# LAT Frequency Flowdown

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### Summary

(Colin Hill writing 4-22-19)

In this posting I present the results of "flowdown" (i.e., optimization) calculations for the frequency channel distribution of the CMB-S4 LATs.

### Setup and Assumptions

I use the noise calculator for the CMB-S4 LATs posted at https://cmb-s4.org/wiki/images/Lat-noise-190311-py.txt

This noise calculator allows one to vary the sky fraction surveyed (fsky) and the number of optics tubes of each type (LF, MF, UHF). For reference, LF = 27 and 39 GHz, MF = 93 and 145 GHz, UHF = 225 and 280 GHz. I do not consider the 20 GHz channel (ULF) in this work, as its resolution is low enough as to likely be irrelevant for LAT science. I consider fsky = 0.1, 0.2, and 0.4, and all possible optics tube configurations with 18 total tubes (assuming there is 19th tube used for the ULF). I also impose a constraint that there is at least one MF tube, since these are clearly the main 'science frequencies'. This yields a total of 3420 possible configurations. Note that I assume two identical copies of the LAT, in order to reduce the computational complexity. If desired, one could run a full optimization of all 36 optics tubes.

I also run calculations using a modified version of the S4 noise calculator (via Matthew Hasselfield) that allows for the possibility of "XHF" optics tubes, where XHF = 281 and 350 GHz. The atmosphere for these tubes is assumed to match CCAT site specifications.

I include Planck data (30 - 353 GHz) in all calculations, as these channels are useful on large angular scales where the S4 atmospheric noise is large.

### Methodology

I focus on temperature-based observables in the following: thermal SZ, kinematic SZ, CMB lensing via the TT quadratic estimator, and the CMB TT power spectrum. I model the temperature sky at all frequencies from 27 to 353 GHz (13 channels, or 15 when including XHF tubes) using the simulated sky maps described in Sec. 2 of https://arxiv.org/abs/1808.07445 . These maps are also available at https://lambda.gsfc.nasa.gov/toolbox/tb_cmbsim_ov.cfm

I use simple Galactic-emission-thresholded sky masks that leave the cleanest 10%, 20%, or 40% of the sky that is visible from Chile. These are identical to the sky masks used in https://arxiv.org/abs/1808.07445

For each fsky and optics tube configuration option, I use a harmonic-space ILC code to obtain "post-component-separation" noise power spectra for either the blackbody CMB temperature or Compton-y fields, using the modeled sky power spectra and the per-frequency noise power spectra computed using the S4 calculator (as well as Planck noise, assumed to be white).