Analysis of thermally-induced effects in Planck Low Frequency Instrument

Abstract

The Planck mission will provide full-sky maps of the Cosmic Microwave Background with unprecedented angular resolution (~ 10') and sensitivity (DT / T = 10^-6). This requires cryogenically cooled, high sensitivity detectors as well as an extremely accurate control of systematic errors, which must be kept at micro-K level. In this work we focus on systematic effects arising from thermal instabilities in the Low Frequency Instrument operating in the 30-100 GHz range. Our results show that it is of crucial importance to assure "in hardware" a high degree of stability. In addition, we provide an estimate of the level at which it is possible to reduce the contamination level in the observed maps by proper analysis of the Time Ordered Data.Comment: 5 pages, 7 figures. The following article has been submitted for publication in the AIP Proceedings of the Workshop on "Experimental Cosmology at millimeter wavelengths", Cervinia, Italy, 9-13 July 200

Dynamic validation of the Planck-LFI thermal model

“…The first product of our analysis is the sampling of the dynamical thermal transfer function between the focal plane cold end and a set of points on the focal plane itself at three frequencies. We then use such measurements to validate the numerical thermal model of the focal plane, thus confirming (1) the validity of the temperature stability requirements on the Planck/LFI focal plane, which have driven the design of Planck, and (2) the validity of a number of works that have used the estimates of the model [9][10][11][12]. Finally, we use the measured transfer functions to estimate the improvement in the stability of the receivers over the LFI requirements and provide an explanation of such improvements in terms of non-ideal thermal contacts.…”

“…In fact, fluctuations in the LFI 20 K stage introduce a potentially serious systematic effect in the LFI science as they may mimic brightness changes in the beam as the satellite scans through the sky [15]. While the pseudo-correlation design of the LFI receivers suppress to first order thermal fluctuations [9], residual effects must be damped to extremely low levels (see table 1). The combination of radiometer susceptibility, expected fluctuations in the 20 K stage, and thermal damping from the instrument were the key factors in the instrument design to ensure adequate stability.…”

Planck-LFI: design and performance of the 4 Kelvin Reference Load Unit

“…We then set limits to each component to comply with the total requirement (both temperature stability and absolute temperature). The main quantities related to the 4KRL performance are the 1/ f knee frequency, which is linked to difference between the reference and sky temperature [23], the temperature stability [25] and the 4KRL sensitivity to spurious RF components that could contaminate the reference signal. The requirements apply to the signal as measured at the input of the hybrid coupler in the FEM, where it is "mixed" with the signal from the sky.…”