The caption to go with it:
WORKMEN ASSEMBLE GIANT PALOMAR MOUNTAIN TELESCOPE
PALOMAR MOUNTAIN, CALIF. - The sky end of the huge telescope tube as it appeared in upright position in the yoke assembly in the observatory atop Palomar Mountain in Southern California recently. The workman standing beside the telescope tube gives an impression of the size of the unit, installation of which was completed April 11. The 200-inch mirror for the telescope, which is now being polished at California Institute of Technology is expected to be completed in the latter part of 1940.
Of course the mirror wasn't finished by the end of 1940. World War II eventually interrupted work on the glass, which wasn't finished at Caltech until October 1947. When this photo was taken the upper part of the telescope, which houses prime focus, had not yet been installed.