How interferometry can stabilize and diagnose high resolution spectrographs

To study exo-Earths with Doppler shifts, one needs extremely accurate spectrograph dimensional stability. This requirement is out of reach for most spectrographs. We describe a technique known as Externally Dispersed Interferometry (EDI) which places a small Michelson interferometer in series with the spectrograph input beam. This imprints an extremely period comb spectrum on the input stellar spectrum, which thereby creates moire patterns. These are stellar spectral features heterodyned down to lower frequencies. These broad moire are dramatically less sensitive to positional drifts or defects in the imaging detector. The moire patterns shift in phase proportionally with Doppler velocity, and have been used to discover exoplanet HD102195 and HD87646 in 2005 and 2016. Recently we have discovered how two different simultaneous signal components in our data counter-rotate in phase under an unwanted wavelength drift ∆x, and have modified our data analysis to combine these signals to cancel the net reaction to drift or pixel positional error. We can reduce the net response to a spectrograph error by 1000 times! This 1000x gain in stability multiplies any conventional gain (ie. from vacuum tank, thermal insulation, or fiber scrambling), to produce an extremely ideal net stability, satisfying the 1/100000 pixel requirement to study exo-Earths.

Speaker: David Erskine, Lawrence Livermore National Labs