### SYNCHROBALLISTIC PHOTOGRAPHY

#### Andrew Davidhazy Rochester Institute of TechnologyImaging and Photographic Technology Department School of Photographic Arts and Sciences

Synchroballistic, photofinish, panoramic, peripheral, and strike recording camera systems all share a common bond in that they are variations on a theme dependent on imaging systems within which the subject images and the recording medium move past a stationary slit "shutter". The system in general is sometimes referred to a "strip" photography.

In conventional photographs of high speed projectiles in flight one constantly struggles to reduce the exposure time to a low enough value which will yield images of acceptable sharpness. Since the resolution of fine detail in a subject is a function of the relationship between image motion with respect to the film and the exposure time, large image velocities with respect to the film call for correspondingly short exposure times given the desire to resolve a particular detail in the object.

One way to minimize image motion is to "pan" the camera with the moving object. This allows longer exposure times than if the subject simply moved at right angles accross the field of view of the stationary camera. Yet another method is to move the film slightly in the direction of image motion during the time that the subject is photographed. Both of these methods generally depend on accurate synchronization between the exposure and the presence of the event within the field of view of the camera.

Synchroballistic photography overcomes both the problem of synchronization and exposure time by making the image of the object pass over an open slit while at the same time the recording medium passes below the slit at approximately (or exactly if possible) speed as the image. Since in this manner the image does not move with respect to the film a sharp record can be secured even at relatively long exposure times, these being a function of the width of the slit and the rate of film movement.

As in other applications, in synchroballistic photography it is also useful to know the rate at which an object will be moving at right angles to the optical axis. Then, the Film Velocity which must be set into the camera is simply the Object Velocity divided by the Optical Reduction of the camera system. Exposure Time will be the Slit Width divided by the Film Velocity.

This allows one to calculate the required F# to be used given a particular film and lighting combination.

Or, a test can be simply run at some apparently logical Film Velocity. Upon analyzing the image the appropriate correction to the previous speed can be made by examining the record for length vs. widthwise distortion compared to the ratio of the subject's length vs. width. If the record appears stretched out, let's say by a factor of about 2 times vs. the original's proportions then the film velocity needs to be reduced by a factor of two. Conversely, if the record appears compressed, this is an indication that the film was moving too slowly and its velocity needs to be increased for the next run. By the way, the film must move within the camera in the opposite direction than that in which the subject moves so that the _image_ of the subject and the _film_ will move in the _same_ direction.

Regardless of the apparent distortion, however, a synchroballistic record is very often an excellent means for determining the velocity, attitude and rotation rate of a projectile. For this to be possible some precautions need to be taken in terms of the placement of markings of known dimensions and orientation on the test missile.

For example, the average velocity of a missile over a distance equal to its length can be computed if the Film Velocity is known. It is equal to the length of the real missile divided by the value of the length of its image divided by the Film Velocity. The units for the missile length may be different than the units for image length and film velocity but these two must be in the same units of length.

By taking the trouble of incorporating a straight line extending from the missile tip to its tail, rotation rate can also often be determined. This is done by analyzing the image in terms of small segments extending from tip to tail and noting the position of the line with respect to the edges of the missile body. If the line invariably appears centered between the edges and if the original line was in fact straight, then the assumption which can be made is that there was no rotation as the missile's image moved from tip to tail accross the slit of the strip camera. However, if the mark appears to move from one edge to the other as one examines the record from tip to tail, then rotation took place. The rate is simply a function of the number of degrees of displacement which would cause the noticed movement of the line from one edge to the other divided by the number which results from dividing the distance over which the movement is measured by the film velocity.

Further, adding lines at right angles to the above line, changes in the attitude of the missile can also be estimated by noting the degree to which these line are no longer paralell to each other and the distance ( or time ) over which this effect is measured. In addition, vibration of the missile can sometimes also be detected and measured. The fact that the synchroballistic record not only delivers a sharp record without major synchronization problems is of itself noteworthy. That it can also deliver so much additional quantitative information is probably simply amazing.

You are invited to make your own synchroballistic photograph by simply attaching the aluminum film grabber clamp onto the white tab extending from the side of the Polaroid camera. Use the allen wrench to make a firm connection. Turn the power to the camera _ON_ briefly to take up the slack in the steel cable then turn camera _OFF_. Now, turn the missile sled power pack _ON_ if the sled is not already running. Wait for it move as far from the camera as possible. Turn the power to the camera _ON_ once more and wait for the white tab to completely exit the side of the camera. Turn the power to the camera _OFF_ at this time. Pull out the dark film processing tab from the camera side trying not to disturb the alignment of the camera. Wait 40 seconds and then peel the print from the negative. If you took the trouble to time the length of time it took the film to go through the camera you can figure out the Film Velocity since one exposure is equal to about 4 inches. Assuming that it took 10 seconds from the start to the finish of the exposure your film moved at 4 inches divided by 10 seconds or .4 inches per second. If the real rocket measures 8 inches and its record measures let's say 2 inches, then the rocket traveled a distance equal to its real length or 8 inches in 2 inches divided by .4 inches/second or 5 seconds for a "missile speed of 1.6 inches per second".

If you have any questions about this demonstration or synchroballistic photography in general contact me, Prof. Andrew Davidhazy, Rochester Institute of Technology, Imaging and Photographic Technology, P.O. Box 9887, Rochester, NY 14623 (716)475-2592