Those streaks in that pic are the path of a star that UT passed. Given they know the velocity, if they time the period where the light from the star is occluded, they get a measure of how wide the object is at that elevation.

This is what an occultation looks like: https://www.youtube.com/watch?v=7Y_qc8Ifhso

Crazy they are able to map a few of those together so precisely to get this data.