How to add a camera to turtlebot?

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It really depends on what you want to do with your camera.

It's for example very likely that you will want to add your camera to the tf tree of your robot.

Why ? Because to use your camera data alongside other sensors you will need to position each of the sensors in one similar coordinate framework. (example: rtabmap_ros can build a 3D map of an environment using RGBD camera, Odometry and 2D laser data, but in order to combine this data, you have to position each of the sensors in a common framework: this framework is often completed with what you will find in the wiki articles, in the "Required tf Transforms" and "Provided tf Transforms" sections)

How to do that ?

You can for example broadcast a tf, or edit the urdf file of your robot. For the turtlebot 2, here are some information about how to find the urdf files.

On the other hand, if you plan on doing image processing using open CV, you don't need to do that to use your camera. If the camera is not already supported by a ROS Package (understand: not launched with a ROS node and its data seen through a ROS message topic being published), you'll need to connect it yourself with a new package to the ROS network.