| They've used the blazar J1924-2914 to calibrate[1] for Sagittarius A*. From the paper[2]: It can be seen from Table 1 and Figure 4 that the inner jet structure determined by 1.3 mm VLBI agrees well with the reported inner jet orientation at lower frequencies (e.g., Shen et al. 2002) and from the conclusions: For the first time, an AGN jet is spatially resolved with 1.3 mm VLBI with robust closure phase measurements. At pc scales, the inner jet direction is oriented toward the northwest (P.A.= −53◦) with respect to the assumed core, consistent with previous results. The paper from 2002 can be found here[3]. edit: Also do note that the EHT is kind of a "virtual" telescope. It's using existing radio telescopes[4] to collect data, and these telescopes have been built and used for other stuff and so are well characterized. [1]: https://en.wikipedia.org/wiki/Event_Horizon_Telescope#J1924-... [2]: https://arxiv.org/abs/1208.4402 [3]: https://arxiv.org/abs/astro-ph/0209234 [4]: https://eventhorizontelescope.org/array |