Simons Observatory

Simons Observatory site. Credits: Deborah Kellner.
​General description of the project

The Simons Observatory (SO) is a new generation CMB experiment, gathering 60,000 TES detectors on two types of telescopes: 3 Small Aperture Telescopes (SATs) and one Large Aperture Telescope (LAT). This hybrid was shown to be the most efficient approach to study the early and late Universe [Barron et al 2018]. 5 years of observations with the SATs will give constraints on the primordial gravitational waves with a precision on the tensor-to-scalar ratio of sigma(r)~0.003 (see Fig. 1). An internal or external delensing will further improve this performance [Namikawa et al 2021]. The LAT will constrain the formation and evolution of large scale structures through the observation of O(20,000) SZ clusters and gravitational lensing across fsky~40% of the sky. Through kSZ, the LAT will also study the reionization period. As such SO will therefore open a window on the early Universe as well as on the current dynamics of the Universe. 

To reach these scientific goals, galactic emissions such as dust and synchrotron polarized radiation are a major issue. The SATs and the LAT observe in 6 frequency bands from 27GHz to 280GHz, so that SO will be able to characterize and remove the galactic foregrounds [The SO collaboration 2018]. In addition, this will also allow SO to give us a better understanding of the physics at play in our own galaxy [The SO galactic science working group, 2021 in prep].

As of 2021, SO is under construction in the Atacama desert in Chile (Fig. 2), and will be able to capture its first light during 2022.  The nominal duration of the project is 5 years. 

  • Activities at APC 

The activities of SO members at APC are focused on data analysis. We are involved in almost all steps of the analysis presented in Fig. 3. From raw detector data used to get the maps of the sky in each SO frequency to the estimation of cosmological parameters such as the tensor-to-scalar ratio.
We are developing new, efficient and massively parallel mapmaking codes capable of mitigating several instrumental effects, like ground pickup signal to atmospheric noise [El Bouhargani et al. 2022].

We are also working on a statistically robust component separation scheme, capable of removing galactic signals from our CMB map. It gives us an estimation of how the error from the component separation propagates to the cosmological parameters. This method (e.g. Stompor et al. 2016) can be generalised to account for instrumental effects and used for the estimation of other cosmological parameters such as cosmic birefringence.

We are also working on the optimization of the power spectra estimation: SO will observe a partial sky, creating spurious polarization modes once we go to harmonic space. We are trying to mitigate this effect by optimizing the shape of the sky mask. 

Thanks to the arcmin angle resolution from the LAT, the APC team is also focusing on late-time objects such as galaxy clusters through the SZ effects, gravitational lensing, and their cross-correlation with optical and spectroscopic measurements (Rubin observatory, Euclid, DESI). 

  • Past and present members

Present : James Bartlett, Hamza El Bouhargani, Josquin Errard, Kenneth Ganga, Baptiste Jost, Jean-Baptiste Melin, Magdy Morshed, Radek Stompor

Past : Dominic Beck, Clara Vergès

  • References

Paper Forecast :

Whitepaper :

Video YT : (Simons Observatory Explained)


Figure 1 : Projection of SO for primordial spectrum parameters, Source : Simons Observatory official website




Figure 2 : Chart of the study of the CMB. Source :