British Kinematography (1950)

November, 1950 Ross : new still process projector 167 No vertical adjustment is provided in the central column because it is our experience that precise positioning of the projector is unnecessary, particularly when using high-diffusion screens. This makes it possible to provide a really rigid lock to the pan movement, which is a very real advantage. Projector Base. The base of the projector is in the form of a platform on which the operator may stand. It is of heavy cruciform cast construction. The pairs of rubbertyred wheels at each corner may be steered in pairs, or locked off, and four jacks are provided. The details of the base were worked out to permit the projector to be carried in elevating towers available at the studios, and consequently guide rollers are provided along each side, and the wheels are made retractable. Lugs are provided at each corner to permit the projector to be' lifted by chain tackle in the studio roof, and the base is strong enough to withstand the effect of the weight being taken at only three of these lugs. All the main metalwork and the base are of light-alloy, and were made by Turner Brothers, Ltd., Birmingham. Location of Components. One of the features adopted in the design of this projector, which is rather unusual in such equipment, is the extensive use of kinematic principles for the location and mounting of the optical components, the lens mounts being designed by Mr. T. W. Clifford, of Taylor, Taylor & Hobson, Ltd. Briefly, kinematic design recognises that any part can theoretically move in three directions (along the projector, across and vertically) and can also rotate about three corresponding axes. If the part is to be prevented from moving in any one direction, one restraint or clamp, and only one, must be provided ; and for each direction in which it is to move, only one control shall be provided. Examples are location by a ball fitting into a conical hole, a three-legged stool, or a typewriter carriage running on two balls in V-shaped guides. These methods are usually harder to design than the more normal ones, but it is often easier in the finished machine to obtain the required accuracy of positioning when it is impractical to set up specialised machine-tools for construction. More important, perhaps, is that assembly is simple, and, therefore, maintenance is assisted because the components can be taken apart and easily put back in exactly the same position. Mounting of Lenses. Experience has shown that in an optical system of the type used here, it is necessary to keep all the lenses exactly on the optical axis ; if adjustments are provided it is difficult to know which one to alter to remedy any defect, and soon the system gets out of order. The only exceptions are the positions along the axis* of No. 3 lens, and of the objective lens mount — to accommodate the range of focal lengths — and the position of the arc crater with respect to No. 1 lens. All the lenses (in their circular mounts) drop on to locating surfaces on supports, and these are in turn located on the two machined surfaces seen in Fig. 7, running the whole length of the optical bed. The supports are constrained sideways and squared up, by being held against the vertical side of the centre rail visible in the figure ; the adjustable units are merely clamped against this, using the right-hand inclined side of the rail. The problem of accurately mounting an f/2 objective lens weighing 33 lbs., so that it is adjustable in three directions (focus, cross and vertical) and yet may be moved by small remote-control motors without any backlash, has been solved by Mr. Clifford, using a kinematic design.