Bandpass Convention - What does flat mean

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Nov 6 2017, Clem Pryke

For CMB-S4 sims we have a table of bandpasses in CMB-S4 frequency bands v1.99 which specifies the lower/upper edges of assumed tophat bandpasses. In Tophat bands for Data Challenge (Nov 2 2016) Colin specified how to define "flat" tophat - he states "I define this tophat to be such that a single-moded antenna (AΩ scales as λ2) would have uniform response as a function of frequency to a beam-filling Rayleigh-Jeans source."

The Planck bandpass files are specified in units of (relative) spectral radiance (SR) although the WMAP ones are specified in RJ units.

PySM works in spectral radiance units (Jy/Sr) and it makes a lot of sense to do all astrophysics type modelling in these units.

When working in SR units the correct shape for a "flat" bandpass is not flat - it is nu^-2 because real detectors absorb more photons to lower frequencies. Sky models 00 and 05 did this in the creation step. However, apparently in the re-analysis presented in Maximum likelihood search results for Data Challenge 02 flat in SR was assumed and this may be all or part of the cause of the biases found in the results. (Raphael's re-analysis does not need to know the bandpasses.)

As part of the PySM documentation Ben Thorne offers this document - I am not fully able to tell but I think flat for him means flat in SR units?