eee a EEereats pieeSab Das2oe 5 oa Tesolutian through the entire optical system will be approximately 25 ‘to 30 lines per millimeter. ger The lenses are held in special fixed-focus mounts, rather than in the variable-focus mounts provided by the manufacturer. The special mount, shown in Figure 5, is required in view of the close tolerances of resolution and focus that must be met. The mount allows the insertion of shims, of various thicknesses, by means of which the distance from lens to film may be adjusted precisely, Lenses are focused by means of a collimator and resolution target, which are arranged with a light source and the camera as in Figure 6. The shim thickness is chosen to give maximum resolution over the area of the frame. The focal length is then determined by means of a beam-splitting prism, placed between the collimator and the lens. Meas- urement of the separation of the two resulting images, and of the deflection angle of the prism, gives a figure for the lens focal length that is good to better than +0.2 percent. Measurements made over periods of years have shown these focal lengths to remain constant, except for occasional physical damage, to about +0.1 percent. Lenses are always focused in this manner, rather than visually. It has been determined that the position of best photographic focus differs significantly from that for best visual focus. Facilities will be provided so that the U. N. Observers can check the focal lengths of the lenses at Parry Island. vided at the foot of the photo tower. A calibration range will be pro- A platform will hold the camera in posi- tion to photograph an array of photographic (chemical) flash bulbs.— The bulbs camera (1,000focal lengths), and will will be placed about 200 ft from the be activated to give a short burst of light while the camera is operating at rated speed. The Observers will measure the distance from camerato lights, and also the Spacing of the array; measurements of the photographic images will verify the AEC figures as to focal lengths. This check will be carried out for each of the four cameras to be used for U. N. purposes. Since the cameras do not operate at constant speed, means are pro- vided to impress time data upon the film while the detonation is being photographed. The timing marks are derived from a tuning fork and a high- voltage pulser, which cause a spark gap within the camera to discharge 200 times each second, The light from the discharge gives a series of dots - 15 - 7