out in the new clean-room present in our laboratory (ISO-6) where a wire bonder (Bondtec 53XX) and a laser direct litography (Quantum design microwriter ML3 baby) have been recently installed.