The final collimator tubes (40 in Station 52, and 20 in Station 53) were made of stainless steel tubing in accordance with sketches prepared by the User, The problem presented a number of requirements that had to be met: i. The finished structure had to be strong enough to preserve the information which was to be removed from it after the experi- ment, 2. Some adjustment had to be provided to align the structure as a whole after completion to correct for any unequal settle- ment in the foundations or any other misalignment during construction, 3. The. many collimator tubes had to be maintained true to alignment within 15 minutes of arc and 1/8 inch in position, It was decided that it would be impractical to cast the limonite concrete in place in wood forms at the site because it would be difficult if not impossible to set the collimator tubes to the close toler- ances required and to maintain this alignment during the pouring of the limonite concrete and it would be very difficult to provide sufficient reinforcement steel in the space alloted and still leave enough clearance to pour the concrete, It was decided that a complete unit including the basic collimator tubes should be shop fabricated to the close tolerances required and then shipped to the field ready for the installation of the limonite concrete and the final collimator tubes. The shell assembly containing the basic collimator tubes was designed to hold the tubes to true align- ment during shipment and during the placement of the limonite concrete and also to take the designated blast loads, Structural steel back-stay grillages were provided to prevent the blocks from rolling backward under blast, Vertical angular adjustment for the face of the block after installation of the concrete was provided by a set of opposing hand-cperated jacks set between the block and the back-stay grillages. Hand-operated jacks were also installed at the base of the blocks to provide for fore and aft and sideway adjustment. It was also necessary to design special lifting slings and connections to handle the heavy steel units between the fabricator's shop and the final positioning at the Jobsite. units weighed as much as 32 tons each, The The footing for Station 52 con- tained 219 cubic yards of concrete alone, and Station 53 had 78 cubic yards. Because the data furnished on March 3 was somewhat sketchy in places, it was necessary for a representative of NBS to come to the H&N office in Los Angeles on May 16, On May 16 the design data for Engebi was furnished, and on May 19 the data for Runit was provided, 5-164