Astronaut Don Pettit shows how to take long-exposure photos from the ISS

NASA astronaut Don Pettit created his own device to help him take photos of the stars while on the International Space Station — and the results are pretty impressive. In a Reddit thread spotted by Space.com, Pettit describes how he brought a homemade star tracker with him to space, allowing his camera to capture long-exposure photos without the stars leaving any trails behind.

Star trackers are designed to rotate with the Earth — or in Pettit’s case, the ISS — to prevent distortion when taking pictures of the night sky. One of Pettit’s photos, which you can see above, was a 15-second time exposure. He says his tracker completes a rotation every 90 minutes to match the ISS’s pitch rate. “Without this tracker, you can not take photo[s] longer than 1/2 sec without star blur due to the rate of orbital motion,” Pettit writes.

In a separate post, Pettit notes that aligning the tracker on a moving platform isn’t an easy task, adding that he can currently take up to 30-second exposures “without significant star motion.” Taking photos through the ISS’s windows also presents another challenge. “Looking through 4 panes of glass, two of which are 30mm thick, at an angle makes for some distortion and relative optically induced star motion,” Pettit says.