Jean-Michel Lamarre

Photon noise in photometric instruments at far-infrared and submillimeter wavelengths

Photon noise in photometric instruments is computed using two different methods. A first expression, derived from quantum physics, corresponds to the most widely used formulas and is applicable when diffraction phenomena are negligible. The second one is derived from the semiclassical approach of Hanbury Brown and Twiss. Properties of this expression, including consistency with the former one, asymptotical behavior, and scale properties are derived and discussed. The effect of polarization is studied, and practical examples are given.

Planck pre-launch status: The optical system

Planck is a scientific satellite that represents the next milestone in space-based research related to the cosmic microwave background, and in many other astrophysical fields. Planck was launched on 14 May of 2009 and is now operational. The uncertainty in the optical response of its detectors is a key factor allowing Planck to achieve its scientific objectives. More than a decade of analysis and measurements have gone into achieving the required performances. In this paper, we describe the main aspects of the Planck optics that are relevant to science, and the estimated in-flight performance, based on the knowledge available at the time of launch. We also briefly describe the impact of the major systematic effects of optical origin, and the concept of in-flight optical calibration. Detailed discussions of related areas are provided in accompanying papers.

First Measurement of the Submillimeter Sunyaev-Zeldovich Effect

We report the first detection of the Sunyaev-Zeldovich (S-Z) increment on the cosmic microwave background at submillimeter wavelengths in the direction of a cluster of galaxies. It was achieved toward the rich cluster Abell 2163, using the PRONAOS 2 m stratospheric telescope. Together with data from the SuZIE, Diabolo, and ISO-PHT experiments, these measurements, for the first time, give a complete picture of the far-infrared-to-millimeter spectral energy distribution of the diffuse emission toward a cluster of galaxies. It clearly shows the positive and negative parts of the S-Z effect and also a positive signal at short wavelengths that can be attributed to foreground dust in our Galaxy.

Metallic mesh properties and design of submillimeter filters

By using a Michelson interferometer in the asymmetric mode with helium cooled bolometer, we have measured with precision the complex transmittance and reflectance of metallic meshes in the wavelength region 0.7<λ/g<5, where λ is the wavelength, and g is the period of the mesh. For λ≅g we observed a minimum power reflectance smaller than 10−4 at normal incidence. The important variations of the transmittance with the angle of incidence have been thoroughly investigated. Changes around the maximum transmittance are explained by the propagation of diffracted modes. Phase measurements show that a sharp dip appearing at λd with 1<λd/g<1.8 is related to the finite thickness of the mesh. From our measurements as well as other data precedently published it appears that there is a linear dependance between λd/g and the relative width of the slots of the mesh. All these deviations from classical models must absolutely be taken in account when designing high performances far infrared filters.

Beam mismatch effects in Cosmic Microwave Background polarization measurements

Measurement of cosmic microwave background polarization is today a major goal of observational cosmology. The level of the signal to measure, however, makes it very sensitive to various systematic effects. In the case of Planck, which measures polarization by combining data from various detectors, the beam asymmetry can induce a conversion of temperature signals to polarization signals or a polarization mode mixing. In this paper, we investigate this effect using realistic simulated beams and propose a first-order method to correct the polarization power spectra for the induced systematic effect.

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.

Fast Physical Optics Simulations of the Multi-Beam Dual-Reflector Submillimeter-Wave Telescope on the ESA PLANCK Surveyor

We present physical optics simulations of the multi-beam dual-reflector submillimeter-wave telescope on the ESA PLANCK surveyor designed for measuring the temperature anisotropies and polarization characteristics of the cosmic microwave background. The telescope is of a non-conventional Gregorian configuration, with two ellipsoidal reflectors providing a very large field of view at the focal plane where the array of 76 horn antennas feeding low-temperature detectors is located. We analyse the defocusing effects of the system, the polarization properties of the telescope, and the optical performance of the high-frequency channels based on special multi-moded horns operating at 545 and 857 GHz.

Analytical approach to optimizing alternating current biasing of bolometers

Bolometers are most often biased by alternating current (AC) in order to get rid of low-frequency noises that plague direct current (DC) amplification systems. When stray capacitance is present, the responsivity of the bolometer differs significantly from the expectations of the classical theories. We develop an analytical model that facilitates the optimization of the AC readout electronics design and tuning. This model is applied to cases similar to the bolometers in the Planck space mission. We study how the responsivity and the noise equivalent power (NEP) of an AC biased bolometer depend on these essential parameters: bias current, heat sink temperature and background power, modulation frequency of the bias, and stray capacitance. We show that the optimal AC bias current in the bolometer is significantly different from that of the DC case as soon as a stray capacitance is present due to the difference in the electrothermal feedback. We also compare the performance of square and sine bias currents and show a slight theoretical advantage for the latter. This work resulted from the need to be able to predict the real behavior of AC biased bolometers in an extended range of working parameters. It proved to be applicable to optimize the tuning of the Planck High-Frequency Instrument bolometers.

The Planck high-frequency instrument: a third-generation CMB probe and the first submillimeter surveyor

The High Frequency Instrument of the Planck satellite is dedicated to the measurement of the anisotropy of the Cosmic Microwave Background (CMB). Its main goal is to map the CMB with a sensitivity of ΔT/T=2.10-6 and an angular resolution of 5 arcmin in order to constrain cosmological parameters. Planck is a project of the European Space Agency based on a wide international collaboration, including United States and Canadian laboratories. The architecture of the satellite is driven by the thermal requirements resulting from the search for low photon noise. Especially, the passively cooled telescope should be at less than 50K, while a cascade of cryo-coolers will ensure the cooling of the HFI bolometers down to 0.1K. This last temperature will be produced by a gravity insensitive 3He/4He dilution cooler. This will be achieved at the L2 Lagrangian point of the Sun-Earth system. The whole sky will be observed two times in the 14 months mission with a scanning strategy based on a 1RPM rotation of the satellite. In addition to the cosmological parameters that can be derived from the CMB maps, Planck will deliver nine high sensitivity submillimeter maps of the whole sky that will constitute unique data available to the whole astronomical community.

Periodic Faraday Bias and Lock-In Phenomena in a Laser Gyro

A simple theoretical description of coupling phenomena in a laser gyro has been developed. In particular, this allows the calculation of the residual nonlinearity of the response of the laser gyro in the presence of a periodic bias. A laser gyro has been constructed in a CER-VIT block. To solve the lock-in zone problem this detector contains a periodic bias system using the Faraday effect. The results of a number of trials are reported. In particular, we obtain experimental values of residual error due to coupling phenomena for each rotational speed. These results are in good agreement with a simple coupled oscillators theory.