17 research outputs found
Consequences of constant elevation scans for instrumental systematics in Cosmic Microwave Background Experiments
Spin characterization of systematics in CMB surveys- A comprehensive formalism
The CMB -mode polarisation signal -- both the primordial gravitational
wave signature and the signal sourced by lensing -- is subject to many
contaminants from systematic effects. Of particular concern are systematics
that result in mixing of signals of different ``spin'', particularly leakage
from the much larger spin-0 intensity signal to the spin-2 polarisation signal.
We present a general formalism, which can be applied to arbitrary focal plane
setups, that characterises signals in terms of their spin. We provide general
expressions to describe how spin-coupled signals observed by the detectors
manifest at map-level, in the harmonic domain, and in the power spectra,
focusing on the polarisation spectra -- the signals of interest for upcoming
CMB surveys. We demonstrate the presence of a previously unidentified
cross-term between the systematic and the intrinsic sky signal in the power
spectrum, which in some cases can be the dominant source of contamination. The
formalism is not restricted to intensity to polarisation leakage but provides a
complete elucidation of all leakage including polarisation mixing, and applies
to both full and partial (masked) sky surveys, thus covering space-based,
balloon-borne, and ground-based experiments. Using a pair-differenced setup, we
demonstrate the formalism by using it to completely characterise the effects of
differential gain and pointing systematics, incorporating both intensity
leakage and polarisation mixing. We validate our results with full time ordered
data simulations. Finally, we show in an Appendix that an extension of simple
binning map-making to include additional spin information is capable of
removing spin-coupled systematics during the map-making process.Comment: Minimal revisions - some additional references added, typos fixed,
etc. 31 pages, 8 figures, 1 table. Accepted by MNRA
Blind map level systematics cleaning: a quadratic estimator approach
From IOP Publishing via Jisc Publications RouterHistory: received 2021-01-25, accepted 2021-06-09, ppub 2021-07, epub 2021-07-09, open-access 2021-07-09Publication status: PublishedAbstract: We present the first detailed case study using quadratic estimators (QE) to diagnose and remove systematics present in observed Cosmic Microwave Background (CMB) maps. In this work we focus on the temperature to polarization leakage. We use an iterative QE analysis to remove systematics, in analogy to de-lensing, recovering the primordial B-mode signal and the systematic maps. We introduce a new Gaussian filtering scheme crucial to stable convergence of the iterative cleaning procedure and validate with comparisons to semi-analytical forecasts. We study the limitations of this method by examining its performance both on idealized simulations and on more realistic, non-ideal simulations, where we assume varying de-lensing efficiencies. Finally, we quantify the systematic cleaning efficiency by presenting a likelihood analysis on the tensor to scalar ratio, r, and demonstrate that the blind cleaning results in an un-biased measurement of r, reducing the systematic induced B-mode power by nearly two orders of magnitude
Controlling systematics in ground-based CMB surveys with partial boresight rotation
Future CMB experiments will require exquisite control of systematics in order
to constrain the -mode polarisation power spectrum. One class of systematics
that requires careful study is instrumental systematics. The potential impact
of such systematics is most readily understood by considering analysis
pipelines based on pair differencing. In this case, any differential gain,
pointing or beam ellipticity between the two detectors in a pair can result in
intensity leakage into the -mode spectrum, which needs to be controlled to a
high precision due to the much greater magnitude of the total intensity signal
as compared to the -mode signal. One well known way to suppress such
systematics is through careful design of the scan-strategy, in particular
making use of any capability to rotate the instrument about its pointing
(boresight) direction. Here, we show that the combination of specific choices
of such partial boresight rotation angles with redundancies present in the scan
strategy is a powerful approach for suppressing systematic effects. This
mitigation can be performed in analysis in advance of map-making and, in
contrast to other approaches (e.g. deprojection or filtering), results in no
signal loss. We demonstrate our approach explicitly with time ordered data
simulations relevant to next-generation ground-based CMB experiments, using
deep and wide scan strategies appropriate for experiments based in Chile. These
simulations show a reduction of multiple orders of magnitude in the spurious
-mode signal arising from differential gain and differential pointing
systematics.Comment: 6 figures. Updated to match published version with a few minor
clarifications; the main change is a brief discussion of bandpass mismatc
Blind Map Level Systematics Cleaning: A Quadratic Estimator Approach
We present the first detailed case study using quadratic estimators (QE) to
diagnose and remove systematics present in observed Cosmic Microwave Background
(CMB) maps. In this work we focus on the temperature to polarization leakage.
We use an iterative QE analysis to remove systematics, in analogy to
de-lensing, recovering the primordial B-mode signal and the systematic maps. We
introduce a new Gaussian filtering scheme crucial to stable convergence of the
iterative cleaning procedure and validate with comparisons to semi-analytical
forecasts. We study the limitations of this method by examining its performance
both on idealized simulations and on more realistic, non-ideal simulations,
where we assume varying de-lensing efficiencies. Finally, we quantify the
systematic cleaning efficiency by presenting a likelihood analysis on the
tensor to scalar ratio, , and demonstrate that the blind cleaning results in
an un-biased measurement of , reducing the systematic induced B-mode power
by nearly two orders of magnitude.Comment: 30 pages, 12 figures, prepared for submission to JCA