---
title: Next-Generation Event Horizon Telescope
url: https://www.emergentmind.com/topics/next-generation-event-horizon-telescope-ngeht
type: topic
---

# Next-Generation Event Horizon Telescope

The Next-Generation Event Horizon Telescope (ngEHT) is a planned upgrade to the original Event Horizon Telescope (EHT) array, designed to overcome the EHT’s limitations in sensitivity, angular resolution, and dynamic range. The ngEHT will enable horizon-scale imaging and polarimetry of a significantly larger population of supermassive black holes (SMBHs), systematically test strong-field general relativity, and uniquely address the structure and dynamics of SMBH accretion, jet launching, and multi-messenger astrophysics.

## 1. Array Architecture and Global Coverage

The ngEHT will extend the existing EHT by adding 6–10 new high-frequency dishes at strategically selected geographical locations, including NOEMA (Europe), the Greenland Telescope, a site in southern Africa, southern hemisphere (Chile/Argentina), the US Southwest, and potentially Australia or Antarctica [2406.17754]. These additional stations will:

- Fill critical north–south and east–west gaps in the $uv$-plane.
- Increase the maximum terrestrial baseline from $\sim10{,}000$ km (EHT) to $\sim12{,}000$ km, corresponding to a maximum spatial frequency of $\sim12$ G$\lambda$ at 345 GHz.
- Densify intermediate $uv$-coverages (2–6 G$\lambda$ at 230 GHz) by $\approx30$–$50\%$, reducing imaging artifacts and boosting dynamic range.

Three-frequency operation is central, with simultaneous or dual observations at 230 GHz ($\lambda\approx1.3$ mm) and 345 GHz ($\lambda\approx0.87$ mm), and in some designs incorporating an 86 GHz/85 GHz band for robust frequency phase transfer (FPT) and source-frequency phase-referencing (SFPR) [2212.08994, 2302.11776].

## 2. Sensitivity, Resolution, and Bandwidth Upgrades

ngEHT enhancements target the physical limitations of the original EHT in sensitivity and angular resolution by:

- Quadrupling the aggregate observing bandwidth from 4 GHz/pol (EHT) to up to 16 GHz/pol (ngEHT), with multi-band total instantaneous bandwidth often quoted as 16 GHz [2406.17754, 2304.11188].
- Upgrading receivers for lower system noise temperatures, leading to station SEFDs reduced by a factor of $\sim2$ (e.g., SEFD $\sim$1500 Jy at 230 GHz) [2406.17754].
- Achieving theoretical imaging resolutions of
  - $\theta_{230} \simeq \frac{1.3\,\mathrm{mm}}{1.2\times10^7\,\mathrm{m}} \simeq 20\,\mu\mathrm{as}$ (ngEHT)
  - $\theta_{345} \simeq \frac{0.87\,\mathrm{mm}}{1.2\times10^7\,\mathrm{m}} \simeq 15\,\mu\mathrm{as}$
- Reaching baseline thermal noise (per 60 s) of $\sigma_{230} \sim 8$ mJy and $\sigma_{345} \sim 16$ mJy with the nominal SEFDs and $\Delta\nu=16$ GHz [2406.17754].
- Imaging dynamic range improvements from EHT’s $\sim100\!:\!1$ to $\gtrsim500\!:\!1$ (static imaging) and $>1000\!:\!1$ in continuous/dynamical campaigns [2406.17754, 2212.11355].
- Table: Comparative ngEHT Capabilities

| Capability                | Current EHT (230 GHz) | ngEHT (230 GHz) | ngEHT (345 GHz) | Ground+Space (BHEX) |
|---------------------------|:---------------------:|:---------------:|:---------------:|:-------------------:|
| Max Baseline (km)         | $\sim10{,}000$        | $\sim12{,}000$  | $\sim12{,}000$  | $\sim50{,}000$      |
| Resolution $\theta$ ($\mu$as) | 20              | 18              | 13              | 5                   |
| Bandwidth $\Delta\nu$ (GHz)   | 4               | 16              | 16              | 16                  |
| SEFD (Jy)                 | 3000                  | 1500            | 3000            | 1500                |
| Baseline σ (mJy, 60 s)    | $\sim40$              | $\sim8$         | $\sim16$        | $\lesssim8$         |
| Dynamic Range             | $\sim100\!:\!1$       | $\gtrsim500\!:\!1$ | $\gtrsim500\!:\!1$ | $\gtrsim1000\!:\!1$ |
| Accessible sources        | 2 (M87*, Sgr A*)      | $\sim12$        | $\sim12$        | $\gtrsim50$         |

Observations at 345 GHz are crucial for lengthening baselines, surpassing scintillation limits (especially for Sgr A*), and achieving beam sizes necessary for super-resolution and photon-ring analyses [2407.09750, 2210.13498].

## 3. Calibration, Phase Transfer, and Astrometry

The ngEHT incorporates advanced calibration methods leveraging simultaneous multi-band observations:

- Frequency Phase Transfer (FPT) and Source-Frequency Phase Referencing (SFPR) exploit the linearity of tropospheric phase fluctuations to transfer solutions from lower (e.g., 85/110 GHz) to higher (230–345 GHz) bands, extending achievable coherence times by $10^2$–$10^3\times$ (from $\sim10$ s at 230 GHz to $10^3$–$10^4$ s) [2212.08994, 2302.11776].
- The detection threshold at high frequencies is set by SNR at the reference (lower) band, allowing routine detection of sources down to $\sim$10 mJy at 255–345 GHz.
- SFPR permits direct measurement of frequency-dependent core shifts with astrometric precision $\sigma_\theta \sim 3\,\mu\mathrm{as}$, enabling registration of structures across frequency bands and micro-arcsecond-level astrometry for Sgr A*, M87*, SMBH binaries, and cluster galaxies.
- Receiver systems must enable simultaneous or near-simultaneous operation at two or more frequencies; tri-band receivers (e.g., 85/230/345 GHz) with integer frequency ratios are strongly preferred to avoid phase ambiguity [2302.11776].

## 4. Science Case Expansion: Population Studies and New Targets

The ngEHT will transition from imaging only Sgr A* and M87* to direct horizon-scale studies of $\mathcal{O}(10^1$–$10^2)$ SMBHs. Quantitative projections [2212.00779, 2406.17754]:

- Under $\nu=230$ GHz and April conditions, a Phase 1 ngEHT can conduct:
  - Mass (ring-diameter) measurements for $\sim$50 SMBHs
  - Spin constraints for $\sim$30 SMBHs (polarization proxies)
  - Shadow detections (clear ring interior) for $\sim$7 SMBHs
- For each target, the ability to recover the photon-ring radius and mass is governed by the fundamental formula:
  $$
  \theta_\mathrm{ring} = 2\sqrt{27}\,\frac{G M}{c^2 D} \approx 9\,\mu\mathrm{as}\ \Bigl(\frac{M}{10^9\,M_\odot}\Bigr)\ \Bigl(\frac{D}{10\,\mathrm{Mpc}}\Bigr)^{-1}
  $$
- Many new targets have $F_{230} \sim 0.03$–$0.2$ Jy and angular diameters $\gtrsim10\,\mu$as, within ngEHT’s sensitivity and resolution.
- Higher Eddington ratios for these new sources may yield larger optical and Faraday depths (complicating polarimetric spin measurements) [2406.17754].
- Accessing masses and spins with high accuracy depends on independent inclination constraints and robust modeling that includes possible jet contributions.

## 5. Polarimetry, Photon Ring Detection, and Dynamic Imaging

ngEHT will enable a new generation of horizon-scale polarimetric science and time-domain imaging:

- Polarimetric detection of the $n=1$ photon ring in Sgr A* at 345 GHz is feasible using a robust circular-polarization ratio observable,
  $$
  R(u,v) \equiv \frac{V_R(u,v)}{V_L(u,v)}
  $$
  where $V_R$, $V_L$ are the right- and left-circular polarized visibilities. A phase reversal in $R$ at baselines $\gtrsim4$ G$\lambda$ signals the photon ring; detection significance is quantified by the metric
  $$
  \mathrm{SNR}_\mathrm{PR} =
  \frac{\operatorname{Re}\bigl[ \langle R\rangle_\mathrm{long} - \langle R\rangle_\mathrm{short} \bigr]}
       {\sigma_\mathrm{noise}}
  $$
  Simulations predict $\mathrm{SNR}_\mathrm{PR} \sim 2$–3 for a week-long campaign at nominal Phase 1 sensitivity [2407.09750].
- For static and dynamic imaging, the quadrupled instantaneous bandwidth and increased station count deliver dynamic range (DR) enhancements from $\sim100$ to $\gtrsim1000$, critical to optically resolving central brightness depressions (shadows) and tracing transient/faint emission (e.g., hot spots, jet knots).
- The ngEHT Analysis Challenges demonstrate that with $\sim$10 new antennas and 4$\times$ bandwidth, event-horizon-scale “movies” of M87* jet dynamics and Sgr A* variability can be robustly reconstructed [2212.11355].

## 6. Imaging Algorithms, Spectral Reconstruction, and Parameter Estimation

ngEHT advances in both data acquisition and analysis methodology:

- Regularized Maximum Likelihood (RML) and Bayesian imaging pipelines using frequency-joint and polarization-joint constraints outperform independent single-frequency reconstructions. Simultaneous RML multi-frequency imaging naturally recovers both intensity and spectral index ($\alpha$) maps:
  $$
  \ln I_i(x,y) = \ln I_0(x,y) + \alpha(x,y)\,\ln\frac{\nu_i}{\nu_0} + \beta(x,y) \Bigl(\ln\frac{\nu_i}{\nu_0}\Bigr)^2
  $$
  (typically, just the linear term, $\alpha$, is sufficient in ngEHT bands) [2210.12226].
- These methods improve superresolution, spectral mapping, and propagation of $uv$ coverage between frequencies, yielding NRMSE and cross-correlation metrics superior to band-independent methods.
- For parameter estimation, the Fisher-matrix approach yields error estimates on ring diameter, polarization modes, and shadow width, with statistical confidence thresholds of $<20\%$ fractional error for spin proxies.
- Imaging capabilities are extended to fainter, more extended structures (e.g., detecting jet sheath, recollimation nodes, or outflows) by leveraging high dynamic range and expanded $uv$-coverage.

## 7. Instrumental Challenges, Limitations, and Future Outlook

Despite significant capability gains, key limitations and challenges persist:

- High-frequency operations (especially 345 GHz) are weather-limited, requiring excellent atmospheric conditions (low PWV), and the small dish apertures at new sites necessitate wide bandwidth and long integration for deep sensitivity [2304.11188].
- Robust photon-ring detection in realistic data is complicated by blending with direct image features and possible model degeneracies; hybrid imaging methods can yield false positives if not coupled rigorously to physical models [2210.13498].
- Faraday rotation and optical depth effects may compromise polarimetric mass/spin measurements for higher-Eddington sources.
- Full exploitation of dynamic imaging and time-domain science demands highly stable calibration, optimized scan strategies, and improved pipeline automation.
- The envisioned extension to space VLBI (Black Hole Explorer, BHEX) will further increase accessible angular resolution ($\sim 5\,\mu\mathrm{as}$) and sample size (tens more SMBHs).

A plausible implication is that, with sustained hardware, calibration, and algorithmic development, the ngEHT will shift SMBH astrophysics from single-object phenomenology to population-level tests of accretion physics and strong gravity, eventually encompassing both stellar-mass and supermassive regimes over a wide range of cosmic history.

Source: https://www.emergentmind.com/topics/next-generation-event-horizon-telescope-ngeht