This paper presents a vacuum support technique for a Fizeau type interferometer that uses a 1.1 m diameter reference surface. The interferometer uses Computer Generated Holograms (CGHs) for aspheric measurement, illumination optics and imaging optics. To reduce the effects of air turbulence and retrace error, the gap between the reference surface and Unit Under Test (UUT) is limited to 5 mm. The illumination optics consist of two lenses, one of which we call the test plate which includes the reference surface. The 1.1-m diameter illumination optics are quite heavy, 212 kg, and are prone to self-weight deflection problems. We solve this with a vacuum support of the test plate. A vacuum region is created between the two lenses in the illumination optics using a rubber seal. The upper lens of the illumination optics suffers extra deflection caused by vacuum. However, since this lens is in the common path of the interferometer this does not cause significant error in the interferogram. In this paper, a simple experiment on a small surface and analyses on a large model are discussed. In the simple experiment the deflection of flat surfaces having 100mm in diameter and 2.7mm in thickness was measured by a Wyko 6000 interferometer. The region between the two flat surfaces was sealed by an O-ring and a needle was inserted into the O-ring so that the sealed region could be evacuated. This experiment shows correspondence of less than 10 % between the simulation and the experiment. A finite element model simulation of the vacuum support for the 1.1-m test plate is also presented. The nominal surface slope irregularity caused by gravity at the reference surface is 26.88 nm/cm RMS. When a differential pressure is equal to 2300 Pa, the deflection is minimized to 0.75 nm/cm RMS. Based on an optical simulation of the system, this deflection introduces only 8.0 nm RMS into the measurement error. Therefore the vacuum support can be useful for reducing the effect of gravity on the reference surface.