Low Frequency Instrument
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| Low Frequency Instrument | |
|---|---|
| Name | Low Frequency Instrument |
| Mission | Planck |
| Operator | European Space Agency |
| Manufacturers | Thales Alenia Space, Jet Propulsion Laboratory, Rutherford Appleton Laboratory |
| Launch | 2009 |
| Wavelength | 30–70 GHz |
| Detectors | HEMT amplifiers |
| Status | Decommissioned |
Low Frequency Instrument The Low Frequency Instrument was a cryogenic radiometer array flown on the Planck mission that measured microwave anisotropies in the cosmic microwave background. Designed and built by a consortium including Thales Alenia Space, Rutherford Appleton Laboratory, and teams from Jet Propulsion Laboratory, it complemented a companion high-frequency instrument to provide broad spectral coverage from 30 to 70 GHz. Operating near the second Lagrange point, the instrument enabled cross-comparison with data from Wilkinson Microwave Anisotropy Probe and informed foreground separation used by Planck Collaboration analyses.
Overview
The Low Frequency Instrument was one of two principal payloads aboard Planck, the other being the High Frequency Instrument. It targeted the low end of the microwave spectrum—30 GHz, 44 GHz, and 70 GHz channels—providing sensitivity to synchrotron, free–free, and anomalous microwave emission. The instrument employed cryogenic cooling to ~20 K and relied on low-noise amplifiers developed from High Electron Mobility Transistor technology. As part of the European Space Agency science programme, it contributed key data to cosmological parameter estimation, complementing ground-based observatories like Atacama Cosmology Telescope and South Pole Telescope.
Design and Components
The instrument architecture combined a focal-plane array of pseudo-correlation radiometers, waveguide components, and a sorption cooler developed in collaboration with Thales Alenia Space and industrial partners. Each radiometer used a pair of input horns and orthomode transducers to separate polarization states, feeding signals to cryogenic HEMT amplifiers provided by teams including Jet Propulsion Laboratory and Rutherford Appleton Laboratory. The front-end modules were connected via stainless-steel waveguides to warm back-end electronics mounted on the Planck service module. Key subassemblies included feedhorns machined by specialist contractors, polarization splitters designed with expertise from National Radio Astronomy Observatory, and calibration loads developed with input from European Space Research and Technology Centre engineers. Thermal design interfaced with the Planck V-groove radiators and the sorption cooler system used in conjunction with the HFI 4 K stage.
Scientific Objectives and Performance
Primary objectives centered on measuring temperature and polarization anisotropies of the Cosmic Microwave Background across angular scales from the dipole to several degrees, constraining parameters such as the Hubble constant, baryon density, dark matter density, and spectral index of primordial perturbations. The LFI targeted foreground characterization, aiding separation of Galactic synchrotron associated with the Milky Way and polarized emission from compact sources catalogued by Planck Catalogue of Compact Sources. Instrument performance metrics included noise-equivalent temperatures, beam profiles mapped using observations of Jupiter and Mars, and systematic error control validated against signals like the orbital and solar dipole. LFI polarization sensitivity complemented HFI polarization maps, improving constraints on the optical depth to reionization linked to studies by WMAP teams and probes like BICEP2.
Calibration and Data Processing
Calibration exploited celestial standards such as the orbital dipole and planetary calibrators including Jupiter, with beam characterization using transit scans coordinated with ground-based facilities like Effelsberg 100-m Radio Telescope. Time-ordered data were processed by the Planck Collaboration data-processing centers using pipelines for demodulation, glitch removal, 1/f noise mitigation, and map-making implemented with algorithms developed in conjunction with Max Planck Institute for Astrophysics and Imperial College London researchers. Component-separation techniques combined LFI maps with HFI data and external datasets from WMAP and radio surveys to isolate the Cosmic Microwave Background from Galactic foregrounds. End-to-end simulations assessed systematic uncertainties associated with bandpass mismatches, polarization angle calibration linked to measurements by Crab Nebula observations, and straylight from the Sun and Earth.
Operational History and Missions
LFI operated throughout the nominal and extended mission phases of Planck, with routine science surveys beginning after the 2009 launch and commissioning phase. It benefited from coordinated operations with the HFI until the HFI's liquid helium depletion, after which LFI continued data acquisition during an extended period. Data releases from the Planck Collaboration—including the 2013, 2015, and 2018 legacy releases—incorporated progressively refined LFI calibrations and maps. Teams from institutions such as University of Oxford, University of Milano–Bicocca, and Istituto Nazionale di Astrofisica contributed to instrument operations, ground segment support, and scientific data analysis. Post-mission archiving preserved LFI datasets in European data centers for continued access by the community including groups at NASA, NASA affiliates, and university consortia.
Results and Contributions to Cosmology
LFI data played a central role in the joint Planck results that determined cosmological parameters with unprecedented precision, corroborating the Lambda-CDM model and refining the value of the Hubble constant in tension studies involving local distance-ladder measurements by teams like those led by Adam Riess. LFI's foreground maps improved understanding of Galactic synchrotron and anomalous microwave emission tied to studies of the Interstellar Medium by groups at Max Planck Institute for Radio Astronomy and informed polarization analyses relevant to searches for primordial gravitational waves reported by collaborations such as BICEP2 and follow-up studies with Keck Array. Legacy contributions include the LFI-based low-ell polarization constraints on reionization and catalogs of compact sources used across the astrophysical community.