You're looking at the side of a sail boom marked with white epoxy ink to provide extra contrast that will be (relatively) easy to spot with the onboard cameras. I say "relatively" because our cameras have fairly low resolution, and there are a lot of extreme colors in our images: the blackness of space, the silver sail, the gold booms, and possibly, the Earth or sun.

The five tick marks don't signify anything in particular—in total, they're about an inch wide, lengthwise, on the booms. Stephanie Wong of Ecliptic tells me the team chose the tick mark design over a single, inch-long ink block to reduce the likelihood the ink would peel off in one big chunk after being wound so tightly inside the spacecraft.

The team added six fiducials in all. There are two fish-eye cameras that sit on opposite sides of the spacecraft. When the sail is deployed, each camera sees one boom more or less in the center of the frame, with one or more adjacent booms highly distorted near the edge of the image. That means our best chance of seeing any faraway fiducials comes only on the two booms that align with the cameras.

So, every boom has a fiducial with a Planetary logo that completely shows when the booms are fully deployed. And on the two booms that align with the cameras, there's a second fiducial at the halfway mark, which looks like this: