Abstract
We demonstrate a key step along a technical route to achieving cm/s scale accuracy for astronomical spectrographs over long (multi-year) time scales, which is critical for the Doppler characterization of Earth sized exoplanets, and measurement of small cosmic redshift drift over many years. This same technique also enables searching exoplanet atmospheres for biosignificant molecules in direct planet imaging using, otherwise, insufficiently low resolution and drift prone dispersive (grating or prism) spectrographs. Using a method called crossfading for externally dispersed interferometers (EDIs) to get highly robust spectra, we recently demonstrated a factor of 1000 × reduction in the net shift of an EDI measured ThAr line to a deliberate simulated wavelength translation of the detector. This 1000 × gain in disperser stability can be combined with conventional stability gains afforded by fiber scramblers, vacuum tanks, and thermal control, to provide an additional 1 to 3 orders of magnitude reduction in the net point spread function shift drift. Crossfading combines high- and low-delay fringing signals that react oppositely in phase to cancel their net reaction to a detector wavelength drift. This can be implemented by an interferometer addition to a facility spectrograph.
Highlights
We recommend using conventional mitigations when affordable, we propose that dispersive spectrograph stability can be further improved by inclusion of a Michelson interferometer in series, which is externally dispersed interferometers (EDIs) [Fig. 3(a)]
This was a T-EDI measured ThAr line at 4849 cm−1 whose data were artificially shifted by Δx 1⁄4 −0.5 cm−1
The behavior becomes non-linear at larger Δx approaching the half-width at half-max (HWHM) of EDI or conventional native spectrograph, 0.725 cm−1 and 0.27 cm−1 (R 1⁄4 3300 and 9000)
Summary
The net EDI (red curve) is a sum of weighted wavelets, each having different delays. The weights were calculated from the behavior under Δx of a neighboring 4764 cm−1 line acting as ersatz calibrant applied to the 4849-cm−1 subject line.
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