Tape springs have many advantages to develop spaced-based deployable systems because of their simple structures, light
weights, high stiffness, and the abilities of self-deploy and self-lock. Their applications in deployable space structures
make the spaced-based systems fulfill the requirements of light weight and compact launching volume. A new kind of
deployable space telescope under research uses tape springs as the support structure of secondary mirror. Before launch,
the support structure of secondary mirror is folded, and deployed when on orbit. In order to achieve near diffraction
limited imaging quality, the deployment precision must reach the level consistent with the optical compensation system.
We designed an accurate measuring system based on position sensitive detectors (PSDs) to measure the deployment
precision. The measuring system includes 3 laser diodes, 3 two-dimensional PSDs around the primary mirror of the
deployable telescope, and 3 small mirrors around the secondary mirror. The equations linking the PSD readings and the
six-degree-of-freedom displacement of the secondary mirror were deduced. With these equations, the deployment errors
can be resolved.