Rise of the Tomb Raider: Find your way to the Observatory, walkthrough Rise of the Tomb Raider guide, walkthrough

To reach the observatory, you need to pass through a fragment of the geothermal valley. Jump over to the other side of the rift and walk across the brook. There, you find a camp.

Several steps past the camp, you should meet Jonah again. After the cutscene, the man gets abducted.