Planck pre-launch status: Design and description of the Low Frequency Instrument (1001.3321v1)

Abstract: In this paper we present the Low Frequency Instrument (LFI), designed and developed as part of the Planck space mission, the ESA program dedicated to precision imaging of the cosmic microwave background (CMB). Planck-LFI will observe the full sky in intensity and polarisation in three frequency bands centred at 30, 44 and 70 GHz, while higher frequencies (100-850 GHz) will be covered by the HFI instrument. The LFI is an array of microwave radiometers based on state-of-the-art Indium Phosphide cryogenic HEMT amplifiers implemented in a differential system using blackbody loads as reference signals. The front-end is cooled to 20K for optimal sensitivity and the reference loads are cooled to 4K to minimise low frequency noise. We provide an overview of the LFI, discuss the leading scientific requirements and describe the design solutions adopted for the various hardware subsystems. The main drivers of the radiometric, optical and thermal design are discussed, including the stringent requirements on sensitivity, stability, and rejection of systematic effects. Further details on the key instrument units and the results of ground calibration are provided in a set of companion papers.

The Design and Description of the Planck Low Frequency Instrument

The research article details the Low Frequency Instrument (LFI) that was integral to the European Space Agency's (ESA) Planck space mission, dedicated to cosmic microwave background (CMB) investigations. The LFI represents a sophisticated array of microwave radiometers operating at low temperatures to effectively map the cosmos in frequency bands of 30, 44, and 70 GHz, while the High Frequency Instrument (HFI) consort conducts observations in the 100-850 GHz range. This paper outlines the instrument's design, scientific objectives, technical advancements, and the methodologies implemented to mitigate various instrumental systematic effects.

Instrument Architecture and Components

The LFI consists of 11 radiometer chain assemblies (RCAs) and 22 radiometers, configured to cover an extensive portion of the CMB spectrum, particularly targeting the frequency range where foreground emission correlates minimally with the CMB. Each RCA receives signals from orthogonally polarized outputs via the orthomode transducers (OMTs), optimally directing them to enhance sensitivity to polarization. The LFI combines dual amplification stages — a cryogenically cooled frontend at 20K and a warm backend at 300K — linked by waveguides specifically designed to maximize thermal isolation and minimize signal loss.

The front-end modules (FEMs) are the heart of LFI, based on InP HEMT amplifiers which exhibit low noise temperatures that are crucial for accurate CMB measurements. The design of these components underscores the necessity for high sensitivity, with the effective bandwidth for each radiometer significantly contributing to the fidelity of astrophysical signal separation from noise.

Scientific Requirements and Performance

The essential scientific requirements focused on achieving low noise and high stability conditions, enabling the extraction of temperature anisotropies and polarization data from the CMB with sensitivity benchmarks of $\Delta T/T$ values around $2 \times 10^{-6}$. This is complementarily aligned with stringent requirements for angular resolution achieving values as fine as 13' at 70 GHz. The LFI design places significant emphasis on minimizing systematic effects confronting the CMB power spectrum extraction, thereby involving meticulous planning on radiometer sensitivity, knee frequency minimization, and stability measures against 1/$f$ noise contributions through differential receiver configurations.

Thermal and Mechanical Design

Thermal design was crucial, particularly achieving and maintaining operational temperatures for effective noise suppression. The sorption cooler achieved 20 K at the frontend and facilitated further HFI cooling to 4 K. Heat dissipation concerns and functional demands were addressed via lightweight, robust support structures, minimizing parasitic power loss.

Future Implications and Theoretical Contributions

The LFI's innovative design and cooling capabilities offer a quintessential example of deploying state-of-the-art technology in space-based observatories, future-proofing against technological obsolescence while retaining high adaptability for diagnostic data analysis. As a part of the comprehensive Planck mission, LFI advances ontology by deepening the understanding of cosmological parameters, potentially impacting theoretical models and paradigms, such as inflationary theory and cosmic variance bounds.

Summary

The Low Frequency Instrument is a testament to advancing observational astrophysics, synergizing advanced technology with rigorous scientific methods. This combination is essential for addressing cosmological questions, notably those involving the postulated early universe scenarios and the anticipation of analyzing primordial gravitational waves. Further studies emanating from the Planck mission are expected to underpin theoretical conjectures about the universe's evolution, positioning LFI as a cornerstone in astrophysical research and development.