Y. Longval

Planck pre-launch status: The HFI instrument, from specification to actual performance

Context. The High Frequency Instrument (HFI) is one of the two focal instruments of the Planck mission. It will observe the whole sky in six bands in the 100 GHz-1 THz range. Aims: The HFI instrument is designed to measure the cosmic microwave background (CMB) with a sensitivity limited only by fundamental sources: the photon noise of the CMB itself and the residuals left after the removal of foregrounds. The two high frequency bands will provide full maps of the submillimetre sky, featuring mainly extended and point source foregrounds. Systematic effects must be kept at negligible levels or accurately monitored so that the signal can be corrected. This paper describes the HFI design and its characteristics deduced from ground tests and calibration. Methods: The HFI instrumental concept and architecture are feasible only by pushing new techniques to their extreme capabilities, mainly: (i) bolometers working at 100 mK and absorbing the radiation in grids; (ii) a dilution cooler providing 100 mK in microgravity conditions; (iii) a new type of AC biased readout electronics and (iv) optical channels using devices inspired from radio and infrared techniques. Results: The Planck-HFI instrument performance exceeds requirements for sensitivity and control of systematic effects. During ground-based calibration and tests, it was measured at instrument and system levels to be close to or better than the goal specification.

Planck pre-launch status: The optical architecture of the HFI

The Planck High Frequency Instrument, HFI, has been designed to allow a clear unobscured view of the CMB sky through an off-axis Gregorian telescope. The prime science target is to measure the polarized anisotropy of the CMB with a sensitivity of 1 part in 106 with a maximum spatial resolution of 5 arcmin (Cl ~ 3000) in four spectral bands with two further high-frequency channels measuring total power for foreground removal. These requirements place critical constraints on both the telescope configuration and the receiver coupling and require precise determination of the spectral and spatial characteristics at the pixel level, whilst maintaining control of the polarisation. To meet with the sensitivity requirements, the focal plane needs to be cooled with the optics at a few Kelvin and detectors at 100 mK. To limit inherent instrumental thermal emission and diffraction effects, there is no vacuum window, so the detector feedhorns view the telescope secondary directly. This requires that the instrument is launched warm with the cooler chain only being activated during its cruise to L2. Here we present the novel optical configuration designed to meet with all the above criteria.

Planck Pre-Launch Status: HFI Beam Expectations from the Optical Optimisation of the Focal Plane

Planck is a European Space Agency (ESA) satellite, launched in May 2009, which will map the cosmic microwave background anisotropies in intensity and polarisation with unprecedented detail and sensitivity. It will also provide full-sky maps of astrophysical foregrounds. An accurate knowledge of the telescope beam patterns is an essential element for a correct analysis of the acquired astrophysical data. We present a detailed description of the optical design of the High Frequency Instrument (HFI) together with some of the optical performances measured during the calibration campaigns. We report on the evolution of the knowledge of the pre-launch HFI beam patterns when coupled to ideal telescope elements, and on their significance for the HFI data analysis procedure.

Multi-mode horn design and beam characteristics for the Planck satellite

The ESA Planck satellite has begun studying the anisotropies of the cosmic microwave background radiation over the whole sky with unprecedented sensitivity and high angular resolution. The High Frequency Instrument, HFI, on Planck is observing simultaneously in six bands in the range 100 GHz to 857 GHz. The inclusion of non-CMB bands allows for robust removal of foreground sources from the data. This paper is concerned with the design, modeling and predicted performances of the two highest frequency channels centered on 545 GHz and 857 GHz, which use specialized multi-mode feedhorns, and are dedicated to observing these foregrounds. Multi-mode systems have the advantage of increasing the throughput, and thus sensitivity, of the detection assembly when diffraction limited resolution is not required. The horns are configured in a back-to-back setup which transmits the signal through filters to a detector horn. The modeling of the broadband beam patterns on the sky is shown to require careful analysis. Simulations of the complex interactions of the horns is computationally challenging when the detector horn in the relay system is included. The paper describes the approach to modeling these high frequency channels and discusses how the optical requirements on the horn designs are met in terms of spillover, edge taper, illumination of the telescope aperture and beam patterns on the sky.

PILOT: a balloon-borne experiment to measure the polarized FIR emission of dust grains in the interstellar medium

Future cosmology space missions will concentrate on measuring the polarization of the Cosmic Microwave Back- ground, which potentially carries invaluable information about the earliest phases of the evolution of our universe. Such ambitious projects will ultimately be limited by the sensitivity of the instrument and by the accuracy at which polarized foreground emission from our own Galaxy can be subtracted out. We present the PILOT balloon project which will aim at characterizing one of these foreground sources, the polarization of the dust continuum emission in the diffuse interstellar medium. The PILOT experiment will also constitute a test-bed for using multiplexed bolometer arrays for polarization measurements. We present the results of ground tests obtained just before the first flight of the instrument.

Instrumental polarization modelling for the PILOT submm experiment

PILOT is a balloon borne experiment which aims at measuring precisely the polarized emission of the interstellar dust emission, in the submm range (240 and 550 μm). These measurements will be used to reach a better understanding of the galactic magnetic field role in the structure of the Galaxy and the star formation process. They will be useful too for CMB experiments by providing a precise knowledge of galactic foreground emission. Simulations including realistic instrument performances show that after three flights (around 24 hours each), it will be possible to cover the full galactic plane map (±30° in latitude). In addition, several deep surveys will be performed at high galactic latitude. As the level of polarized emission of interstellar dust is less than 5%, an accurate knowledge of the instrumental polarization is mandatory for the data processing and analysis.

ELISA: A small balloon Experiment for a Large Scale Survey in the Sub-millimeter

This paper presents the technical aspects and scientific objectives of the balloon-borne Experiment for Large Infrared Survey Astronomy (ELISA). The emphasis is put upon the synergies existing between the ELISA project and future space missions, both with respect to technical and scientific aspects. ELISA is a small balloon project for an experiment dedicated to measure the Far-Infrared to Sub-millimeter continuum emission of dust over a large fraction of the sky, with unprecedented sensitivity and angular resolution. The primary mirror of the telescope, similar to the one used for the Top-Hat mission, will have a diameter of 1 m, ensuring an angular resolution of about 3.5′. PACS-type bolometer arrays will be used in four photometric bands centered at 170, 240, 400, and 650 μm and providing a 22′×45′ instantaneous field of view per channel. A liquid He cryostat will host the cold optics, including the secondary mirror of the telescope, as well as the detectors, which will be cooled to 0.3 K using an He3 ...

PILOT: optical performance and end-to-end characterisation

PILOT (Polarized Instrument for the Long-wavelength Observations of the Tenuous ISM), is a balloon-borne astronomy experiment dedicated to study the polarization of dust emission from the diffuse ISM in our Galaxy [1]. The observations of PILOT have two major scientific objectives. Firstly, they will allow us to constrain the large-scale geometry of the magnetic field in our Galaxy and to study in details the alignment properties of dust grains with respect to the magnetic field. In this domain, the measurements of PILOT will complement those of the Planck satellite at longer wavelengths. In particular, they will bring information at a better angular resolution, which is critical in crowded regions such as the Galactic plane. They will allow us to better understand how the magnetic field is shaping the ISM material on large scale in molecular clouds, and the role it plays in the gravitational collapse leading to star formation. Secondly, the PILOT observations will allow us to measure for the first time the polarized dust emission towards the most diffuse regions of the sky, where the measurements are the most easily interpreted in terms of the physics of dust. In this particular domain, PILOT will play a role for future CMB missions similar to that played by the Archeops experiment for Planck. The results of PILOT will allow us to gain knowledge about the magnetic properties of dust grains and about the structure of the magnetic field in the diffuse ISM that is necessary to a precise foreground subtraction in future polarized CMB measurements. The PILOT measurements, combined with those of Planck at longer wavelengths, will therefore allow us to further constrain the dust models. The outcome of such studies will likely impact the instrumental and technical choices for the future space missions dedicated to CMB polarization. The PILOT instrument will allow observations in two photometric channels at wavelengths 240 μm and 550 μm, with an angular resolution of a few arcminutes. We will make use of large format bolometer arrays, developed for the PACS instrument on board the Herschel satellite. With 1024 detectors per photometric channel and photometric band optimized for the measurement of dust emission, PILOT is likely to become the most sensitive experiment for this type of measurements. The PILOT experiment will take advantage of the large gain in sensitivity allowed by the use of large format, filled bolometer arrays at frequencies more favorable to the detection of dust emission. This paper presents the optical design, optical characterization and its performance. We begin with a presentation of the instrument and the optical system and then we summarise the main optical tests performed. In section III, we present preliminary end-to-end test results.

Pilot optical alignment

PILOT (Polarized Instrument for Long wavelength Observations of the Tenuous interstellar medium) is a balloonborne astronomy experiment designed to study the polarization of dust emission in the diffuse interstellar medium in our Galaxy. The PILOT instrument allows observations at wavelengths 240 μm (1.2THz) with an angular resolution about two arc-minutes. The observations performed during the first flight in September 2015 at Timmins, Ontario Canada, have demonstrated the optical performances of the instrument.

Pilot, a balloon borne experiment underground tests

PILOT is a balloon borne experiment, which will measure the polarized emission of dust grains, in the interstellar medium, in the sub millimeter range (with two photometric channels centered at 240 and 550 μm). The primary and secondary mirror must be positioned with accuracies better than 0.6 mm and 0.06°. These tolerances include environmental conditions (mainly gravity and thermo-elastic effects), uncertainties on alignments, and uncertainties on the dilatation coefficient. In order to respect these tolerances, we need precise characterization of each optical component. The characterization of the primary mirror and the integrated instrument is performed using a dedicated submillimeter test bench. A brief description of the scientific objectives and instrumental concept is given in the first part. We present, in the second and in the third part, the status of these ground tests, first results and planned tests.