Surface Measurement of a Large Inflatable Reflector in Cryogenic Vacuum

Abstract

The metrology of membrane structures, especially inflatable, curved, optical surfaces, remains challenging. Internal pressure, mechanical membrane properties, and circumferential boundary conditions imbue highly dynamic slopes to the final optic surface. Here, we present our method and experimental results for measuring a 1 m inflatable reflector’s shape response to dynamic perturbations in a thermal vacuum chamber. Our method uses phase-measuring deflectometry to track shape change in response to pressure change, thermal gradient, and controlled puncture. We use an initial measurement as a virtual null reference, allowing us to compare 500 mm of measurable aperture of the concave f/2, 1-meter diameter inflatable optic. We built a custom deflectometer that attaches to the TVAC window to make full use of its clear aperture, with kinematic references behind the test article for calibration. Our method produces 500 × 500 pixel resolution 3D surface maps with a repeatability of 150 nm RMS within a cryogenic vacuum environment (T = 140 K, P = 0.11 Pa).

Research and Application on High Precision Measurement Method of Large Mesh Antenna Surface

“…These corresponding marker points in the multiple 2-D images furtherly are identified through image processing. On this basis, the 3-D coordinate values relative to a space coordinate system are calculated according to the known intrinsic and extrinsic parameters of the camera [13][14][15][16] . As depicted in Figure 2, the bundle adjustment is employed for similar three-dimensional reconstruction in the image processing stage, and then the proportion factor is calculated by using the length of the reference bar as a basis, obtaining absolute three-dimensional reconstruction results for all marker points on the antenna reflector surface.…”

Deflectometry for specular surfaces: an overview

“…Some DM applications tolerate a relatively low-quality setup calibration, which can be performed with the simplest means. This is often the case when measuring small differences with respect to a reference object [e.g., surface deformations (Li W. et al, 2014;Kim et al, 2021;Quach et al, 2022b;Esparza et al, 2022)], or even shearing measurements followed by a reconstruction of a quasi-flat surface (Kewei et al, 2016). However, in general the calibration uncertainty impacts the surface reconstruction quality in a non-trivial way.…”

Deflectometry for specular surfaces: an overview

“…Some DM applications tolerate a relatively low-quality setup calibration, which can be performed with the simplest means. This is often the case when measuring small differences with respect to a reference object (e.g., surface deformations [198,250,251]), or even shearing measurements followed by a reconstruction of a quasi-flat surface [252]. However, in general the calibration uncertainty impacts the surface reconstruction quality in a non-trivial way.…”