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\begin{document}

\title{Enabling 1000x More Sensitive Spectrographs for Exoplanet Search}



\author[1,1]{David Erskine}%
\author[1,1]{Dayne Fratanduono}%
\affil[1]{Lawrence Livermore National Laboratory}%


\vspace{-1em}



  
  \date{June 14, 2024}


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\begin{abstract}
Current diagnostic velocity resolution limits our ability to search for
exoplanets within the habitable zone. We propose a new capability
(Crossfading-EDI or X-EDI) that will boost optical spectrograph
stability and spectral resolution, enabling 100x-1000x more sensitivity
for the exoplanet search. This technique for increasing the stability of
any dispersive (grating or prism) spectrograph to unwanted wavelength
drifts has been simulated on real data and shown to produce a 1000x or
more stability improvement, by use of an interferometer using *pairs* of
overlapping delays placed in series with the disperser, rather than
singly-used delays. This combines fringe shifts having opposite phase
reaction, to cancel drift. This technique, ``Crossfading Externally
Dispersed Interferometry'' (X-EDI) builds upon an earlier singly-
delayed Doppler technique (EDI) demonstrated on a variety of telescopes
including the 5-meter Hale telescope at Mt. Palomar. The single-delay
EDI technique already affords a significant stability enhancement to a
spectrograph, and has been used by others to discover exoplanets in 2006
and 2016. We expect that improving EDI technique further by the use of
crossfading pairs of delays will dramatically improve the instrumental
noise floor due to spectrograph focal point drifts or detector pixel
misplacement, which can limit the detection of small exoplanets over
long (months or years) time scales. The X-EDI has been simulated on EDI
data on a ThAr lamp line measured at the Hale telescope*. A simulated
drift insult was applied. The observed reaction to the line position
drift was reduced 1000x. All spectrographs suffer drift insults of
various kinds, and the X-EDI technique reduces the reaction to these by
moving the fine wavelength determination from the dispersive
spectrograph to the interferometer, which uses the symmetry of delay
pairs to eliminate drift. This technique can greatly improve spectral
precision and stability for (1) Doppler radial velocimetry, and (2)
direct planet imaging using adaptive optics (such as the Gemini Planet
Imager) that feeds a low resolution integral field multi-object
spectrograph. *David J. Erskine, J. Astr. Tele. Instrum. Sys.,
7(2):025006, June 2021. Prepared by LLNL under Contract DE-
AC52-07NA27344.%
\end{abstract}%



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