Optical tests of a 3.7-m-diameter liquid mirror: behavior under external perturbations.

We built and tested a 3.7-m-diameter liquid mirror that rotates on a ball bearing. Although the ball bearing is of a poor quality, the mirror is surprisingly good for one that comprises 1-mm-thick mercury layers. We found no evidence of the strong astigmatism that might have been expected from Coriolis forces. We did not detect effects of turbulence might or vibrations for thin mercury layers, illustrating the necessity of using thin layers: Large liquid mirrors would have had unacceptable optical qualities for layers much thicker than 2 mm.