Telescopes
Radio Interferometry
Combining radio telescopes — angular resolution beyond largest dishes
Radio interferometry combines signals from multiple radio telescopes to synthesize a virtual telescope with diameter equal to the maximum baseline (distance between telescopes). Resolution improvement: VLA 27 antennas, max baseline 36 km — angular resolution of telescope of that size. EHT (Event Horizon Telescope): global network, max baseline ~Earth diameter — imaged supermassive BH. Radio interferometry produces highest-resolution images in astronomy.
- Resolutionλ/B (B = baseline length)
- VLA (New Mexico)27 antennas; max baseline 36 km
- VLBIContinental baselines; even higher resolution
- EHTEarth-sized baselines; ~20 µas resolution
- First major imagerCambridge One-Mile Telescope (Ryle)
- Modern facilitiesVLA, ALMA, MeerKAT, SKA (future)
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Why radio interferometry matters
- Highest resolution. Best angular resolution available.
- Black hole imaging. EHT imaged Sgr A* and M87*.
- Discovery science. Many breakthroughs.
- Cosmology. High-z observations.
- Galactic mapping. Detailed structure.
- Future SKA. Revolutionary radio astronomy.
- Multi-wavelength. Combined with optical/IR for full picture.
Common misconceptions
- Single big telescope better. Interferometer simulates that.
- Interferometers see TV-like images. Mathematical reconstruction.
- Radio images low-resolution. Best resolution in astronomy.
- VLBI is one telescope. Multiple connected by data only.
- Interferometry only for radio. Optical/IR interferometry exists too (CHARA).
- Connect more antennas, see more. More baselines = better u-v coverage; quality image.
Frequently asked questions
How does interferometry work?
Multiple antennas observe same source. Light from source arrives slightly different at each antenna due to baseline geometry. Combine signals (correlate); resulting interference pattern depends on source structure. Reconstruction gives high-resolution image. Equivalent to single telescope of diameter = max baseline.
What's resolution?
Angular resolution θ = λ/D (where D = telescope diameter or baseline). For radio (λ = 1 cm) and baseline = 36 km (VLA): θ = 0.06 arcsec. For EHT (λ = 1.3 mm, B = Earth diameter): θ ~ 20 µas. Best angular resolution in astronomy.
What's VLBI?
Very Long Baseline Interferometry. Antennas separated by hundreds to thousands of km. No physical connection — signals recorded with atomic clocks, correlated later. Achieves very high resolution. Examples: VLBA, EVN, Global VLBI. Continental and intercontinental baselines.
What's the EHT?
Event Horizon Telescope. Global VLBI network observing at 1.3 mm. Earth-sized baselines. Resolution sufficient to image black hole event horizons. Imaged M87* (2019) and Sgr A* (2022). Combines telescopes from Hawaii, Mexico, Spain, Antarctica, Chile, etc. Synchronized observations.
How are images reconstructed?
Each baseline measures one Fourier component of source. Multiple baselines + Earth's rotation → many components. Inverse Fourier transform → image. Algorithms: CLEAN (deconvolution), MEM (maximum entropy), Bayesian methods. Iterative: model, fit, refine.
What are major facilities?
VLA (New Mexico): 27 antennas. ALMA (Chile): 66 antennas, mm/sub-mm. MeerKAT (South Africa): precursor to SKA. NOEMA (France): mm wavelengths. VLBA: 10-station VLBI. SKA (Australia, South Africa, future): largest radio telescope ever — major astronomy frontier.
What are main applications?
(1) Black hole imaging (EHT). (2) Galactic structure mapping. (3) Cosmology — high-z galaxies. (4) Pulsar studies (VLBI). (5) Astrometry — precise position measurements. (6) Gravitational lens imaging. (7) Solar system studies. (8) Star formation. Many discoveries enabled by interferometry.