The CHARA Array:

The sharpest eyes in the world

nic scott

Telescope Systems Scientist

The Array is capable of resolving details as small as 200 micro-arcseconds, equivalent to the angular size of a nickel (0,20€) coin seen from a distance of 10,000 miles (16,000 km).

Preliminary planning: 1985

engineering studies: 1992

Georgia State $5.6-million 5-telescope array: 1994

NSF capital investment to nearly $6.3-million.

University has provided matching funds in a similar amount.

Ground broken: 1996

Keck Foundation $1.5-million to add a 6th telescope: 1998

first fringes: 1999

the Array achieved starlight fringes on its 331-meter baseline, the longest baseline (by a factor of three) ever achieved by an optical interferometer: 2000

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)

  • 0.20 mas at R (650 nm)
  • 0.52 mas at H (1.67 μm)
  • 0.66 mas at K (2.13 μm)
















State of the art of single aperture telescopes





16x sharper than Hubble

Optical Interferometry

Image credit: ESO

V = \frac{I_{max}-I_{min}}{I_{max}+I_{min}}

Fringe spacing

Fringe orientation

Fringe Depth

Binary separation

Binary position angle

Binary magnitude difference

All we have to measure is light from a distant how can we find out so much about them?





What can we measure from light?


  • Intensity ("flux")
    • photometry
      • magnitudes
      • transits
  • Wavelength ("color")
    • spectrum
      • radial velocity
      • composition
    • incandescence
    • emission
    • absorption
  • Interference
  • Polarization
  • Timing

Old but interesting. Perhaps a reader is ready to modernize the list?? I believe this is from Shapley's book "Beyond the Observatory."


“Thirty Deductions from a Glimmer of Starlight”

Harlow Shapley

… let us return to a consideration of what modern astronomers can find out about a single star. I shall list here thirty facts that we can now deduce from appropriate studies of a single star image. The first eighteen of these facts can be discovered about any star.

1.    The position in the sky with reference to other stars.

2.    The apparent magnitude (brightness) with reference to stellar or artificial standards.

3.    The color index (found by comparing the brightness in various spectrum intervals—that is, measuring the color tint: reddish, yellowish, greenish, or bluish).

4.    The variability in light; it may be zero.

5.    The spectral class in two dimensions.

6.    The variability, if any, in spectral class.

7.    The chemical composition of the stellar atmosphere and the consequent nature of the atomic transformations that maintain the radiation.

8.    The approximate age.

9.    Whether it is a single or double (found in various ways).

10.    The existence and strength of its magnetic field.

11.    The involvement with interstellar nebulosity.

12.    The speed of rotation.

13.    The tilt of the rotational axis.

14.    The speed in the line of sight, and variations, regular or irregular in that speed.

15.    The cross motion—measurable only if the distance of the star is small or the speed is great.

16.    The surface temperature.

17.    The total luminosity (candle power).

18.    The diameter.

The next eight facts can also be learned if the star is an eclipsing binary—a double star whose light varies because the two members of the system periodically eclipse each other.

1.    The mean density of the two components.

2.    The period of revolution.

3.    The geometry of the eclipse—and whether it is total or partial.

4.    The degree of darkening at the lib.

5.    The ratio of the sizes of the two components.

6.    The eccentricity of the relative orbit.

7.    The inclination of the orbital plane.

8.    The approximate distance.

And about a Cepheid variable—a star that varies in light periodically because of pulsations—four additional facts can be found.

1.    The shape of the light curve.

2.    The period of pulsation.

3.    The population of membership.

4.    The approximate distance.

magnetic fields

For many stars, we can now add: the frequencies and amplitudes of its oscillations (asteroseismology)

presence of planets

Let's consider the state-of-the-art in "single star images" - resolved interferometric imaging in the optical can yield:
* Stellar shapes - eg. particularly oblateness for rapid rotators
* Surface brightness distribution - including star spots, stellar limb darkening, gravity darkening
* Interior temperature structure of the upper atmosphere (by inverting the limb darkening)
* For rapid rotators, gravity darkening can directly indicate the degree of convective versus radiative energy transfer
* And the star spots can make Bryan happy by giving insight into magnetic fields 😎

Fizeau & Michelson

Brown & Twiss



1920 - measured Betelgeuse (with Pease)

1956 - HBT effect, correlation b/t coherent photons

Intensity interferometry

The Rayleigh criterion and the tyranny of the atmosphere

\Theta = 1.22 \lambda/d
\approx \lambda / r_0

but in atmosphere, turbulent cells limit resolution to 

Fizeau & Michelson

Brown & Twiss



1920 - measured Betelgeuse (with Pease)

1956 - HBT effect, correlation b/t coherent photons

detail lost to the atmosphere can be regained through interferometric analysis 

Intensity interferometry




a method to obtain diffraction-limited resolution across the full aperture of a large telescope

 long exposure

speckles blur

produce Airy pattern

true images are impossible, only centrosymmetric objects can be reconstructed

speckle pattern is the Fourier transform of telescope pupil

autocorrelation of speckles

(in Fourier space)


(time-averaged intensity)

Lanthermann et al. 2022



MIRC-X disperses light across several spectral channels and operates in the J and H-bands. MYSTIC operates in the  K-band. MIRC-X/MYSTIC can be used simultaneously to create images of stellar surfaces and circumstellar disks.  The precision closure phases are well suited to detecting faint binary companions.

Three spectral modes are currently available in the H-band: prism R=50 (8 spectral channels), prism R=102, and grism R=190. A higher resolution grism with R=1170 is available (with approval)


Anugu 2020


MYSTIC, the Michigan Young STar Imager at CHARA is a K-band, cryogenic, 6-beam combiner. All-in-One 6T or high sensitivity 4T gravity chip

A 4-telescope mode for MYSTIC using an integrated optics component designed for the VLTI-GRAVITY experiment is under development and will provide better sensitivity for the faintest targets.

MYSTIC is available in the K-band using a low-resolution R=49 prism. Additional spectra modes (prism R=22, grism R=278, grism R=981, grism R=1724)

Setterholm et al. 2023

Pannetier et al. 2021


(Stellar Parameters and Imaging with a Cophased Array)

SPICA-FT H-band 6-beam ABCD combiner by VLC Photonics, inspired by GRAVITY

low-resolution mode uses MIRC-X for fringe-tracking

The goal of the SPICA project is to provide a large and homogeneous set of stellar parameters across the HR-diagram. The survey aims to measure the angular diameters of 1000 stars.

Low-resolution spectrograph for measuring precise angular diameters (R=150, 50 channels over 650–950 nm).

For a sub-sample of bright stars, medium (R=4300) and high (R=13,200) spectral resolution modes will be available for spectral imaging of stellar surfaces and environments and kinematic studies.



(CHARA Array Integrated Optics Testbed)

collaboration with Leibniz Institute for Astrophysics Potsdam (AIP)

ULI optics for JHK bands

(Siliprandi, Labadie, Madhav, Dinkelaker, Thompson, Benoît)

Currently have two injection stages, CRED1 arrives this year

Coupling ratio evolutions of (input coupling waveguide) and R2(λ) in blue diamonds of a writing beam combiner characterized by coupling input light in the two external PT1 and PT2 inputs to obtain two experimental measurements of the 3 dB asymmetric directional coupler (a, b) and one for each photometric asymmetric Tap directional couplers, as Tap 1 (c) and Tap 2 (d)

Siliprandi et al. 2022


(CHARA Michelson Array Pathfinder)

Mobile Telescope Transport (TR116)

Ligon et al. 2022







The full Michelson Array would offer

12 total positions, creating 66 possible baselines.



science drivers

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

There are some 250 known exoplanets with host stars accessible to CHARA (Dec>-30 deg, V < 7). 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 shape of the silhouette of the planet can be measured in long baseline observations made during transits.

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.

~50% of the stars you see are binaries

~25% are triples

-on avg there's an exoplanet per star

~10% of Sun-like stars likely host rocky exoplanets, more for lower-mass stars

-40-50% of Kepler/K2/TESS objects of interest may have companions

-biases occurrence rates & underestimates exoplanet radii by 50%









Solar Masses


  • get transmission spectrum of the atmosphere
  • measure different radii at different wavelegnths to give clues to composition

NGC 4151

Early results from CHARA already show that the innermost dusty region in NGC 4151 is aligned perpendicularly to the jet axis.

Be stars

Be stars

Spotted magnetic stars

Interacting binaries

Expansion curve of Nova Del 2013.

following slides stolen from EXOPAG 23:

future plans

  • Telescope dichroic replacement
  • more automated alignment and tracking
  • new/upgraded labAO system
  • TEMA replacement
  • new telescope drives, cylinder drives
  • fibers to all telescopes (PM fused silica,          1550nm H-band 1st, 1350 nm – 1470 nm metrology)
  • nuller "Achromatic photonic tricouplers for application in nulling interferometry" - Martinod et al. 2021
  • W5/Channel 13 site (1100m)
    • 0.16 mas at H, 65 μas at R
  • double-pass delay (90m tracking delay)
  • Narcissus Mirror for SILMARIL

  • The Michelson Array
  • 2m central telescope?
  • up to 300 nights over three years of open access time via (NOIRLab).

  • Snapshot Imaging Mode to encourage new investigations. 

  • Explore the apparent variability of known exozodis
    • long-term monitoring
    • ​clues to source and formation of the dust
  • Expand strong exozodi sample
    • ​leveraging LBTI and prior surveys​​
    • from ~100 \(\rightarrow\) ~1000 objects  
  • Use spectral dispersion to resolve the thermal/scattered dilemma
  • Risk mitigation for coronagraphy/starshade missions
  • Target selection and characterization for mid/large missions (HWO, Plato, etc)
    • ​exozodis likely to be dominant noise source
  • Precision diameters and fundamental astrophysics
  • Planewave:
    • 2-3 yrs until a 2 meter
    • $2.5 million ea
    • > 2 magnitudes deeper
    • move existing telescopes to CMAP sites?
      • 1m outriggers?
    • 6x2m + 4x1m array? 45 baselines!
  • JWST plenary talk:
    • “imaging interferometers are coming online”
  • How to get more hands at the Array/instrumentation interest
    • REU options
    • Shadow-a-Scientist programs
    • Collaborations/Partners
  • Funding
    • NSF wants astronomers to compete for MSIPs with MSRI funds
    • Workshops on centers of excellence, partnerships with industry (TIP)
    • Could chara get solar panel funding?
    • Grow local and state politician interest/investment?

Some thoughts from AAS 241

CHARA intro

By Nic Scott

CHARA intro

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