Guðmundur K. Stefánsson, PhD

Hi! I am a Henry Norris Russell Postoctoral Fellow in Astrophysics at Princeton University. I work on developing and using next-generation instruments to discover and characterize exoplanets around nearby low-mass stars.

Research: Instrumentation

Precision Photometry with Engineered Diffusers

Precision ground-based photometric facilities play a critical role in confirming and characterizing transiting exoplanet candidates from space-based surveys such as the Transiting Exoplanet Survey Satellite (TESS). In Stefansson et al. 2017, I led the design and implementation of an Engineered Diffuser for the ARCTIC instrument on the 3.5m Telescope at Apache Point Observatory. Produced using specialized nanofabrication techniques, Engineered Diffusers are micro-structured optical devices capable of molding the image of a star into a broad and stable top-hat shape, minimizing photometric errors due to detector, seeing, and scintillation effects. Using the Engineered Diffuser on the 3.5m Telescope, we have demonstrated some of the highest photometric precisions achieved from the ground, or 62ppm precision in 30 minute bins.

As Engineered Diffusers are inexpensive, diffusers have now been installed and used at a number of different observatories to characterize a number of transiting planets (see Observation section below). If you are interested in trying a diffuser on your telescope, check out my Python package iDiffuse, and/or don't hesitate to send me an email.

Relevant Papers:

Precision Radial Velocities in the Near Infrared and Optical

I am a part of the instrument and science teams of two next-generation precision radial velocity (RV) spectrographs designed to detect and characterize planets in the habitable-zones around nearby stars: the near-infrared (NIR) Habitable-zone Planet Finder (HPF) on the 10m Hobby-Eberly Telescope, and the NASA/NSF funded NEID Spectrograph for the 3.5m WIYN Telescope at Kitt Peak Observatory.

The main science goal of HPF is to carry out a 5-year survey to detect and characterize terrestrial planets in the Habitable-zone around nearby mid-to-late M-dwarf stars within ~15pc. I currently maintain the HPF RV extractions, where we have demonstrated that HPF is capable of 1.53m/s RV precision on the nearby bright M-dwarf Barnard's Star (Metcalf et al. 2019), the highest precision RVs in the NIR. The main science goal of NEID is to detect and characterize terrestrial planets in the habitable-zone around nearby FGK stars with a goal RV precision of 30cm/s.

For newer updates, check out the HPF blog and the NEID blog.

Relevant Papers:

Research: Observation

Discovering New Planets Orbiting Nearby M-dwarf Stars

Using diffuser-assisted photometry and precision radial velocities with HPF, I have led or directly contributed to a number of efforts to detect and characterize new planet candidates orbiting nearby stars, with a particular focus on planets orbiting nearby M-dwarfs.

Relevant Papers:

Characterization of the Properties of Nearby M-dwarf Planet Systems

Using diffuser-assisted photometry and precision Radial Velocities (RVs) with HPF and other spectrographs, I am currently leading efforts to study the orbital and atmospheric properties of planets orbiting nearby M-dwarfs.

In particular, I am interested in using the Rossiter-McLaughlin Effect to characterize the spin-orbit alignment of planets, a powerful probe of their formation and subsequent dynamical evolution. As is shown in the animation to the left, this method relies on measuring the angle between the stellar rotation axis and the stellar equator. As a star rotates, one hemisphere is blueshifted towards the observer, and the other hemisphere is redshifted. As a planet crosses in front of the stellar disk, the planet successively blocks differently blue and redshifted light from the star resulting in a detectable signature in the RVs during the transit. The shape and size of this signature is strongly sensitive to the obliquity of the planet, the angle between the stellar equator and the planet orbit.

Relevant Papers:

Press Releases

You Can See That From Here

Nanofabricated Engineered Diffusers Enable some of the Highest Precision Photometry from the Ground (Stefansson+2017, ApJ).


First Author

9 Stefansson, et al. 2020, “A Mini-Neptune and a Radius-Valley-Planet Orbiting the Nearby M2 dwarf TOI-1266 in its Venus-Zone: Validation with the Habitable-zone Planet Finder“. In AJ: 160, 6 (Nov, 2020). arxiv: 2006.11180.

8 Stefansson, et al. 2020, “The Habitable-zone Planet Finder Reveals A High Mass and a Low Obliquity for the Young Neptune K2-25b“, In: AJ 160, 192 (Oct, 2020). arxiv: 2007.12766.

7 Stefansson, et al. 2020, “A sub-Neptune sized planet transiting the M2.5-dwarf G 9-40: Validation with the Habitable-zone Planet Finder“, In: AJ, 159, 100 (March 2020). arxiv: 1912.00291.

6 Stefansson, et al. 2018, “Diffuser-assisted Photometric Follow-up Observations of the Neptune-sized Planets K2-28b and K2-100b”. In: AJ 156, 266 (July 2018). arXiv: 1807.04420.

5 Stefansson, et al. 2018, “Extreme precision photometry from the ground with beam-shaping diffusers for K2, TESS, and beyond”. In: SPIE Conference Series. Vol. 10702. SPIE. Aug. 2018, p. 10702.

4 Stefansson, et al. 2017, “Toward Space-like Photometric Precision from the Ground with Beam-shaping Diffusers”. In: ApJ 848, 9 (Oct. 2017), p. 9. arXiv: 1710.01790.

3 Stefansson, et al. 2016, “A Versatile Technique to Enable Sub-milli-Kelvin Instrument Stability for Precise Radial Velocity Measurements: Tests with the Habitable-zone Planet Finder”. In: ApJ 833, 175 (Dec. 2016), p. 175. arXiv: 1610.06216.

2 Stefansson, et al. 2016, “Ultra-stable temperature and pressure control for the Habitable-zone Planet Finder spectrograph”. In: SPIE Conference Series. Vol. 9908. SPIE. Aug. 2016, p. 990871.

1 Stefansson, et al. 2011, "Samþætting vökvarása og ljósrása á örflögum (English: "Fabrication of integrated optical and microfluidic devices)" In: Raust (2011).

2nd & 3rd Author

14 J. Lubin, P. Robertson, G. Stefansson, J. Ninan, S. Mahadevan, M. Endl, E. Ford, J. T. Wright, C. Beard, C. Bender, W. D. Cochran, S. A. Diddams, C. Fredrick, S. Halverson, S. Kanodia, A. J. Metcalf, L. Ramsey, A. Roy, C. Schwab, R. Terrien "Stellar Activity Manifesting at a One Year Alias Explains Barnard b as a False Positive". In: AJ (accepted), (May, 2021). arxiv: 2105.07005.

13 S. Mahadevan, G. Stefánsson, P. Robertson, R. C. Terrien, J. P. Ninan, R. J. Holcomb, S. Halverson, W. D. Cochran, S. Kanodia, L. W. Ramsey, A. Wolszczan, M. Endl, C. F. Bender, S. A. Diddams, C. Fredrick, F. Hearty, A. Monson, A. J. Metcalf, A. Roy, C. Schwab "The Habitable-zone Planet Finder Detects a Terrestrial-mass Planet Candidate Closely Orbiting Gliese 1151: The Likely Source of Coherent Low-frequency Radio Emission from an Inactive Star". In: ApJL (accepted), (Feb, 2021). arxiv: 2102.02233.

12 C. Cañas, G. Stefansson, S. Kanodia, et al. 2020, "A warm Jupiter transiting an M dwarf: A TESS single transit event confirmed with the Habitable-zone Planet Finder". In: AJ, 160, 147 arXiv: 2007.07098.

11 S. Kanodia, C. Cañas, G. Stefansson et al. 2020, "TOI-1728b: The Habitable-zone Planet Finder confirms a warm super Neptune orbiting an M dwarf host". In: ApJ, 899, 29 (Aug, 2020). arXiv: 2006.14546.

10 P. Robertson, G. Stefansson, Suvrath Mahadevan, et al. 2020, "Persistent starspot signals on M dwarfs: multi-wavelength Doppler observations with the Habitable-zone Planet Finder and Keck/HIRES". In: ApJ, 897, 125 (July 2020). arXiv: 2005.09657.

9 J. Ninan, G. Stefansson, S. Mahadevan, et al. 2020, "Evidence for He I 10830 A absorption during the transit of a warm Neptune around the M-dwarf GJ 3470 with the Habitable-zone Planet Finder". In: ApJ, 894, 97 (May 2020). arXiv: 1910.02070.

8 J. Ninan, S. Mahadevan, G. Stefansson et al. 2019, "Impact of crosshatch patterns in H2RGs on high precision radial velocity measurements: Exploration of measurement and mitigation paths with HPF". In: Accepted for Publication in ISPA 2018 (2019). arXiv: 1903.06614.

7 S. Kanodia, A. Wolfgang, G. Stefansson, et al. 2019, "Mass-Radius relationship for M dwarf exoplanets: Comparing nonparametric and parametric methods". In: ApJ 882, 38 (Sep. 2019), p. 14. arXiv: 1903.00042.

6 C. von Essen, G. Stefansson, M. Mallonn, et al. 2019, "First Light of Engineered Diffusers at the Nordic Optical Telescope Reveal Time Variability in the Optical Eclipse Depth of WASP-12b". In: A&A, 628, 11 (August 2019). arXiv: 1904.05362

5 C. Cañas, G. Stefansson, A. J. Monson, et al. 2019, "TOI-150: A transiting hot Jupiter in the TESS southern CVZ". In: ApJL 877, 29 (June 2019), p. 8. arXiv: 1902.09710.

4 P. Robertson, T. Anderson, G. Stefansson, et al. 2019, “Ultrastable environment control for the NEID spectrometer: design and performance demonstration”. In: JATIS, 015003 (Jan 2019). arXiv: 1902.07729.

3 Y. Li, Stefansson, G., P. Robertson, et al. 2017, “A Candidate Transit Event around Proxima Centauri”. In: Research Notes of the American Astronomical Society 1, 49 (Dec. 2017), p. 49. arXiv: 1712.04483.

2 C. Bender, P. Robertson, Stefansson, G. et al. 2016, “The instrument control software package for the Habitable-Zone Planet Finder spectrometer”. In: SPIE Conference Series. Vol. 9913. SPIE. Aug. 2016, p. 991338.

1 Slovinsky, I., Stefansson, G., Kossoy A., et al. 2013, “Propagation Loss of Long-Range Surface Plasmon Polariton Gold Stripe Waveguides in the Thin-Film Limit”. In: Plasmonics 8.4 (2013), pp. 1613–1619.


19 R. C Terrien, J. P Ninan, S. A Diddams, S. Mahadevan, S. Halverson, C. Bender, C. Fredrick, F. Hearty, J. Jennings, A. J. Metcalf, A. Monson, A. Roy, C. Schwab, G. Stefansson"Broadband Stability of the Habitable Zone Planet Finder Fabry-Pérot Etalon Calibration System: Evidence for Chromatic Variation". In: AJ, 161, 252, (June, 2021). arxiv: 2103.08456.

18 A. Seifahrt, J. L. Bean, J. Stürmer, D. Kasper, L. Gers, C. Schwab, M. Zechmeister, G. Stefánsson, B. Montet, L. A. Dos Santos, A. Peck, J. White, E. Tapia "On-sky commissioning of MAROON-X: A new precision radial velocity spectrograph for Gemini North". In: SPIE, Vol 11447, (June, 2021). arxiv: 2106.02157.

17 S. Kanodia, S. Halverson, J. P. Ninan, S. Mahadevan, G. Stefansson, A. Roy, L. W. Ramsey, C. F. Bender, S. Janowiecki, W. D. Cochran, S. A. Diddams, N. Drory, M. Endl, E. B. Ford, F. Hearty, A. J. Metcalf, A. Monson, P. Robertson, C. Schwab, R. C. Terrien, J. T. Wright "A Harsh Test of Far-field Scrambling with the Habitable-zone Planet Finder and the Hobby-Eberly Telescope". In: ApJ, 912, 15, (May, 2021). arxiv: 2103.05148.

16 Quang H. Tran, Brendan P. Bowler, William D. Cochran, Michael Endl, Gudmundur Stefansson, Suvrath Mahadevan, Joe P. Ninan, Chad F. Bender, Samuel Halverson, Arpita Roy, Ryan C. Terrien "The Epoch of Giant Planet Migration Planet Search Program. I. Near-Infrared Radial Velocity Jitter of Young Sun-like Stars". In: AJ (accepted), (Jan, 2021). arxiv: 2101.11005.

15 Arvind F. Gupta, Jason T. Wright, Paul Robertson, Samuel Halverson, Jacob Luhn, Arpita Roy, Suvrath Mahadevan, Eric B . Ford, Chad F. Bender, Cullen H. Blake, Fred Hearty, Shubham Kanodia, Sarah E. Logsdon, Michael W. McElwain, Andrew Monson, Joe P. Ninan, Christian Schwab, Gudmundur Stefansson, Ryan C. Terrien. "Target Prioritization and Observing Strategies for the NEID Earth Twin Survey". In: AJ (accepted), (Jan, 2021). arxiv: 2101.11689.

14 Christian Schwab, Andrew J. Monson, Shubham Kanodia, Emily Lubar, Andrea S. J. Lin, Colin Nitroy, Samuel Halverson, Qian Gong, Ryan C. Terrien, Joe P. Ninan, Chad Bender, Cullen Blake, Frederick R. Hearty, Suvrath Mahadevan, Michael W. McElwain, Paul M. Robertson, Arpita Roy, Gudmundur Stefansson. "The NEID spectrometer: fibre injection system design". In: SPIE, (Dec, 2020).

13 Shubham Kanodia, Joe P. Ninan, Andrew J. Monson, Suvrath Mahadevan, Colin Nitroy, Christian Schwab, Samuel Halverson, Chad F. Bender, Ryan Terrien, Frederick R.Hearty, Emily Lubar, Michael W. McElwain, Lawrence. W. Ramsey, Paul M.Robertson, Arpita Roy, Gudmundur Stefansson, Daniel J. Stevens. "Ghosts of NEID's past". In: SPIE, (Dec, 2020). arxiv: 2012.00182.

12 Keri Hoadley, Christopher Martin, Brian D. Metzger, Mark Seibert, Andrew McWilliam, Ken J. Shen, James D. Neill, Gudmundur Stefansson, Andrew Monson, Bradley E. Schaefer. "A blue ring nebula from a stellar merger several thousand years ago". In: Nature, 587, 387-391 (Nov, 2020). arxiv: 2011.09589.

11 Christian Obermeier, Jana Steuer, Hanna Kellermann, Roberto P. Saglia, Thomas Henning, Arno Riffeser, Ulrich Hopp, Guðmundur Stefansson, Caleb Cañas, Joe P. Ninan, Suvrath Mahadevan, Howard Isaacson, Andrew W. Howard, John H. Livingston, Johannes Koppenhoefer, Ralf Bender. “Following the TraCS of exoplanets with Pan-Planets: Wendelstein-1b and Wendelstein-2". In: A&A, 639, 130 (Jul, 2020). arXiv: 2005.13560.

10 Arpita Roy, Sam Halverson, Suvrath Mahadevan, Gudmundur Stefansson, Andrew Monson, Sarah E. Logsdon, Chad F. Bender, Cullen H. Blake, Eli Golub, Arvind Gupta, Kurt P. Jaehnig, Shubham Kanodia, Kyle Kaplan, Michael W. McElwain, Joe P. Ninan, Jayadev Rajagopal, Paul Robertson, Christian Schwab, Ryan C. Terrien, Sharon Xuesong Wang, Marsha J. Wolf, and Jason T. Wright. “Solar Contamination in Extreme-precision Radial-velocity Measurements: Deleterious Effects and Prospects for Mitigation”. In: AJ, 159, 161 (April, 2020). arXiv: 2002.09468.

9 Kristine W. F. Lam, Judith Korth, Kento Masuda, Szilárd Csizmadia, Philipp Eigmüller, Gudmundur Stefansson, Michael Endl, Simon Albrecht, Rafael Luque, John H. Livingston, Teruyuki Hirano, Roi Alonso Sobrino, Oscar Barragán, Juan Cabrera, Ilaria Carleo, Alexander Chaushev, William D. Cochran, Fei Dai, Jerome de Leon, Hans J. Deeg, Anders Erikson, Massimiliano Esposito, Malcolm Fridlund, Akihiko Fukui, Davide Gandolfi, Iskra Georgieva, Lucá Gonzalez Cuesta, Sascha Grziwa, Eike W. Guenther, Artie P. Hatzes, Diego Hidalgo, Maria Hjorth, Petr Kabath, Emil Knudstrup, Mikkel N. Lund, Suvrath Mahadevan, Savita Mathur, Pilar Montañes Rodríguez, Felipe Murgas, Norio Narita, David Nespral, Prajwal Niraula, Enric Palle, Martin Pätzold, Carina M. Persson, Jorge Prieto-Arranz, Heike Rauer, Seth Redfield, Ignasi Ribas, Paul Robertson, Marek Skarka, Alexis M. S. Smith, Jan Subjak, Vincent Van Eylen. “It takes two planets in resonance to tango around K2-146”. In: AJ, 159, 120 (March, 2020). arXiv: 1907.11141.

8 Andrew J. Metcalf, Tyler Anderson, Chad F. Bender, Scott Blakeslee, Wesley Brand, David R. Carlson, William D. Cochran, Scott A. Diddams, Michael Endl, Connor Fredrick, Sam Halverson, Dan D. Hickstein, Fred Hearty, Jeff Jennings, Shubham Kanodia, Kyle F. Kaplan, Eric Levi, Emily Lubar, Suvrath Mahadevan, Andrew Monson, Joe P. Ninan, Colin Nitroy, Steve Osterman, Scott B. Papp, Franklyn Quinlan, Larry Ramsey, Paul Robertson, Arpita Roy, Christian Schwab, Steinn Sigurdsson, Kartik Srinivasan, Gudmundur Stefansson, David A. Sterner, Ryan Terrien, Alex Wolszczan, Jason T. Wright, and Gabriel Ycas. “Stellar Spectroscopy in the Near-infrared with a Laser Frequency Comb”. In: Optica, 6, 2 (Feb 2019), p. 233. arXiv: 1902.00500.

7 S. Kanodia, S. Mahadevan, L. W. Ramsey, Stefansson, G. K., A. J. Monson, F. Hearty, S. Blakeslee, E. Lubar, C. F. Bender, J. P. Ninan, D. Sterner, A. Roy, S. P. Halverson, and P. M. Robertson. “Overview of the spectrometer optical fiber feed for the habitable-zone planet finder”. In: SPIE Conference Series. Vol. 10702. SPIE. Aug. 2018, p. 10702. arXiv: 1808.00557.

6 J. P. Ninan, C. F. Bender, S. Mahadevan, E. B. Ford, A. J. Monson, K. F. Kaplan, R. C. Terrien, A. Roy, P. M. Robertson, S. Kanodia, and Stefansson, G. “The Habitable-Zone Planet Finder: improved flux image generation algorithms for H2RG up-the-ramp data”. In: SPIE Conference Series. Vol. 10709. Aug. 2018.

5 S. Halverson, R. Terrien, S. Mahadevan, A. Roy, C. Bender, Stefansson, G. K., A. Monson, E. Levi, F. Hearty, C. Blake, M. McElwain, C. Schwab, L. Ramsey, J. Wright, S. Wang, Q. Gong, and P. Roberson. “A comprehensive radial velocity error budget for next generation Doppler spectrometers”. In: SPIE Conference Series. Vol. 9908. SPIE. Aug. 2016, 99086P. arXiv: 1607.05634 [astro-ph.IM].

4 P. M. Robertson, F. R. Hearty, T. B. Anderson, Stefansson, G. K., E. I. Levi, C. F. Bender, S. Mahadevan, S. P. Halverson, A. J. Monson, L. W. Ramsey, A. Roy, C. Schwab, R. C. Terrien, M. J. Nelson, and B. Blank. “A system to provide sub-milliKelvin temperature control at T 300K for extreme precision optical radial velocimetry”. In: SPIE Conference Series. Vol. 9908. SPIE. Aug. 2016, p. 990862.

3 C. Schwab, N. Jovanovic, T. Feger, M. Bakovic, Y. V. Gurevich, J. Stürmer, R. Apodaca, L. Vanzi, S. Rukdee, J. S. Lawrence, D. W. Coutts, N. Cvetojevic, S. Mahadevan, Stefansson, G. K., S. P. Halverson, and O. Guyon. “Adaptive optics fed single-mode spectrograph for high-precision Doppler measurements in the near-infrared”. In: SPIE Conference Series. Vol. 9912. SPIE. July 2016, p. 991274.

2 F. Hearty, E. Levi, M. Nelson, S. Mahadevan, A. Burton, L. Ramsey, C. Bender, R. Terrien, S. Halverson, P. Robertson, A. Roy, B. Blank, K. Blanchard, and Stefansson, G. “Environmental control system for Habitable-zone Planet Finder (HPF)”. In: vol. 9147. SPIE. 2014,

1 S. Mahadevan, L. W. Ramsey, R. Terrien, S. Halverson, A. Roy, F. Hearty, E. Levi, Stefansson, G. K., P. Robertson, C. Bender, C. Schwab, and M. Nelson. “The Habitable-zone Planet Finder: A status update on the development of a stabilized fiber-fed near-infrared spectrograph for the for the Hobby-Eberly telescope”. In: Ground-based and Airborne Instrumentation for Astronomy V. Vol. 9147. SPIE. July 2014, 91471G.

Teaching and Mentoring


  • Sinclaire Jones (undergraduate at Princeton)
  • Marissa Maney (now graduate student at Harvard)
  • Yiting Li (now graduate student at UCSB)


Spring 2014

  • Astro 11 (008) Elementary Astronomy Laboratory, Instructor of Record.
  • Rooftop and Planetarium TA

Fall 2013

  • Astro 11 (004) Elementary Astronomy Laboratory, Instructor of Record.
  • Astro 1 (001) Astronomical Universe, taught by Prof Ravi Kopparapu.
  • Astro 1 (004) Astronomical Universe, taught by Prof Eric Feigelson.
  • Astro 10 (001) Elementary Astronomy, taught by Prof Alex Wolszczan.

Spring 2013

Physics 2V Experimental Labs in Physics 2V. Supervisors: Prof S. Þ. Ingvarsson and A. Ólalfsson

Fall 2012

Physics 305G Classical Mechanics, taught by Prof P. Jakobsson.


About Me

I am an astrophysicist who develops and uses next-generation instruments to better detect and characterize planets orbiting nearby stars, with a particular focus on planets orbiting low-mass stars. To study these worlds, I use two precision radial velocity spectrographs I helped design and deploy—the near-infrared Habitable-zone Planet Finder (HPF) on the 10m Hobby-Eberly Telescope, and the optical NEID spectrograph on the 3.5m WIYN Telescope—along with the diffuser-assisted photometry technique, a low-cost method I have shown is capable of approaching space-quality photometry from the ground.

I am currently a Henry Norris Russell Fellow at the Department of Astrophysical Sciences at Princeton University. I obtained my PhD in Astronomy and Astrophysics at Penn State University in September 2019, where I completed my thesis "Extreme Precision Photometry and Radial Velocimetry from the Ground" supervised by Professor Suvrath Mahadevan as a NASA Earth and Space Science Fellow (2016-2019). I completed my Bachelor's degree in Physics at the University of Iceland in 2013 where I worked in the Leosson lab in experimental nanophotonics with applications in biosensing, and spent a summer at Stanford University as part of the Summer International Honors Program.

If you have any questions, please don't hesitate to email me at gstefansson - at -