The CHARA Array:

building towards the Michelson Array

Nic Scott

Telescope Systems Scientist

Georgia State University, Atlanta
Director: Douglas Gies

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

Center for High Angular Resolution Astronomy

• 1984 - "CHARA" established
• 1994 - NSF funding awarded
• 1996 - Mt. Wilson ground broken
• 1998 - Keck funding added
• 1999 - First fringes between S1/S2
• 2001 - First starlight fringes on longest baseline
• 2003 - Construction completed
• 2005 - First journal paper - dia. of Regulus (McAlister)
• 2007 - Image of surface of Altair (Monnier)
• 2010 - Open access program initiated
• 2025 - 274 papers published so far

CHARA Timeline

https://chara.gsu.edu/astronomers/journal-articles

The CHARA Array is capable of resolving details as small as 200 micro-arcseconds, equivalent to the angular size of a coin seen from a distance of 10,000 miles (16,000 km).

The CHARA Array

Eisenhauer et al 2023

34 to 331m baselines

• 15 total baselines

Spatial resolution

• 0.20 mas R (650 nm)

• 0.52 mas H (1.67 μm)

• 0.66 mas K (2.13 μm)

Evans et al. 2024, ApJ, 971, 190

Polaris

Roettenbacher et al., 2017, ApJ, 849, 120.

sigma Geminorum

Roettenbacher et al. 2016, Nature, 533, 217

zeta Andromedae

Imaging stars

Zhao et al. 2008, 684, 95

Algol

Baron et al. 2012, ApJ, 752, 20

Kloppenborg et al. 2010, Nature, 464, 870

Ashley Elliott 2024

Diameters of Stars

693 stars, σθ < 5%

Binary & Multiple Stars

Castor A and B

Torres et al. 2022

Ashley Elliott (LSU)

• Resolved the inner binary components
• Masses & 3D orbits
• Mutual orientation of components

Imaging Luminous Stars

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

Anugu et al. 2023

• H & K-band images asymmetric distribution, distorted shape
• NIR spectroscopy: fading increased towards shorter wavelengths

→ implicates dust formation from stellar ejecta as explanation for the fading & unusual appearance.  

post-AGB star AC Her

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

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

Anugu et al. 2023

Martin et al. 2023

• Binary surrounded by large gas/dust disk
• First 3D orbit for system
• ​Cavity in the center not created by the tidal action of the central binary

• Planet formed in disk would be relatively stable
• Central cavity could be the result

Ibrahim et al. 2023

Disks Around Young Stars

T Tau type disks and other Young Stellar Objects (YSOs) are the birthplaces of planets.

Herbig Be star HD 190073

V1925 Aql

SU Aur

Labdon et al. 2023

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

• Inclined & warped disk
• Flux mainly from the far side of the disk
• Near side partially obscures the central star
• Dust emission indicates formation of a disk wind from the boundaries of the warped disk

NGC 4151

Active Galactic Nuclei

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

• Central structure resolved: 0.5 mas
• Ring-like structure, i = 40°
• Perpendicular to radio jet
• K-band flux probably from dust sublimation region on the face of torus surrounding black hole

CMAP

(CHARA Michelson Array Pathfinder)

See Rainer's talk to follow

Ligon et al. 2022

"...technical improvements in both hardware and software will continue indefinitely."

"One rationale for the facility is to serve as a testbed for new developments in optical interferometry."

S4

S3

ALOHA – Univ. Limoges
Single-mode PM fibers
λ=810 nm,  240 m long
Laying on the ground
Connect S1+S2
On-sky fringes
Magri, J. et al., MNRAS, 536, 266 (2025)

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

258m to S2

557m to E1

S2

S1

The CHARA Michelson Array (CMA)

17 to 1100m

• CMAP: up to 36 baselines [6+3]

Max spatial resolution

• 0.06 mas R (650 nm)

• 0.16 mas H (1.67 μm)

• 0.20 mas K (2.13 μm)

CMAP→ CMA depends upon beam transport success

1100m

• up to 12 sites
• 2-4 meter telescopes
• linked via optical fiber
• expanded beam combining lab
• up to 66 possible baselines

• AGN: dust sublimation and accretion disk could be resolved, binary black holes
• High contrast/contact/interacting binaries - Outflow/ejection/dimming events, Be star circumstellar disks relations
• Red giant ages
• Star and planet formation: imaging enabled by greater UV coverage, sensitivity, & longer baselines
• Massive stars
• Cool stars
• HR diagram diameters: extended into late type stars and pre-MS contraction
• Solar System Science:  Asteroids bright enough are too big for original Array, CMAP+Silmaril → break discrepancies between radar and reflectance
• Other mission support: HWO, JWST and ELT candidate finding and follow-up, Space mission technology testbeds?

CHARA/CMA Science Cases?

• Taurus star forming region (d=140 pc)
• Measure contraction of young stars to the MS.
• Need  < 0.2 mas to achieve this.

Known exoplanets from the NASA Exoplanet Archive. Over-plotted yellow rectangle region of planets that could be characterized with CHARA.

• Central 2m telescope
• 1m's → 2m's
• Km+ baselines
• AO/ople/lab upgrades
• Off-target phase tracking
• Nuller
• Future beam transport​
• Michelson Array

Future plans

• CMAP fringes
• Telescope dichroic upgrade
• Automated alignment/tracking
• Upgrade labAO system
• Optimizing multi-wavelength simultaneous observations
• Dual-Star Phase Referencing
  • ​MIRC-X + Mystic to allow longer integration of science fringes ~ tens of mins ~ 6 magnitudes
• Fibers to all telescopes
• W5 site (1100m)
  • extended/double-pass delay
• Up to 300 nights over three years of open access time via NOIRLab.

Near future plans

Total of 296 internal PI + CoI.

• 255 open access investigators awarded time
• 152 open access investigators applied but were not awarded
• Total ~ 700 investigators awarded or applied

Community Access Observing Time at the CHARA Array

Available through NOIRLab call for proposals.

NOIRLab Open Access

"New Visions in Optical Interferometry"

2.5 day workshop at the new Astronomy Discovery Center at Lowell Observatory March 16-20, 2026

backup

Beam Combiners

CHARIOT - 2 beam, K-band IO testbed

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.

• Verify diameter measurements between eclipsing binaries and singles
• 8% of M dwarfs are photometrically variable by 2% in V, R, I. 92% are stable
• Study flares, spots
• Magnetically active M dwarfs have very large spots that could be imaged

20pc sample → 600 M dwarfs

• volume complete sample to 20pc
• How many could chara resolve/image?

Diameters and resolved surfaces of M dwarfs

http://recons.org/published.annrev.pdf
https://aasnova.org/2018/07/11/3796/

Gardner et al. 2022

Binary & Multiple Stars

ARMADA astrometry survey to search for triple systems among known intermediate mass binaries.  

•  ~ 20 - 50 μas residuals
• potentially could detect Jupiter-mass exoplanets in binaries.

new orbits of 12 companions around early F- to B-type binaries,

• 9 new detections
• 3 first astrometric detections of known RV companions.

Gardner et al. 2021

Complementary radial velocity measurements for some of these systems yield very reliable mass estimates.

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