Our motivation is to produce smaller, more light efficient, and less expensive spectrographs for searching for exoplanets, and for high resolution spectroscopy in general.

The techniques we are developing can thus benefit a wide variety of scientific fields that use spectroscopy to study nature, or remote sensing technology to study our own planet.

Doppler method
The search for exoplanets is one of the more exciting scientific endeavors currently underway. The primary detection technique is the Doppler method. The gravitational tug of the planet on the parent star as it orbits the star causes the star to wobble. The wobble is a few meters per second in magnitude, and can vary over time scale of a few days, months or years. The wobble causes a Doppler shift in the wavelength of the stellar spectrum.

Narrow absorption lines require high resolution

The stellar spectrum has very many narrow absorption lines caused by the passage of starlight through its own atmosphere. These absorption lines are occur at well defined wavelengths associate with particular atomic species, and thus can be used as "markers" to measure the shift of the spectrum from one night to the next. However, these absorption lines are exceedingly narrow. Thus a very high resolution spectrograph (approximately 1 part in 50,000) is ordinarily required to see them.

High resolution spectrographs are large and expensive
The problem is that in order to produce these high resolutions for a wide beam of light, such as that from a large telescope, a prism or diffraction grating must throw the light over a long distance of several meters. Hence large optics must be mounted several meters apart and kept in precise alignment in spite of changes in temperature and air convection internal to the spectrograph. This makes the instrument very heavy and expensive (million of dollars).

Worse for the future, the spectrograph size scales in proportion to the telescope aperture. The 10 meter diameter Keck telescope has a high resolution spectrograph the size of a kitchen. The proposed thirty meter telescope would require an even larger spectrograph (14 meters, basketball court?), one so massive that its weight threatens the strength of the telescope mounting structure. Under a tight budget, it may be too costly to build.

Similar severe weight and space restrictions are placed on spacecraft designers who need to include spectrographs on probes to planets or to study our own Earth.

EDI allows smaller spectrographs

Our EDI technique allows lower resolution spectrographs, which are smaller and less costly, to perform precision Doppler measurements and high resolution spectroscopy over a wide bandwidth, in spite of the inability to directly resolve the absorption lines with the grating spectrograph alone.

The EDI can thus reduce the size and cost of the minimum spectrograph needed for Doppler work and high resolution spectroscopy. Secondly, the spectrograph designer can now choose optical components and make design tradeoffs that maximize light efficiency, instead of optimizing high resolution, and let the interferometer "clean up" the signal later. In this way we believe an EDI equipped spectrograph could be more light efficient. While an idealized high resolution spectrograph has the ultimate performance photon for photon, the EDI presents the designer a valuable set of new options for accomplishing the task under budget, size or mass constraints. site maintained by
David Erskine

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