Interferometric fringes from an observation of 3C 273 with the VLTI

An interferometric observation of the quasar 3C 273 with the VLTI. The columns marked UT1, UT2, and UT4 show the data from the individual 8.2-metre Unit Telescopes, which were then combined interferometrically. The resultant fringes are shown in the column marked VLTI. The bright feature is the Paschen-α emission line from atomic hydrogen gas, redshifted to a wavelength of 2.17 µm due to the quasar’s distance of about two billion light-years.

This image is a rare example of a five-second exposure that reveals the fringes. In real use, multiple much shorter 0.3-second observations must be made. It is thanks to the new “blind observation mode” that these shorter exposures can be successfully combined.