For this shot, taken at night when the kids were tucked up in bed and it was dark outside, I suspended the pendulum in front of our living room windows, mounted the camera on a tripod and placed it at an angle to the window, to keep the flash from reflecting off the window.

The camera was set to a 2 second exposure, to capture the movement, an aperture of f20 to ensure focus and also to ensure the light from the 2nd flash wouldn't overpower the image and the in camera flash set to rear curtain sync. To capture the various images of the pendulum, I manually triggered the 2nd flash unit (set on automatic exposure, but with it assuming an f4.0 aperture) repeatedly during the exposure.

The theory behind the setup was that the main flash on the camera body was set to expose correctly for the pendulum at the end of the shot and the less powerful strobes from the 2ndary flash would provide additional "ghosting" snapshots of the pendulum as it oscillated. The orange/yellow in the crystal was from the ambient tungsten lighting in the room.