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Method 2: Shooting with a Gun

 

Equipment used: 

Equipment Setup:

Description: We filled the tank 3/4 to 4/5 full of tap water.  We then tied a partially air-filled balloon to a couple weights and placed the weights in the bottom of the tank.  The flash unit was placed on one side of the tank and we inserted a variable resistor in place of the auto-thyristor.  We dialed this variable resistor to the lowest resistance to get the shortest possible flash duration.  We wanted the smallest possible flash duration because the popping of a balloon underwater is an extremely high speed phenomenon and would blur with greater flash durations.  Opposite of the flash unit, on the other side of the tank, a mirror was placed to reflect light from the flash and increase the visibility of the balloon while it was popping.  This flash we hooked to the output of the parallel-port interface on the Toshiba 1600 to enable ourselves to make extremely fine adjustments in time delay, necessary due to the fast and precise nature of the balloon burst after the gun it shot.  The Toshiba 1600 connected through the parallel-port interface to a microphone which would pick up the sound of the gun, RWS Diana, shooting, initiating the time delay.  The gun was mounted above the tank so it would shoot directly down into the balloon.   A protective sheet of plastic was placed over the tank so the splash from the superpoint pellet and the balloon bursting would not enter the gun's barrel.  Another protective sheet was placed in the bottom of the tank to prevent the superpoint pellets from cracking the tank.  The camera was placed approximately one meter away from the subject, the balloon, and stood on a tripod. 

To capture a burst we would synchronize opening the shutter with shooting the gun.  The camera was set on bulb through the entire experiment, so the flash alone provided the light for the image.  We selected bulb shutter speed because it allowed manual control of whether the shutter was opened or closed, and after the first set of pictures we knew there was not too much thermal noise due to the length the shutter remained open.  By using the camera on bulb shutter speed we could be positive that the shutter was open to capture the image.  When the gun shot, the vibrations in the air were picked up by the microphone, which transferred the signal to our interface box.  From the interface box the signal entered the Toshiba 1600, which then initiated a time delay averaging at 11 to 12 milliseconds.  After the time delay was complete, the computer output a signal to the interface box which was sent to the flash unit, causing the flash unit to discharge.  For all our photographs, the flash unit had a variable resistor inserted in place of its auto-thyristor and dialed to the least resistance possible so that the flash length was always the same and always as short as possible.  We chose these settings because the burst was a very high speed phenomenon that would be blurred with a longer flash duration, and we wanted sharp, precise photos.  

 

Diagram: Side View, from behind the camera

Diagram: Top down view

 

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