# Building A Larger Array

The Schwarzschild radius for Sgr A* is 10 microarcseconds, an exceedingly small size even by astronomical standards. EHT observations to date have achieved an a resolution of better than 60 microarcseconds -- about the angular size of an orange on the moon. We are working to include other millimeter telescopes into the EHT array in order to improve the resolution of the EHT and eventually be able to produce images of the black holes in Sgr A*, M87, and other sources.

Resolution: The angular resolution of a baseline is given by λ/B, where B is the projected separation between the antennas. Finer angular resolution can be obtained by observing at shorter wavelength and by increasing the distance between telescopes.

We have obtained detections to date on baselines between Hawaii (SMA + JCMT + CSO), California (CARMA), and Arizona (SMT), with an angular resolution corresponding to 6 Schwarzschild radii for Sgr A*. ALMA will double this angular resolution. Future EHT observations may be able to obtain a resolution as fine as 1.5 Schwarzschild radii.

 Baseline Resolution at 230 GHz Resolution at 345 GHz CARMA - SMT 300 μas 200 μas Hawaii - SMT 58 μas 39 μas Hawaii - ALMA 28 μas 19 μas Plateau de Bure - South Pole 23 μas 15 μas
Table showing the resolution achievable on Sgr A* with current and future EHT baselines.

Image fidelity: As more telescopes are added to the EHT, we will be able to produce images of the emission around black holes. In general, the fidelity of images produced by an interferometric array increases as additional telescopes are added to the array.

Radio astronomers use the term "uv coverage" to refer to the projected baseline lengths and orientations for which data are obtained. The east and north projection of each baseline as measured in units of the observing wavelength are referred to as "u" and "v", respectively. As the Earth rotates, the projection of each baseline in the plane normal to the direction to the source changes such that each baseline sweeps out an arc in the uv plane. Each location in the uv plane corresponds to one Fourier component of the image on the sky. The ability to reconstruct the sky image improves with increasing uv coverage.

The improving uv coverage as telescopes are added EHT array.