Beams and C(l)

— Eric Hivon (email: hivon@ap.fr) 2018/07/16 09:09

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The beam computed at 150GHz, at a location offset by 10cm in the X (=co-scan) direction is now also available, on top of the central beam and Y=10cm beams generated earlier this month.

Two different effects are now visible on the power spectra:

• an overall rotation of the polarization by 1.74 deg, showing up as a strong EE to BB leakage when the full IQU beam is used instead of assuming a copolar beam (something we did not see in the (X,Y)=(0,0) and (0,10cm) cases)
• the ell-dependent leakage from EE to BB seen previously is now larger than before.

Same exercise as on July 02, this time considering a beam at 150GHz located 10cm in the Y (=cross-scan) direction from the focal plane center (in a focal plane of 45cm in diameter).
We see this time a much larger contamination of BB, which looks like an ell-increasing leakage from EE to BB, probably due to the m=4 components (=square-like shape) of the beam map. As for the central beam, the pure copolar and full IQU beam models return the same power spectra.

First GRASP simulations by Karl Young of PICO main beam responses at 150GHz at the center of focal plane, for 2 orthogonal detectors.

The FWHMs were found to be ~4.9arcmin, the elongations (ratio of the major and minor axes) 1.01, and the ratios of U/I about 1.e-4 for both beams, meaning that the beams are very much co-polar, with very little spurious rotation (~<0.5 arcmin).

The same exercise would have to be repeated at a more representative position (ie, off-axis) where systematics (=elongation of beam and rotation of polarization) could be larger.

The plot below shows how the various spectra are affected for either the purely copolar beam (ignoring the U beam component, green dashes) or the “full” beam (red dashes), as computed by QuickPol assuming a scanning strategy adapted from July 1st 2018 baseline (α=30º, T_prec=10h, β=65º, T_spin=1mn, sampling = 151Hz, 2 years of observation, N_side=1024).

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