Inflatable membrane primary optics for space telescopes are a smart approach in the context of saving flight payload weight and volume. The Orbiting Astronomical Satellite for Investigating Stellar systems (OASIS) adopted the membrane architecture for primary optics (primary antenna, A1) to have 20 meter diameter collection area with operation bands at the terahertz frequency. The membrane is made of Kapton or Mylar film with an aluminized surface, and the balloon (transparent surface + aluminized surface) is inflated to work as the convex mirror. In order to leverage the carrying volume advantage of inflatable optics, it must be folded during launch and deployed in orbit. The thin membrane film can crumple easily when it is folded, and it should be ironed out when the telescope is deployed for observation. We studied the microroughness and mid-to-high spatial frequency characteristics of the membrane via optical metrology to evaluate the surface properties. Because it is not of traditional shape and material, it is impossible to test with an off-the-shelf interferometer and profilometer. Moreover, the defect spatial frequency of interest is a few hundred microns to millimeters range, so the measurable field and dynamic range need to be in range of a few centimeters with microns resolution. To meet those requirements for metrology, we developed a flexible optics testbed utilizing deflectometry. The microroughness and mid-to-high frequency properties are measured with a white light interferometer and proposed methodology. The test results show that the candidate membrane is suitable for OASIS and this reliable test will guide the further design study of A1 assembly and optical system error budget.