Exploring the Cosmos: Enabling One-Thousand Times More Sensitive Spectrographs for Exoplanet Search

David Erskine | 22-LW-020

Project Overview

Spectrographs are used across science, engineering, medicine, forensics, and remote sensing for national security. A fundamental challenge is that spectrographs can suffer an unknown wavelength drift error. Furthermore, spectrographs used for mobile and especially airborne remote sensing need both compactness and stability. Changing temperature and air pressure, vibration and accelerations present in these platforms can create wavelength drifts. 

The approach developed on this project was to insert an interferometer prior to the spectrograph to imprint a periodic sinusoidal grid that multiplies the input spectrum. Termed externally dispersed interferometry (EDI), this multiplication heterodynes otherwise unresolvable high frequency signals down to low frequency beat signals that are resolvable by the spectrograph. In this project we have identified two signal components that react oppositely to a spectrograph wavelength drift under heterodyning. An improvement in the stability (resistance to wavelength drift) of up to 1000 times was demonstrated in EDI data taken with a single delay. The use of a single delay, not needing a pair of delays, is a recent and significant improvement over our previous multiple-delay approach since it accepts a simpler apparatus. An additional advance was discovered during testing EDI on the Keck Planet Finder (KPF) spectrograph: a novel and simple way to diagnose a spectrograph blur width and asymmetry across its whole bandwidth at once, by using a common laboratory light source. This technique could be useful commercially, especially to the precision optical community.

Mission Impact

This research advances the Laboratory's Core Competency in Lasers and Optical Science and Technology. This project showed our externally dispersed interferometry (EDI) technique can boost spectrograph stability against wavelength drifts 1000x times by inclusion of a small interferometer. Hence this allows spectrographs to be used in rough environments where previously not practical to maintain instrument rigidity such as a mobile platform (airborne surveillance for national security) suffering changes in temperature, pressure, vibration and accelerations. Previous research showed EDI can boost the effective spectral resolution 2 to 20 times, depending on delay choice. Since spectrograph length scales with resolution, this allows using a lower resolution physically smaller spectrograph (having 8x to 8000x less volume) to meet a given final resolution goal. This again allows use in mobile platforms where small instrument size is critical. The mobile applications include use in medicine and forensics, and many other science and engineering uses.

Publications, Presentations, and Patents

Erskine, David J. 2022. " Spectrograph Stabilization using a Single-delay Interferometer." US Patent application IL-13759, filed Dec 8, 2022.

David J. Erskine and D. Fratanduono, "Enabling 1000x More Sensitive Spectrographs for Exoplanet Search" (Poster Presentation, Am. Geophys. Union, poster# U43B-08, New Orleans, LA, December 13-17, 2021). 

David J. Erskine, and D. Fratanduono, "Enabling 1000x More Sensitive Spectrographs for Exoplanet Searches" (Poster Presentation, Am. Astro. Soc. 239th mtg., poster# 633, Salt Lake City, UT, Jan 9-13, 2022). 

David J. Erskine, D. Fratanduono, E. Davies, E. Linder, J. Edelstein, E. Wishnow, M. Sirk, and R. Ozer, "Crossfaded Externally Dispersed Interferometer Testbed for 1000x Improved Doppler Spectrograph Stability" (Poster Presentation, AAS Exoplanets IV, May 1-6, 2022, poster# 102.69, Las Vegas, NV, 2022).

David J. Erskine, J. Edelstein, E. Wishnow, E. Davies, M. Sirk, R. Ozer, and D. Fratanduono, "Externally Dispersed Interferometer Testbed Diagnosing Keck Planet Finder Spectrograph High Resolution Performance" (Poster Presentation, Ground-based and Airborne Instrumentation for Astronomy IX, SPIE 12184, poster# 165, Montreal, Canada, July 17-22, 2022). https://doi.org/10.1117/12.2627160.

David J. Erskine, E. Wishnow, E. Linder, E. Davies, M. Sirk, R. Ozer, D. Fratanduono, and J. Edelstein, "Spectrograph Stabilization of 500x using a Single-delay Interferometer" (Poster Presentation, Ground-based and Airborne Instrumentation for Astronomy IX, SPIE 12184 , poster# 169, Montreal, Canada, July 17-22 2022). https://doi.org/10.1117/12.2628388

David J. Erskine, J. Edelstein, E. Wishnow, M. Sirk, E. Linder, and D. Fratanduono, "Performance boosting and characterization of high resolution spectrographs using externally dispersed interferometry" (Poster Presentation, Amer. Astro. Soc. 241st mtg., Seattle, WA, Jan 8-12, 2023). https://aas241-aas.ipostersessions.com/default.aspx?s=96-4D-05-8F-F2-B1…

David J. Erskine and Eric V. Linder, "Redshift Drift: Measuring the timescale of the universe" (Presentation, Timescales in Astrophysics, New York University Abu Dhabi, UAE, Jan 16-20, 2023).

David J. Erskine, J. Edelstein, E. Wishnow, M. Sirk, E. Linder, and D. Fratanduono, "Stability boosting and characterization of high resolution spectrographs using an externally dispersed interferometer" (Presentation, Extreme Precision Radial Velocity 5, Santa Barbara, CA, March 27-30, 2023).

David J. Erskine, "How interferometry can stabilize and diagnose high resolution spectrographs" (Invited Oral Presentation, Eastbay Astronomical Society, Oakland, CA, June 24, 2023).

David J. Erskine,  J. Edelstein, E. Wishnow, M. Sirk, E. Linder, and D. Fratanduono, "1000x stability boosting, and characterization, of high resolution spectrographs using externally dispersed interferometry" (Presentation, CASIS -Imaging Sciences - Workshop, Livermore, CA, August 2-3, 2023).