Better method for only computing cells with a large tau: start with the largest tau, nearest the particle, and proceed to progressively smaller taus until we reach a threshold, when we stop.
Better method for only computing cells with a large tau: start with the
largest tau, nearest the particle, and proceed to progressively smaller
taus until we reach a threshold, when we stop.

sbird/fake_spectra,sbird/fake_spectra,sbird/fake_spectra