Finding secret spacecraft is what Felix Schöfbänker does.

In June, the Austrian amateur astronomer spied a secret Chinese spaceplane. Now, Mr. Schöfbänker has spotted a mysterious U.S. military satellite.

Using a 14-inch Dobsonian telescope optimized for tracking satellites, Mr. Schöfbänker compared the images he collected with data from various spy satellites launched by the Pentagon, he said in an interview with Space.com.

While most of his recent finds have been known crafts deployed by the National Reconnaissance Office — including Boeing’s Future Image Architecture radar satellites and Keyhole, or KH-11, orbiters — one satellite stood out, he said.

“In July, I managed to get a look at a satellite called ‘USA 290,’ which is suspected of possibly being another KH-11,” Mr. Schöfbänker told Space.com. “But my image shows a different kind of design, which doesn’t look like a typical KH-11.”

In Mr. Schöfbänker’s image of the mysterious USA 290, a strange, 16-foot-long panel is visible, a feature not seen on KH-11 satellites. “It could be a solar panel, although I think it’s less likely due to the fact that the panel seems to be fixed on the rest of the structure,” he said.

Adding to the satellite’s mystery, USA 290’s movements deviated from the norm. “KH-11s are launched into sun-synchronous orbits,” Mr. Schöfbänker said. “USA 290 wasn’t.”

Despite capturing images of potentially classified material for the second time this summer, Mr. Schöfbänker is unconcerned about any repercussions: “I don’t think that most countries would be too concerned about amateurs like me imaging their spacecraft. Most big countries have their own observatories dedicated to this kind of imaging.”