Setup

Our setup, as shown in the diagram below, was arranged with an air gun, a light bulb, and a BB trap in a straight line.  The distance from the gun to the trap was about 3 meters with the bulb placed halfway in between.  Perpendicular to this and across from the light bulb, we set up our rotating mirror table along with our computer and flash.  The mirror was approximately half a meter from the light bulb.  Secured above the rotating mirror by three ring stands and metal rods was a laser.  The ring stands were secured to the table by C-clamps.

In front of the mirror (as close as we could get it), we set up our camera: the Nikon D1.  On top of the camera, we mounted a flash which had a phototransistor taped to the top of it.  We then connected the camera to a TV monitor so that we could more easily see our pictures as we took them.  A black backdrop was hung behind the bulb and the mirror.  In the diagram, the Gameport Interface Box is described as the “Yellow Box”.

 

Triggering, Timing, and Imaging

In one of the most complex triggering systems yet seen in High Speed Imaging this year, we attached a flash onto the top of the Nikon D1.  On top of this flash we attached an extender and on top of the extender there was a breadboard.  This breadboard had a single photoreceptor that announced to the computer that the flash has gone off and triggers “PB 0” (Push Button 0).  This activated PB0 which allowed for the signal from PB1 to be received.  PB1 was connected to a laser pointer that was making continuous contact with the photocell at the bottom of the rotating mirror.  The pushbutton was not activated until the screw on the side of the rotating mirror broke the beam following the signal from PB0⁵.

            The Crosman 2100 had a solenoid attached to the trigger that was pulled when two situations were satisfied:

1.      A “red button” safety trigger had to be depressed by an experimenter in order for the gun not to go off accidentally.

2.      The computer had to send a signal, via annunciator 0 to the optoisolator, giving the solenoid enough power to trigger the gun. 

This signal was given when both PB0 and PB1 had been activated.

            Once the gun had been fired, the computer waited for a set period of time before activating annunciator 2.  Annunciator 2 led back to the Nikon D1, where it opened the shutter at the precise moment that the bullet reached the bulb and started the shattering of the airtight environment.  All of the computer work was done using Ball and Mirror 2004B and the Gameport Interface box.  The Gameport Interface Box was a yellow box that connected to the Gameport interface on the Apple II.  This was the box that contained the Push Button inputs and the Annunciator outputs that the computer controls through binary programming.

            In order to learn how long the flashes should be delayed and for the camera shutter to remain open, we used a program called RBallTimer 2004B.  We knew that the “bang” that the gun makes would occur when the BB was exiting the gun because it was then that there was no longer a force acting on the BB (except gravity), which allowed the BB to start slowing down.  This created the “bang”, very similar to a sonic boom heard by Concord airliners.  Armed with this knowledge, we used a sound trigger to send a signal to the computer which had been, via the program RBallTimer 2004B, measuring the time since it had sent the signal to trigger the gun to the solenoid and stopping its measurement when it receives the signal from a sound trigger.  We performed this sequence three times (triggering the gun which triggers the sound trigger and recording the time between) and taking the average we found the time the gun takes to “go off”.  In conjunction with the frequency of the rotating mirror – measured via a stroboscope – and the flight time of the BB – which we varied during our experiments – we were able to input timing and with some fine adjustments, shatter a lit bulb on film.

            Imaging was done using the rotating mirror method.  We set up a rotating mirror with the Nikon D1 perpendicular to the subject, aimed the camera at the mirror, and found the effective angle (i.e. the angle where the camera, aimed at the mirror, is able to see the bulb) of the mirror.  Using this effective angle, we were able to aim the camera and trigger its shutter in such a way that the camera was only exposed while the mirror was viewing the bulb to achieve a sweep photograph of the dimming of a light bulb after being hit by a BB.  Our rotating mirror setup consisted of a tape player motor on the inside of a wooden box.  The mirror was attached to the motor, and the motor was on its lowest frequency setting (as chosen by a gear on the underside of the box).  Using a stroboscope, we found this frequency to be 4.7 rotations per second.  The mirror itself was a front-coated mirror so that there was only one reflection instead of two (one coming from the actual mirror coating, the other from the glass on top of the mirror) making our picture easier to read and more true-to-life.
            We were using the rotating mirror method to achieve our sweep photos.  A sweep photo is a picture in which there is a prolonged exposure of a moving object on one piece of film, giving the impression that the photo is “sweeping” or moving across the film.  In our case, we used a rotating mirror to make the bulb appear to be moving, so as the filament burnt out, it would get dimmer progressively across the film.  This effectively allowed us to analyze the change in intensity of the bulb via the Photo Analysis method as a function of time.