Atmospheric model verification using BICEP/Keck data

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September 23, 2020 - Colin Bischoff and Reijo Keskitalo


In this post we study atmospheric noise correlations in two scansets (approximately 55 minutes) of Keck array data acquired with five co-positioned cameras observing at 100, 150 and 220 GHz.

It is expected, that as CMB-S4 SAT:s integrate down the instrumental noise across large numbers of detectors, the dominant noise component in the final analysis will have an atmospheric origin. For that reason, it is exceptionally important to forecast the extent to which the atmospheric noise in various detectors is statistically independent. The rate at which we acquire these independent measurements of the atmosphere will dictate our mapping speed.

Dataset like the BICEP/Keck data of concern here has never been used to verify the TOAST atmospheric simulation module before. Compared to earlier verification campaigns with POLARBEAR, ACT and SPT, these data uniquely offer

  • detector line-of-sights separated by up to 16 degrees
  • small aperture telescope with resulting near-field geometry
  • multiple co-positioned cameras


Each array of detectors has a square layout of single frequency detectors. Two arrays observe at 100 GHz, one at 150GHz and two at 220GHz. The deck angle around the boresight is different between the two studied observations, 20150614C06_dk023 and 20150714B04_dk338:

Fpgeom.20150614C06 dk023.png

Fpgeom.20150714B04 dk338.png

Holes in the grids are left by the ad hoc data cuts implemented in this analysis.

Time-ordered data

The calibrated but unfiltered timestreams show prominent correlations across all frequencies:

Tod 20150614C06 dk023.pairsum unfiltered.png

Interestingly enough, the 150 and 220GHz data above seem to decouple somehow at around 2000 seconds.

Tod 20150714B04 dk338.pairsum unfiltered.png

The dominant features of the above plots are long time scale fluctuations that are typically filtered out early in the data processing. Here we show the same TOD after filtering each half-scan with a 2nd order polynomial and each scanset with a 10th order ground-synchronous polynomial.

Tod 20150614C06 dk023.pairsum filtered.png

Tod 20150714B04 dk338.pairsum filtered.png