HFI-systematics

Like all experiments, Planck HFI had a number of specific issues that needed to be tracked to verify that they were not compromising the data. While these are discussed in appropriate sections, here we gather the systematic effects affecting the TOIs together to give brief summaries of the issues and refer the reader to the appropriate sections for more details.

• Cosmic rays – unprotected by the atmosphere and more sensitive than previous bolometric experiments, HFI saw many more cosmic ray hits than its predecessors. These were detected, the worst parts of the data flagged as unusable, and "tails" were modelled and removed. This is described in the section on glitch statistics as well as in the 2013 HFI glitch removal paper.
• "Elephants" – cosmic rays also hit the HFI 100-mK stage and cause the temperature to vary, inducing small temperature and thus noise variations in the detectors. These elephants are removed with the rest of the thermal fluctuations, described directly below.
• Thermal fluctuations – HFI is an extremely stable instrument, but there are small thermal fluctuations. These are discussed in the timeline processing section on thermal decorrelation.
• Random telegraphic signal (RTS) or "popcorn noise" – some channels were occasionally affected by what seems to be a baseline that abruptly changes between two levels, which has been variously called popcorn noise or random telegraphic signal. These data are usually flagged. This is described in the section on noise stationarity.
• Jumps – similar to (but distinct from) popcorn noise, small jumps were occasionally found in the data streams. These jumps are usually corrected, as described in the section on jump corrections.
• 4-K cooler-induced EM noise – the 4-K cooler induced noise in the detectors with very specific frequency signatures, which can be filtered. This is described in the 2013 HFI DPC Paper; their stability is discussed in the section on 4-K cooler line stability.
• Compression – on-board compression is used to overcome our telemetry bandwidth limitations. This is explained in Planck-Early-IV.
• ADC – Planck-2013-VII reported that the HFI raw data show apparent gain variations with time of up to 2% due to nonlinearities in the HFI readout chain. In the 2013 data release (Planck-2013-VIII) a correction for this systematic error was applied as an apparent gain variation at the map making stage. The 2013 maps relied on an effective gain correction based on the consistency constraints from the reconstructed sky maps, which proved to be sufficient for the 2013 cosmological analysis based on temperature only.

For the 2015 data release we have implemented a direct ADC correction in the TOI. The ADC effect and its correction, and its validation through end-to-end simulations are described in sections 2 and 5 of . An improved correction for the ADC affect using the SRoll map-making algorithm was implemented for the 2018 release, and is described in Planck-2016-XLVI and Planck-2020-A3.

• Noise correlations – correlations in noise between detectors seems to be negligible, except for two polarization-sensitive detectors in the same horn. This is discussed in the 2013 HFI Glitch removal paper.
• Pointing – the final pointing reconstruction for Planck is near the arcsecond level. This is discussed in the 2013 HFI DPC Paper.
• Focal plane geometry – the relative positions of different horns in the focal plane are reconstructed using planets. This is also discussed in the 2013 HFI DPC paper.
• Main beam – the main beams for HFI are discussed in the 2013 Beams and Transfer function paper.
• Ruze envelope – random imperfections, or dust on the mirrors, can mildly increase the size of the beam. This is discussed in the 2013 Beams and Transfer function paper.
• Dimpling – the mirror support structure causes a pattern of small imperfections in the beams, which generate small sidelobe responses outside the main beam. This is discussed in the the 2013 Beams and Transfer function paper.
• Far sidelobes – small amounts of light can sometimes hit the detectors from just above the primary or secondary mirrors, or even from reflections off the baffles. While small, when the Galactic centre is in the right position, this can be detected in the highest frequency channels, and so is removed from the data. This is discussed in the 2013 Beams and Transfer function paper and also in the 2013 Zodiacal emission paper.
• Planet fluxes – comparing the known flux densities of planets with the calibration on the CMB dipole is a useful check of calibration for the CMB channels, and is the primary calibration source for the submillimetre channels. This is done in the 2013 Mapmaking and Calibration paper.
• Point source fluxes – as with planet fluxes, we also compare fluxes of known, bright point sources with the CMB dipole calibration. This is done in the 2013 Mapmaking and Calibration paper.

The systematic effects affecting the maps were corrected in open loop in the 2015 release, using ground measurements and modelisation. For the 2018 release, almost all known systematics affecting the maps have been corrected. Using together ground-based and extracted-from-the-sky determination of the parameters of these systematic effects, we correct these at the mapmaking level using the SRoll generalized polarized destripper.

Previous Releases: (2013) and (2015) HFI systematic effects

2013 & 2015 HFI systematic effects

Like all experiments, Planck-HFI had a number of specific issues that it needed to be tracked to verify that they were not compromising the data. While these are discussed in appropriate sections, here we gather them together to give brief summaries of the issues and refer the reader to the appropriate section for more details.

• Cosmic rays – unprotected by the atmosphere and more sensitive than previous bolometric experiments, HFI saw many more cosmic ray hits than its predecessors. These were detected, the worst parts of the data flagged as unusable, and "tails" were modelled and removed. This is described in the section on glitch statistics as well as in the 2013 HFI glitch removal paper.
• "Elephants" – cosmic rays also hit the HFI 100-mK stage and cause the temperature to vary, inducing small temperature and thus noise variations in the detectors. These elephants are removed with the rest of the thermal fluctuations, described directly below.
• Thermal fluctuations – HFI is an extremely stable instrument, but there are small thermal fluctuations. These are discussed in the timeline processing section on thermal decorrelation.
• Random telegraphic signal (RTS) or "popcorn noise" – some channels were occasionally affected by what seems to be a baseline that abruptly changes between two levels, which has been variously called popcorn noise or random telegraphic signal. These data are usually flagged. This is described in the section on noise stationarity.
• Jumps – similar to (but distinct from) popcorn noise, small jumps were occasionally found in the data streams. These jumps are usually corrected, as described in the section on jump corrections.
• 4-K cooler-induced EM noise – the 4-K cooler induced noise in the detectors with very specific frequency signatures, which can be filtered. This is described in the 2013 HFI DPC Paper; their stability is discussed in the section on 4-K cooler line stability.
• Compression – on-board compression is used to overcome our telemetry bandwidth limitations. This is explained in Planck-Early-IV.
• Noise correlations – correlations in noise between detectors seems to be negligible, except for two polarization-sensitive detectors in the same horn. This is discussed in the 2013 HFI Glitch removal paper.
• Pointing – the final pointing reconstruction for Planck is near the arcsecond level. This is discussed in the 2013 HFI DPC Paper.
• Focal plane geometry – the relative positions of different horns in the focal plane are reconstructed using planets. This is also discussed in the 2013 HFI DPC paper.
• Main beam – the main beams for HFI are discussed in the 2013 Beams and Transfer function paper.
• Ruze envelope – random imperfections, or dust on the mirrors, can mildly increase the size of the beam. This is discussed in the 2013 Beams and Transfer function paper.
• Dimpling – the mirror support structure causes a pattern of small imperfections in the beams, which generate small sidelobe responses outside the main beam. This is discussed in the the 2013 Beams and Transfer function paper.
• Far sidelobes – small amounts of light can sometimes hit the detectors from just above the primary or secondary mirrors, or even from reflections off the baffles. While small, when the Galactic centre is in the right position, this can be detected in the highest frequency channels, and so is removed from the data. This is discussed in the 2013 Beams and Transfer function paper and also in the 2013 Zodiacal emission paper.
• Planet fluxes – comparing the known flux densities of planets with the calibration on the CMB dipole is a useful check of calibration for the CMB channels, and is the primary calibration source for the submillimetre channels. This is done in the 2013 Mapmaking and Calibration paper.
• Point source fluxes – as with planet fluxes, we also compare fluxes of known, bright point sources with the CMB dipole calibration. This is done in the 2013 Mapmaking and Calibration paper.
• Time constants – the HFI bolometers do not react instantaneously to light; there are small time constants, discussed in the 2013 Beams and Transfer function paper.
• ADC correction – the HFI analogue-to-digital converters are not perfect, and are not used perfectly. Their effects on the calibration are discussed in the 2013 Mapmaking and Calibration paper.
• Bandpass – the transmission curves, or "bandpasses" have shown up in a number of places. This is discussed in the 2013 spectral response paper.