Center for High Angular Resolution Astronomy
Georgia State University, Atlanta
Director: Douglas Gies

Array
Mount Wilson Observatory, CA
Director: Gail Schaefer
16 staff members onsite
Operations funded through the NSF, GSU, collaboration partners

Peter Tuthill - 2024 Michelson Prize

Stephen Ridgway - Fizeau Prize in 2024, Fellow of the AAS in 2026

Theo ten Brummelaar - retired from active work in 2025 after 32 years on the Array
William G. Bagnuolo, Jr. - passed away on 2025 March 12 at the age of 77

Heven Renteria - new operator

Robert Klement returned as a research scientist

Georgia State University

The CHARA Array is operated by the Center for High Angular Resolution Astronomy at Georgia State University in Atlanta.

The two-telescope CLASSIC beam combiner

University of Michigan

The MIRC-X H-band combiner. A six-telescope cryogenic K-band beam combiner, MYSTIC

University of Exeter

The upgrades to the six-telescope MIRC-X combiner

l’Observatoire de la Côte d’Azur

SPICA combines all six-telescopes and provides a range of spectral dispersions at visible wavelengths.

Sydney University

Precision Astronomical Visible Observations (PAVO) instrument

Australian National University

The (PAVO) visible beam combiner

Université de Limoges

the ALOHA fiber experiment.

Kyoto Sangyo University

Pushing the sensitivity limits of the Array in order to resolve the cores of Active Galactic Nuclei.

National Optical-Infrared Astronomy Research Laboratory

Open access time at the CHARA Array is available to the astronomical community through the National Optical-Infrared Astronomy Research Laboratory (NOIR Lab). 

The CHARA Consortium

CHARA Science Meeting 2025 - Nice, France

CHARA/MROI Science Meeting and New Visions in Optical Interferometry Workshop 2026 - Socorro, NM

workshop supported by 

next meeting: Pasadena, CA ~March 2027

Fizeau  

Michelson

Brown & Twiss

1850s

1890s

1920 - measured Betelgeuse (with Pease)

1956 - HBT effect, correlation b/t coherent photons

Intensity interferometry

details can be gained through interferometric measures

Image credit: ESO

diameter

Lawson 2003, S&T

Science Drivers

• multiple star systems
• time domain astronomy
• YSOs/planetary formation
• evolved stars
• starspots/surface imaging
• exozodis
• AGN
• HWO/PLATO
• exoplanet hosts

The Gaia orbits give the center-of-light motion of unresolved binaries, and a single resolved CHARA observation is sufficient to determine the full orbit and masses. High angular resolution observations also reveal how interacting stars are transformed by mass exchange.

Imaging the Surface of Altair
Monnier et al. 2007

Zhao et al. 2008

the CHARA Array

ELT (for scale)

Spatial resolution

• 0.20 mas at R (650 nm)

• 0.52 mas at H (1.67 μm)

• 0.66 mas at K (2.13 μm)

34 to 331m

• 15 baselines
• 10 closure triangles

Sigma Geminorum

Roettenbacher+2017

TelAO

Anugu+2026

Optical Path Length Equalizers (OPLE) system

• original system: VME (>20yo) 
• modern Linux-based system was installed and tested - 2021 to 2024
• tracking uncertainties ~ 12 μm

LabAO

MIRC-X/MYSTIC

• 6T, H & K-bands (1.6, 2.3 μm)
• Spectral resolution: R=50 to 1700
• Sensitivity: H = 7.5 mag
• J & H-bands simultaneous 4-beam (→ 6-beam soon)
• images of stellar surfaces and circumstellar disks, precision closure phases, faint binary companions
• PI: Monnier (Univ. Michigan), Kraus (Exeter)

SPICA (Stellar Parameters and Imaging with a Cophased Array)

• 6T,  600 – 900 nm
• Spectral resolution: R=140, 4000, 14000
• Sensitivity: V = 6.5 mag
• goal → large, homogeneous set of stellar parameters across the HR-diagram, angular diameters of 1000 stars
• visible-light combiner (SPICA-VIS)
• H-band IO 6-beam ABCD combiner (SPICA-FT) on MIRC-X
• SPICA-FT → group- & phase-delay tracking for SPICA-VIS
• PI: Mourard (Observatoire Côte d’ Azur)

Setterholm+2023

Anugu+2020

Pannetier+2021

Mourard+2026

Lanthermann+2022,  2026, SPIE 14148-81

Silmaril

• 3T simultaneous H/K combiner
• Bulk optics / designed for high sensitivity
• CRED-1
• R=35
• pair of narcissus mirrors to reduce background
• J-band dichroic → Niro detector (from Classic) as angle tracker (Strider) 
• expect limiting magnitude of H=10-11
• PI: ten Brummelaar (GSU), Tuthill (Sydney)

CHARIOT (CHara ARray Integrated Optics Testbed)

• 2T fiber-fed K-band combiner (4T planned)
• based off of FLUOR/JouFLU, designed for high precision
• CRED-1
• connectorized ULI integrated optic combiner 
• current limiting magnitude of K=5.2
• PI: Labadie (Universität zu Köln)

Dinkelaker+2026, SPIE 14148-17

CHARIOT

(CHARA Array Integrated Optics Testbed)

collaboration with Leibniz-Institut fur Astrophysik Potsdam, University of Cologne, Heriot-Watt University

ULI optics for JHK bands

Apr 2024 - First fringes

Mobile Telescope Transport (TR116)

CMAP

(CHARA Michelson Array Pathfinder)

Ligon+2022

S4

S3

ALOHA – Univ. Limoges
Single-mode PM fibers
λ=810nm,  240m long
Laying on the ground
Connect S1+S2
On-sky fringes Magri+2024
CMAP
Single-mode PM fibers
λ=1.6μm, 650m long
Trench: 18 inches deep

20 ← 34 - 331 → 579 m

S3

S4

Single-mode PM fibers
λ=1.6 μm, 650 m long
Trench: 18 inches deep

1100 m

W5

S3

S4

SCIENTIFIC RESULTS

Exoplanet Systems

• angular size, parallax, and bolometric flux → stellar radius, Teff, and luminosity

Stellar Pulsations

• Cepheid variable stars → period - luminosity relation → distances of local galaxies  
  • ​distances, angular diameters, binarity, orbits, masses, limb darkening, imaging
• ​independent verification for asteroseismology - particularly important for evolved, core helium-burning stars.

Circumstellar Environments

• planet-forming disks of Young Stellar Objects
• Be star decretion disks - gas from the rapidly spinning star is launched outwards into an ionized gas disk
• massive stars become very luminous and variable as they grow to become cool supergiants.
• asymmetrical mass loss can result from energetic processes in binary systems

Binary and Multiple Star Systems

• fundamental means to measure stellar masses.

• angular orbit + distance estimate → total mass of components

• angular orbit + radial velocity orbits → individual masses + independent distance measurement

  →  stellar evolution processes and interactions between binary stars

Interferometric Methods

29 refereed papers over the past 2 years (294 total)

6 PhD dissertations (61 total)

Exoplanet Systems

• 51 Eridani system - resolved planet in 32 yr orbit, PARSEC evolutionary models ​→ stellar mass and age, & a planetary mass of 4MJup Elliott+2024

• missing companion of ups And Gardner+2024

Stellar Pulsations

• Polaris faint companion detected, 29 yr orbit, Polaris mass 5.13 ± 0.28M⊙, spotted surface image Evans+2024
• various Cepheid model - observation mismatches

Circumstellar Environments

• inner disk of Herbig Ae star HD 163296 → possible tidal wake from wide proto-planet Setterholm+2025
• binary disk system Ibrahim+2026
• T Tauri HD 143006 - inner/outer disk misalignment mystery Codron+2025
• post-mass transfer Be star, smallest orbit yet resolved by the Array = 0.663 mas Rivinius+2026
• imaging of fast nova V1674 Her and slow nova V1405 Cas Aydi+2025

Gallenne+2024

Bras2026

Chowhan+2026

29 refereed papers over the past 2 years

(294 total)

Binary and Multiple Star Systems

• Pleides member star, Atlas: orbit, mass, and rotational distortion of evolved primary Torres+2025

• interferometry + spectroscopy of 6 solar-mass binaries in the Hyades cluster Torres+2025

Interferometric Methods

• first experiments in dual field interferometry at the CHARA Array Anugu+2026

  • α Psc → hierarchical triple system

• observations of Vega via 2x 240m fibers at 810nm from S1 & S2 Magri+2025

• starspot locations on rotating stars using spherical harmonics

  • open-access code harmonix → reconstruct the intensity map based upon a simulation of observations Dholakia&Pope2026

Ashley Elliott 2024

Diameters of Stars

Ashley Elliott (LSU) has compiled interferometric measurements from CHARA and more to create an empirical HR diagram.

Angular Dia. + Parallax  → Linear Radius

Diameter + Bolometric Flux → Teff

SCIENTIFIC RESULTS

Yu et al. 2023

interferometric angular diameters provide key benchmark data to evaluate calibrations of effective temperature from large-scale spectroscopic and photometric surveys - SED/CHARA

693 stars, σθ < 5%

Torres et al. 2022

Torres et al. 2024

Binary & Multiple Stars

Castor A and B

• resolved the inner binary components
• masses and 3D orbits
• mutual orientations of the different components.

HD 284163

hierarchical quadruple system

• orbits of the inner (2.39 d) and first outer (43.1 yr) are    nearly orthogonal
• Accurate masses including that of the secondary
• (0.5245 ± 0.0047 M⊙) is now the lowest mass star with a dynamical mass measurement in the Hyades cluster.

Caballero et al. 2022

Gleise 486

• M3.5 V star at ~8 pc
• transit every 1.467 days.
• MIRC-X → angular size of the the host star
• physical radius and effective temperature.  
• transit light curve → ratio of planetary to stellar radius
• exoplanet diameter
• HPRV captured the reflex motion of the star and led to an exoplanet mass
• model of the interior structure and possible atmosphere of this other world in the solar neighborhood.

Exoplanet Systems

Interferometric observations of exoplanet host stars provide the means to determine the detailed stellar characteristics that are required to find the exoplanet properties.

• Radius and Teff of host
• Mass + age from evolutionary tracks
• Size of habitable zone
• Radius of transiting planets

Anugu et al. 2023

Exoplanet Systems

Planet formation is generally considered in the context of young stars, but mass loss in older stars may also play a role in making planets at the end of a star's life.  

Circumbinary disk with close to a polar alignment with respect to the binary orbit.

• Any planet formed in the disk would be relatively stable.
• central cavity in the disk could be result of such a planet.

If so, represents the first example of a polar circumbinary planet.

Martin et al. 2023

Image credit: Dr Mark A. Garlick / markgarlick.com

post-AGB star AC Her 

• binary system
• surrounded large disk of gas and dust.
• Anugu determined the first 3D orbit for AC Her
• first for any post-AGB system

→ the large cavity in the center of the circumbinary disk is not created by the tidal action of the central binary.

Imaging Luminous Stars

The cool hypergiant star RW Cep experienced a Great Dimming event in 2022:

• imaged near photometric minimum
• H & K-band images show an asymmetric intensity distribution and a distorted shape
• NIR spectroscopy found fading increased towards shorter wavelengths
• implicates dust formation from stellar ejecta as the explanation for the fading and unusual appearance.  

Anugu et al. 2023

Patchy appearance results from dust created by a huge ejection from the star

Illustration credit: NASA, ESA, and E. Wheatley (STScI)

Anugu and colleagues are continuing to monitor the star with CHARA to explore how the surface appearance changes as the star brightens again.  

Ibrahim et al. 2023

Disks Around Young Stars

The process of planet formation involves the development of instabilities in the disk that can be followed through time series interferometric observations at very high angular resolutions.

luminous Herbig Be star HD 190073

•  YSO disk is viewed almost face-on (i<20°)
• clear view of the full extent of the inner gas disk.
• discovered a bright spot in the disk that migrated by 27° over of 32 days  

V1925 Aql

NGC 4151

Active Galactic Nuclei

bright central region of the active galactic nucleus of the Seyfert galaxy NGC 4151.  

• Central structure resolved at the 0.5 mas scale.
• ring-like structure viewed at an inclination of 40°
• perpendicular to the radio jet
• K-band flux probably originates in a dust sublimation region on the face of the torus surrounding the black hole.

• 468 proposals submitted since 2010

• 132 unique OA PIs

• 90 distinct institutions

• up to 300 nights over 3 years of open access time via NOIRLab

• dedicated to community growth in US and beyond

• staff assist new investigators with planning observations, collecting data, and reducing data

• calibrated oifits files provided

NOIRLab Open Access

• started in 2025
• 1n per month available

  open to all observers

• confirmation of high impact findings
• feasibility exploratory
• following-up on events
• completion to allow publication

New Fast Turnaround Snapshot Imaging Mode

​Priority for observers who do not have existing time and for targets not already included in programs with awarded time.

https://chara.gsu.edu/observers/applying-for-chara-time

Exoplanets

Clusters & Ages

AGN

Engineering

Program Stats (2010-2026)

<5% loss to technical issues

Future Plans

• telescope dichroic & STST upgrade ($1.4m NSF MRI1 funded)

  • optimize simultaneous observations in visible + NIR

• AOB upgrades ($16k GSU funded)
• automated alignment/tracking
• painting/AC at domes
• continue quantum initiatives
• exploring the feasibility of a B-band combiner (440 nm) → 0.13 mas
• next-gen AO systems
• fibers to all telescopes
• W5/Channel 13 site (1100m)
  • expanded / double-pass delay
• nulling instrument(s)
• central 2m telescope
• upgrade existing 1m telescopes to 2m telescopes
• The Michelson Array

2028

2028+

New improvements

• removes single-point failures
• modular
• easy to troubleshoot
• COTS industrial components
• allows simultaneous movements
• motor state polling
• 5ms delay b/t commands
• expected to save 30-45mins per 7hrs observing

student Noy Hakobyan (UC Irvine)

• drives (4EL/1AZ done)
• cylinder control
• environmental sensing
• GSU data archive was expanded in 2025 to 500 terabytes of RAID storage
• web portal to search the archive
• remote data reduction machine

AO Board (AOB) / Telescope Manager (TEMA) upgrades

AO upgrades

Current sensitivity limits:​

• Tiptilt, AO: V = 10-12 mag​

• Visible/NIR fringe recording: 8.0 mag​

• Sensitivity improvements → K = 10-11 mag​

• WFS / control max 1kHz / 13-18Hz

• Injection into fibers at telescope

• STST Strehl~55–70% (H), 5-15% (R)

Next-gen AO system (planning):

• new DM, WFS(s), ADC
  • 90-100 actuators (50-60 sub-apertures)
• current beam dia or compress/re-expand?
• DM at M4 or move elsewhere (M2?)
• UV LGS
• reduce maintenance & alignment
• Strehl from 12-15% → >50% in R-band

20 to 1100m

• 0.06 mas at R

• 0.16 mas at H

• 0.20 mas at K

The full Michelson Array would offer 12 total positions, creating 66 possible baselines.

Image an exoplanet during transit

There are some 250 known exoplanets with host stars accessible to CHARA.

The new baselines will enable resolution of solar-like stars out to about 70pc in H-band.

Numerical simulations of the transiting hot-Jupiter in HD189773 indicate that the silhouette of the planet can be measured in long baseline observations made during transits.

The CHARA Array is supported by the US NSF under grants AST-2018862, 2034336, 2407956, 2511059.

The role of the CHARA Array continues to expand...

• from a university program to community research interests
• providing student access and training opportunities
• serving public outreach activities and those of the Mount Wilson Institute
• supporting observational astrophysics programs and innovative instrument development
• commissioning of new beam combiners, testbeds, the use of fiber optics beam transport for interferometry

We plan to further expand new frontiers in instrumentation through collaboration and to support long-term initiatives for interferometry and the development and testing of new technologies that will provide the foundation for future very large long-baseline interferometers.

Eaton Fire Jan 2025

• Jan 7 - fire broke out in high winds
• Jan 9 - fire reached MWO 
• Jan 24 - staff back on-site 

Station Fire (2009)

Bobcat Fire (2020)

Eaton Fire (2025)