What's a good calibration object for the extrinsic calibration (rotation + translation) of a depth camera (no color sensor, only depth)? With RGB, people seem to typically use chessboard pattern or some kind of markers. Is there something similar that would work without RGB?
You can use a 3d Marker, e.g. a cube or a another arrangement of three planes. Finding these planes in the Pointcloud/Depth Image is not hard and provides a 6D Pose (if you can distinguish the planes which can be done by using non-90-deg angles).
If you also add visual markers (e.g. checkerboard pattern) to the planes, you can first use a 2d Camera to measure the relative poses of the planes.