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
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)
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)
CMAP
(CHARA Michelson Array Pathfinder)
See Poster #2008
Rainer Koehler
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
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
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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
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Sensitivity improvements → K = 10-11 mag
- WFS / control max 1kHz / 13-18Hz
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Injection into fibers at telescope
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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).
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[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).
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[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
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
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.
→ 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
EAS26
By Nic Scott
