Setting requirements on out-of-band rejection for next-generation CMB experiments. Application to the LiteBIRD instrument

Introduction to the show: ident: Astrophysics. The day in astrophysics: observations, models and what the data forces.

Vera: We're looking at the paper: "Setting requirements on out-of-band rejection for next-generation CMB experiments. Application to the LiteBIRD instrument".

Jocelyn: Here's the gist of the paper. Next-generation cosmic microwave background experiments have very stringent constraints to achieve the required sensitivity to target polarization B modes The method developed is applied to the LiteBIRD Medium and High Frequency Telescopes In order to determine the impact of out-of-band power, we model the instrument’s optical response and the spectral emissions of the sky and of the instrument itself The requirement derived consist in attenuation factors (in dB) in frequency subdomains The study employs a global approach to set out-of-band rejection requirements by modeling the instrument’s optical response and the spectral emissions of the sky and of the instrument itself The methodology involves propagating assumed out-of-band attenuation coefficients, A L,C,H i,s through the entire detection chain to compute impacts on detector noise equivalent power (NEP) and component separation efficiency This process requires assuming detailed modeling of the instrumental response without any filtering scheme and a modeling of the astrophysical sky The analysis is conducted in two sub-analyses: static analysis, which assesses the impact on detection chain and thermal balance, and dynamic analysis, which addresses fluctuations in out-of-band power Requirements from Detection Chain Analysis The requirements derived from the detection chain focus on two main effects of out-of-band power: the increase of the detector NEP and the modification of component separation efficiency The requirement derived consists in attenuation factors (in dB) in frequency subdomains For instance, Table 3 shows requirements on the three attenuation factors AL i, AC i, AH i for the ten MFT and HFT channels, derived from the requirement ∆NEP ≤ 0.14 aW/√Hz The most stringent requirements are for the high out-of-band (AH factor) Requirements from Thermal Cooling Budget Analysis The thermal cooling budget analysis quantifies the impact of out-of-band power on the thermal balance of the instrument, assessing compliance with cooling power capability The total heat load is clearly dominated by the HOB domain, and requirements are set on AL and AH for this domain For example, Table 5 shows requirements on AL and AH from thermal heat load onto the FP for MFT and HFT The analysis concludes that position s1, between lens L2 and the focal plane, is the most critical because of the majority of the heat load in the HOB domain comes from the HWP, tube 5K and lens L2 Requirements from Dynamic Analysis The dynamic analysis addresses thermal variations along scanning both from sky intensity variations and from instrument thermal fluctuations The requirement derived is that AC and AH should be at minimum 35 dB and 15 dB respectively, which is not very stringent This conclusion was established for HFT channel at 402 GHz but we decided to keep the same requirement for others channels in a conservative approach The main result is that the most stringent requirements on the attenuation factors come from the impact on the detection chain: detector NEP increase and less efficient component separation Also, the highest attenuation factors required are for the high out-of-band, named AH Regarding the impact on the instrument thermal balance in the static case and the dynamic study, we concluded that it is much less stringent or even negligible in many cases The next step is to properly define the filtering scheme A Detector NEP calculation The photon NEP, due to the distribution of arrival times of photons at the detector input, is defined as The thermal carrier NEP inherent to the bolometer power dissipation is calculated using equation (A.3) The readout NEP is estimated with equation (A.4) Technical details on the HWP non-ideality study The out-of-band effect is modelled as a constant excess in the bandpass profiles out of the DIB frequency range The TOD is computed as equation (B.1) The residual map is computed as mres = mout − mtempl, for each frequency channel and each level of out-of-band excess The HWP non-ideality is negligible at this stage of the requirement forecasting, given the current HWP configuration Solid angle calculation The solid angle under which a point A belonging to the HWP sees the sky is computed using equation (C.5) The optical extent is expressed as S omega = Sπθ squared where S is the HWP surface and θ the baffle aperture angle Power detailed calculation The total power received in each domain is written at the bottom while the repartition between all emissive components are shown with bare-plot The incident Ped c,i power on a detector for the LOB, DIB, COB and HOB domains is shown in Figure 7 The total power in the HOB domain is about three orders of magnitude above the total power in the three other domains Total 1.78 5.62 4.23 3.32 Table 6 Synthesis of the requirements on the attenuation factors AL (blue), AC (orange) and AH (red) in dB, derived from each analysis, either detailed per frequency channel or common for all The most stringent requirements are on the high out-of-band (AH factor) Position s MFT ν

GHz: HFT ν

GHz: Section Case s0 s1 s2 s3 s4 100 119 140 166 195 195 235 280 337 402 AL

dB: Section Case s MFT ν

GHz: HFT ν

GHz: AC

dB: Section Case s MFT ν

GHz: HFT ν

GHz: AH

dB: Table 6 Synthesis of the requirements on the attenuation factors AL (blue), AC (orange) and AH (red) in dB, derived from each analysis, either detailed per frequency channel or common for all The most stringent requirements are on the high out-of-band (AH factor) Position s MFT ν

GHz: HFT ν

GHz: AL

dB: Section Case s MFT ν

GHz: HFT ν

GHz: AC

dB: Section Case s MFT ν

GHz: HFT ν

GHz: AH

dB: Table 6 Synthesis of the requirements on the attenuation factors AL (blue), AC (orange) and AH (red) in dB, derived from each analysis, either detailed per frequency channel or common for all <ref:2606.

Paper summary: Vera: To get into the details of this paper, they model how incident power hits a detector assuming no filtering at all and then they set requirements based on a specific increase in detector noise equivalent power, which is constrained to less than zero point one four aW/√Hz >

Jocelyn: That sounds like the static analysis part where you look at the detection chain impact, but what about the separation efficiency part? How does that factor into their requirements? >

Vera: They also look at how out-of-band power modifies component separation efficiency, and they find that the most stringent requirements come from this detection chain analysis >

Subrahmanyan: I'm interested in where they land on the factors themselves. The text mentions that for example, Table three shows requirements on three attenuation factors A L i, A C i, and A H i for the ten Medium Frequency Telescopes and High Frequency Telescopes > <ref:2606.31778#pg1>

Jocelyn: And they point out that the highest requirements are actually for the high out-of-band, which they call the AH factor >

Vera: Yeah, that makes sense. Then there's another part to it where they look at thermal cooling budget analysis to see if the instrument can still handle all that heat load >

Subrahmanyan: The thermal analysis focuses on how out-of-band power affects the thermal balance, and they say the total heat load is clearly dominated by the HOB domain >

Jocelyn: And for that domain, they set specific requirements on A L and A H because of where most of that heat comes from, specifically from the lens L2 and tube 5K > <ref:2606.31778#pg1>

Vera: They conclude that position s1, which is between lens L2 and the focal plane, is the most critical spot because it's where a majority of the heat load in that HOB domain originates >

Subrahmanyan: It sounds like they’ve mapped out exactly where the physical constraints are tightest for a real instrument design >

Jocelyn: Then they have this dynamic analysis to look at thermal variations, and their requirement there is that AC and AH should be at minimum thirty-five dB and fifteen dB respectively > <ref:2606.31778#pg1>

Vera: Which they say isn't super stringent, especially for the HFT channel at four hundred two GHz, but they kept it conservative for all other channels >

Subrahmanyan: So so far, it sounds like the paper establishes a set of constraints based on modeling everything from sky emissions to instrument response and then applying those constraints across different physical effects >

Jocelyn: It really sets the stage for how you should design those filters before you even start building anything >

Conclusion: Vera: So, looking at the whole paper, "Setting requirements on out-of-band rejection for next-generation CMB experiments. Application to the LiteBIRD instrument," it's about establishing these specific attenuation factors in dB based on modeling how out-of-band power affects both the detection chain and the thermal load >

Jocelyn: The authors are L. Mousset, L. Montier, J. Aumont, F. Columbro, P. de Bernardis, J. Errard, C. Franceschet, S Giardiello, T Ghigna and H Hubmayr and G Jaehnig and S Masi and F Piacentini G Pisano A Rizzieri G Savini C Tucker from the LiteBIRD Collaboration >

Subrahmanyan: What this really means for cosmology is that it gives experimentalists a clear set of rules to design their filters so they don't end up with a detector that is too noisy or has poor separation between the signals they are looking for >

Vera: The main result is that the most stringent requirements come from the detection chain impact, specifically the increase in detector noise equivalent power and less efficient component separation >

Jocelyn: And they confirm that for the thermal balance, especially in dynamic or static cases, it's much less stringent or even negligible in many parts of the instrument than people might expect >

Subrahmanyan: The paper's limitation is that they model HWP non-ideality as negligible at this stage, which means their requirement forecasting stops before we really dive into how imperfect the mirrors themselves are >

Vera: So what they do not do is fully incorporate every imperfection in the optical elements, and that’s what the next step would be to properly define a filtering scheme >