https://phys.org/news/2018-08-nasa-planet-hunting-tess-comet-science.html

In addition to the comet, the images reveal a treasure trove of other astronomical activity. The stars appear to shift between white and black as a result of image processing. The shift also highlights variable stars—which change brightness either as a result of pulsation, rapid rotation, or by eclipsing binary neighbors. Asteroids in our solar system appear as small white dots moving across the field of view. Towards the end of the video, one can see a faint broad arc of light moving across the middle section of the frame from left to right. This is stray light from Mars, which is located outside the frame. The images were taken when Mars was at its brightest near opposition, or its closest distance, to Earth.

These images were taken during a short period near the end of the mission's commissioning phase, prior to the start of science operations. The movie presents just a small fraction of TESS's active field of view. The team continues to fine-tune the spacecraft's performance as it searches for distant worlds.