Planck 2013 results. IX. HFI spectral response

TL;DR

The paper addresses the need for accurate relative spectral responses of Planck HFI detectors to enable reliable component separation and unit conversion. It presents a comprehensive ground-based spectral characterization using a scanning Fourier transform spectrometer, derives detector-, band-, and sub-band-level spectra, and formulates unit conversion and colour-correction coefficients, including CO, dust, and SZ components. The results show strong agreement between ground-based spectral characterizations and in-flight data, with exceptional out-of-band rejection (better than $10^{8}$) and validated cross-checks against zodiacal light, CO, SZ, and dust analyses; remaining disparities (notably at 143 GHz for dust and in CO isotopologue comparisons) are acknowledged and being investigated. The validated spectral response data and the accompanying UcCC tools are published for Planck data analysis, enabling accurate maps, component separation, and astrophysical inferences in the Planck legacy dataset, including polarization analyses in future work.

Abstract

The Planck High Frequency Instrument (HFI) spectral response was determined through a series of ground based tests conducted with the HFI focal plane in a cryogenic environment prior to launch. The main goal of the spectral transmission tests was to measure the relative spectral response (including out-of-band signal rejection) of all HFI detectors. This was determined by measuring the output of a continuously scanned Fourier transform spectrometer coupled with all HFI detectors. As there is no on-board spectrometer within HFI, the ground-based spectral response experiments provide the definitive data set for the relative spectral calibration of the HFI. The spectral response of the HFI is used in Planck data analysis and component separation, this includes extraction of CO emission observed within Planck bands, dust emission, Sunyaev-Zeldovich sources, and intensity to polarization leakage. The HFI spectral response data have also been used to provide unit conversion and colour correction analysis tools. Verifications of the HFI spectral response data are provided through comparisons with photometric HFI flight data. This validation includes use of HFI zodiacal emission observations to demonstrate out-of-band spectral signal rejection better than 10^8. The accuracy of the HFI relative spectral response data is verified through comparison with complementary flight-data based unit conversion coefficients and colour correction coefficients. These coefficients include those based upon HFI observations of CO, dust, and Sunyaev-Zeldovich emission. General agreement is observed between the ground-based spectral characterization of HFI and corresponding in-flight observations, within the quoted uncertainty of each; explanations are provided for any discrepancies.

Figures (24)

• Figure 1: Band-average spectral transmission for each of the HFI frequency channels.
• Figure 2: Sample bolometer spectra from HFI 100GHz detector 1a (the HFI detector naming scheme is discussed in Sect. \ref{['sec:Data']}). The black dots represent data from individual spectra while the coloured solid line represents the average of approximately 100 individual spectra.
• Figure 3: HFI detector spectral SNR for the scan-averaged HFI detector spectra prior to taking the ratio with the scan-averaged reference spectra. Also shown is the SNR for the scan-averaged reference bolometer spectra over the same spectral region.
• Figure 4: Example ratioed spectrum (black), filter spectrum (red), waveguide model (blue), and final output spectral response (violet) for HFI 100 GHz detector 1a. The transition frequencies are shown by the vertical dashed lines. While the in-band spectral response is determined by the ratioed spectrum, the out-of-band transmission is determined by the waveguide model and filter data.
• Figure 5: The detector spectral transmission profiles for the HFI 100 GHz detectors. Plot (a) shows the in-band region on a linear vertical scale with plot (b) showing a wider spectral region on a logarithmic vertical scale. The inset within plot (b) shows the same spectra over the full spectral range available with the same units as the main plot for all axes. The same conventions hold for Fig. \ref{['fig:HFI143']}--Fig. \ref{['fig:HFI857']}.