 The projection dome was separate from the outer dome , and was 23 meters ( 75 @.@ 4 ft ) in diameter . Made of curved aluminum sheets , it was lap jointed to create a continuous spherical surface . The sheets were painted white and perforated with 2 @.@ 5 @-@ millimeter ( 0 @.@ 1 inch ) holes , which were designed to let sound through and reduced echoes in the cavernous space . 
