New Developments at the CHARA Array

Nic Scott

 

EAS 2026 - Lausanne

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

The 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 16,000 km.

 Spatial resolution: 0.20 - 0.66 mas at R - K (0.6 - 2.13 μm)

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

Anugu+2026

MIRX

MYSTIC

SILMARIL

SPICA

CHARIOT

PAVO

VIS BEAMS

METROLOGY

STS/STST

BEAM SAMPLERS

BEAM Reduction

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

CMAP

(CHARA Michelson Array Pathfinder)

See Poster #2008

Rainer Koehler

Ligon+2022

Mobile Telescope Transport (TR116)

20 ← 34 - 331 → 579 m

S3

S4

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

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

  • 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

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

  • 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.

 

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+

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, ADC
  • current beam dia or compress and re-expand?
  • DM at M4 or move elsewhere (M2?)
  • UV LGS
  • reduce system overhead
  • Strehl from 12-15% → >50% in R-band

CHA

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.

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

[1] Eisenhauer et al. ARA&A 61, 237–285 (Aug. 2023).
[2] Wright, H., “Explorer of the Universe,” AIP (1994).
[3] ten Brummelaar et al. ApJ 628, 453–465 (July 2005).
[4] McAlister, H. A. and McAlister, S. J., “The 2009 Station Fire Threat to Mount Wilson Observatory,” Amazon (2019).
[5] Gies et al. (SPIE) Conference Series 13095, 1309502 (Aug. 2024).
[6] Elliott et al. PASA 41, e043 (Sept. 2024).

[7] Balmer et al. ApJ 1001, L26 (Apr. 2026).
[8] Gardner et al. AJ 171, 30 (Jan. 2026).
[9] Ertel et al. PASP 137, 031001 (Mar. 2025).
[10] Evans et al. ApJ 971, 190 (Aug. 2024).
[11] Evans et al. ApJ 972, 145 (Sept. 2024).
[12] Gallenne et al. A&A 693, A111 (Jan. 2025).
[13] Chowhan et al. MNRAS 548, stag719 (May 2026).
[14] Setterholm et al. AJ 169, 318 (June 2025).
[15] Ibrahim et al. ApJ 947, 68 (Apr. 2023).
[16] Ibrahim et al. ApJ 998, 10 (Feb. 2026).
[17] Codron et al. MNRAS 541, 1600–1612 (Aug. 2025).
[18] Rivinius et al. A&A 694, A172 (Feb. 2025).
[19] Anugu et al. ApJ 973, L5 (Sept. 2024).

[20] Anugu et al. ApJ 974, 113 (Oct. 2024).
[21] Mobeen et al. A&A 686, A260 (June 2024).
[22] Aydi et al. Nature Astronomy 10, 271–280 (Feb. 2026).
[23] De Furio et al. ApJ 990, 54 (Sept. 2025).
[24] Danner et al. ApJ 988, 113 (July 2025).
[25] Torres et al. ApJ 990, 107 (Sept. 2025).

[26] Torres et al. ApJ 971, 31 (Aug. 2024).
 

References

[27] Richardson et al. ApJ 977, 78 (Dec. 2024).
[28] Lau et al. ApJ 963, 127 (Mar. 2024).
[29] Holdsworth et al. ApJ 977, 185 (Dec. 2024).
[30] Shepard et al. ApJ 977, 236 (Dec. 2024).
[31] Torres et al. ApJ 977, 43 (Dec. 2024)

[32] Anugu et al. JATIS 12, 015008 (Jan. 2026).
[33] Shuai et al. AJ 170, 344 (Dec. 2025).
[34] Anugu et al. AJ 171, 253 (Apr. 2026).
[35] Magri et al. MNRAS 536, 266–273 (Jan. 2025).
[36] Roettenbacher et al. Nature 533, 217–220 (May 2016).
[37] Dholakia et al. PASP 138, 054504 (May 2026).
[38] Mourard et al. A&A , submitted (2026).
[39] Anugu et al. AJ 160, 158 (Oct. 2020).
[40] Setterholm et al. JATIS 9, 025006 (Apr. 2023).
[41] Mourard et al. (SPIE) Conference Series 13095, 1309503 (Aug. 2024).
[42] Lanthermann et al. (SPIE) Conference Series 12183, 121830N (Aug. 2022).

[43] Lanthermann et al. (SPIE) Conference Series 13095, 1309505 (Aug. 2024).
[44] Anugu et al. (SPIE) Conference Series 13095, 130951B
(Aug. 2024).
[45] du Foresto et al. SPIE (2003).
[46] Lhomé et al. SPIE (2012).
[47] Scott et al. JATIS 02(02), 1340005 (2013).
[48] Scott et al. SPIE (2014).
[49] Siliprandi et al. Appl. Opt. 63, 159–166 (Jan 2024).
[50] Gies et al. [Bulletin of the American Astronomical Society], 51, 226 (Sept. 2019).
[51] Rajagopal et al. (SPIE) Conference Series 13095, 130951N (Aug. 2024).

[52] Mourard et al. A & A (2026), accepted

backup slides

Exoplanet Systems

Gardner+2026 searched for the NIR signal of the hot Jupiter planet orbiting ups Andromeda

  • 1.7MJup mass ~ 4 mas separation, flux should be detectable with CHARA
  • 2021 MIRC-X → hints at the expected separation
  • 2023 MIRC-X and MYSTIC → no evidence of a planetary flux signal

→ may indicate that the NIR flux is lower than predicted.

Ashley Elliott developed the RADPy code

→ reliable angular diameters of stars from CHARA observations

 

51 Eridani system

  • resolved massive planet in 32 yr orbit

PARSEC evolutionary models

​→ stellar mass and age, & a planetary mass of 4MJup

github.com/spaceashley/radpy

Elliott+2024

Stellar Pulsations

Evans+2024 investigated the mass of Polaris

CHARA observations made around periastron of a faint companion (2016 - 2021)

  • 1300x fainter Polaris in H

radial velocity + angular position measurement → 29.4 yr orbit + Gaia distance → Cepheid mass of 5.13 ± 0.28M⊙

  • luminosity > predicted
  • additional structure beyond a simple limb darkened disk
  • SURFING code image reconstruction revealed a surface with several bright spots
    • magnetic spots or convective supergranules?

similar analysis of the Cepheid AW Per

  • triple system: 6.8 ± 0.9M⊙ Cepheid and a close binary companion of two similar mass stars → total mass 8.8 ± 0.5M⊙

Relative orbit of Polaris companion from multiple observations

False color image of Polaris in H

Chowhan+2026 PAVO observations of the “secondary red clump” star κ Cyg that exhibits solar-like oscillations

  • angular size + Gaia distance → 8.65±0.10R⊙
  • oscillation frequencies predicted are poor match to NASA TESS observations

→ mismatch - incomplete treatment of convective boundary mixing in the interior models

Gallenne+2025 triple system Cepheid SU Cyg

  • derived mass of the Cepheid is 4.859 ± 0.058M⊙
  • orbits a close pair with masses of 3.595 ± 0.033M⊙ and 1.546 ± 0.009M⊙
  • component velocities + angular orbit → independent distance estimate of 926.3 ± 5.0 pc
    • most accurate distance ever measured for a Cepheid variable
    • evolutionary models fail to account for the apparent over-luminosity of the Cepheid for its derived mass

→ revisions to the model tracks are needed.


 

Stellar Pulsations II

Bras 2026 Cepheid η Aql

  • angular size variation & limb darkening variation over pulsation period
  • variation in limb darkening coefficient ~2x predicted by current stellar atmospheres model for Cepheids,

→ physical model of the center-to-limb intensity variations may be incomplete

Circumstellar Environments

Setterholm+2025 evolution of inner disk of Herbig Ae star HD 163296 - MIRC-X and VLTI PIONIER

  • unresolved central star, uniform background, & structured disk
  • bright spot migrates clockwise over 44d

> Keplerian motion at that separation

→ might be related to a tidal wake from wider proto-planet

 

 

 

Ibrahim+2026 H and K-band survey of 17 Herbig Be stars 

  • MWC 34016 → binary system with a resolved disk surrounding the brighter component and possibly with a smaller disk surrounding the companion

→ binary disk system

Rivinius+2025 spectroscopic survey of Be stars in the post-mass transfer stage before the donor star shrinks to subdwarf

  • 6 binaries, including HD 698 (V742 Cas) 
  • 58.4d, 0.663 mas orbit

→ the smallest orbit yet resolved with the CHARA Array.

 

 

Circumstellar Environments II

hypergiant star RW Per during and after the Great Dimming

Anugu+2024 RW Per, R = 5 AU in the bright state

→ one of the largest diameter stars

 

 

hypergiant star ρ Cas20, R = 3 AU.

  • photospheric, limb-darkened angdia = 2.09 ± 0.2 in 2.0-2.3 μm
  • 2.91 ± 0.19 mas in 2.3-2.4 μm

→ latter range contains many CO lines, add to the overall opacity and shifts line forming region to higher in the atmosphere

Codron+2025 disk structure around T Tauri HD 143006

  • inner and outer disks are misaligned by ~39°
  • dark shadow regions in the outer disk cast by the misaligned inner disk

→ no inner binary companion is detected, so the origin of the misalignment remains a mystery

Circumstellar Environments III

2002: luminous red nova V838 Mon erupts:

high mass star + low mass companion merger → dust outflow enshrouding 

 

 

Aydi et al. 2025 ToO observations with MIRC-X of 2 novae explosions in 2021

interferometric imaging + contemporary spectroscopy + high E γ-ray measurements Fermi fast nova V1674 Her

  • 2 and 3 days after the eruption

→ thermonuclear explosion on WD → fast bipolar outflow + slow equatorial expansion  

slow nova V1405 Cas

  • 53, 55, and 67 days after explosion
  • ellipsoidal structure that is smaller in the final observation

→ outflow concentrated in the orbital plane and that appears smaller as the net optical depth decreases with expansion and the inner regions are revealed.

 

Mobeen+2024

→ merger led to jet formation → bipolar structure of the dust emission

Binary and Multiple Stars

Torres+2025 binary star Atlas, in the Pleiades cluster, age = 104 Myr

  • 291d angular orbit + radial velocity measurements → mass 5.04 ± 0.17M⊙ and 3.64 ± 0.12M⊙.
  • brighter star is rapid rotator
  • rotationally distorted shape + spectroscopic projected rotation velocity → equatorial rotational velocity of 233 ± 45 km s−1 (77% of the critical)​

→ Rotational distortion of evolved primary


 

Mass - abs mag for Hyades cluster members

inset - shows how the PARSEC code tends to overestimate the V fluxes

Relative orbit of companion to Atlas

Torres+2024 interferometry + spectroscopy of 6 solar-mass binaries in the Hyades cluster

  • CHARA + Palomar Testbed Interferometer (PTI) + a large set of RV measurements to derive masses with sub-1% accuracies 
  • good agreement with the predicted mass – luminosity relation
  • but PARSEC models slightly overestimate the V-band fluxes of such stars

→ perhaps due to some missing component of opacity code calculation

Interferometric Methods

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

  • 5" fov into MIRC-X/MYSTIC combiners individually

→ yields measurements of the differential positions

  • fringe track on one target and integrate on the other (fainter) target
  • observations of the α Psc multiple system.
  • B component is itself a close binary

α Psc → hierarchical triple. 

 

Anugu et al. (2026)

ρ = 1.85"

Dholakia&Pope2026 starspot locations on rotating stars using spherical harmonics 

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

Ludovic Grossard & François Reynaud ALOHA (Astronomical Light Optical Hybrid Analysis)

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

Outer A-B