Campuses:
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| Name | Description | Effect | References | Mitigation | SRF | |
|---|---|---|---|---|---|---|
| Pair Differencing | ||||||
| Bandpass mismatch | Edges and shapes of the the spectral filters vary from detector to detector. Can be thought of as spectral-source-dependent gain variation between detectors. | T → P, P → P leakage | HFI 2015 : Calibration & Maps, HT, GP, RB Bandpass paper, Planck low-ell, ECO-Systematics | a) Calibration b) The Planck papers give a prescription for correction and a map-making algorithm 'SRoll'. Ranajoy(RB) has developed a technique to filter out the spurious signal demonstrated in ECO paper and the independent paper will be out shortly. c) full I/Q/U maps for individual detectors d) polarization modulation | 4 | |
| Beam mismatch | Beam shapes differ between two detectors which are pair-differences to measure P. Planck beam mismatch induced a noticeable contamination of the TE spectrum by TT, which had to be modelled and corrected for, and the chromatic variation of beam shape certainly was important, but the physical optics modelling was never enable to tell us. | T→P, P→P | EH Quickpol, CW Beams, CW, Scan Strategy, ECO-Systematics Shimon et al, 2008 BICEP/Keck VII (Tolan) | a) Optical modelling, Point Source Measurement b) Analytical techniques demonstrated by Eric Hivon(EH) using Quickpol and by Chris Wallis(CW). Real space convolution having knowledge of the beams and correction demonstrated in ECO-Systematics by RB(independent paper expected soon). c) purification methods for cleaning E leaked into B d) polarization modulation | 4 | |
| Gain Mismatch | Gain mismatch between detectors in pair differencing creates T→P leakage | T→P | Rosset et al 2010 (Planck HFI) | (see Gain Stability below) | ||
| Optics | ||||||
| Instrumental Polarization | T→P leakage by optical elements in the instrument (reflectors, lenses) | T→P | a) Physical Optics Simulations, Unpolarized point sources scans b) Didier thesis (2016) for mitigation technique | |||
| Cross Polarization | Q↔U rotation by the optical elements in the instrument | E→B | Physical Optics Simulations, Polarized point source scans | |||
| Sidelobes: Reflector Spillover | Rays / beam edges missing the primary or secondary mirrors. Can propagate to the sky before completing all of their intended “bounces” and/or terminate on non-reflective surfaces adding extra loading and picking up signals far off the main beam axis (i.e. the galaxy). Spillover can be highly polarized. | Extra loading and spurious polarization power | Tauber 2010 | Beam Maps, Physical Optics Simulations | ||
| Sidelobes: Stray reflections | Reflections (sometimes coupling to what are assumed to be absorptive elements) can send rays to off-axis angles | beam shape | QUIET instrument paper | Optical Modelling, Beam maps | ||
| Sidelobes: Surface roughness | Random surfacer errors remove power from the main beam and redistribute it over large angles. The process results in a smooth beam shoulder often referred to as a Ruze envelope. | beam shape | Planck 2013 beam paper | Measured with interferometric/photogrammetry techniques | ||
| Beam ghosting | Off-axis beam response caused by internal reflections. Often associated with non-idealities in for example anti-reflection coatings or filters. | beam shape, T→P | Bicep systematics, SPIDER systematics, Sean Bryan thesis | Far field beam maps | ||
| Scattering | Diffuse scattering of light on various optical elements, in some cases due to surface impurities. This is an issue for ACTPol | beam shape, T→P | Chiang thesis | Detailed modelling and testing of each cold optical element | ||
| Sidelobes: diffraction | Light diffracts on edges that vary on scales that are comparable or smaller than the wavelength of light in question. Diffraction often results in a polarized signal. | T→P | Fraisse et al SPIDER, Essinger thesis ABS | Can be mitigated by adding smoothly varying transitions. | ||
| Pointing systematic error & distortion | Detector pointing at position different from flight telemetry data. | Level of leakage demonstrated in the ECO paper for different cases. | Technique similar to bandpass leakage correction by filtering out spurious signal developed by RB, to be published soon. | |||
| Pointing Jitter | Random Pointing Error mixes T, E and B at high \ell | T→P, P→P | Miller et al 08 for impact on lensing, Didier thesis for E->B | |||
| Chromatic beam shape | Beam shape is a function of source SED: measured using a planet, used to build a window function to correct CMB power spectrum. | T→P, P→P, miscalibration of window function | Planck 2013 VII | Physical optics modeling | Note: this was a most important effect for Planck | |
| Detectors | ||||||
| Time Constant Accuracy & Stability | Uncertainty in time constants of the detectors (either from measurements or variation with time) creates uncertainty in polarization angle, pointing and beam size. Incoming power variation creates loop gain variation hence time constant variation. For a constant spin-rate spacecraft this effect is exactly degenerate with the beam shape. | P→P, beam size error, pointing error | Irwing & hilton, | Stimulator flash, Lab measurement | ||
| Readout Cross Talk | Power in one detector leaks into other detectors | T→P, P→P, T supression, high-\ell beam distorsion, frequency mixing | Dfmux: Dobbs et al 2011, Darcy Barron thesis | Electrical model, Measure mixing matrix with point source observation, cosmic ray events | ||
| Polarization Angle | Uncertainty in polarization calibration creates E→B leakage | E→B | Sacrifice cosmic birefringence limits to zero TB,TE,EB Improvement of calibration sources: difficult across broad bands and large. Onboard calibrator? | |||
| Gain Stability | Time-variation of gain (dT/dI) or sensitivity (dP/dI) coming from various sources (bath temperature variations, optical power drifts | Miscalibration, T→P, P→P | Irwing & hilton, der Kuur et al | Repeated calibration measurements with point sources, measure loop gain variations (IV curves), estimate optical power drifts through temperature monitoring, bias steps dipole provided best on-orbit gain measurement | ||
| Detector Non-Linearity | Detector has limited range to convert incoming power to current (i.e. gain change with large power variations) | Miscalibration | Irwing & hilton, Rostem et al, Salatino et al | Detector design with high loop gain | ||
| Residual correlated cosmic-ray hits | Detectors used to form a difference have correlated cosmic ray hits below detection threshold | Correlated noise / additive polarization power | ] | Thermal design of detectors to minimize effect | ||
| ADC non-linearity | ] | |||||
The importance of a good scanning strategy and focal plane design is to be emphasised and their design will be influenced by their effect on systematics.