Ground-testing method of a zero gravity wavefront for space telescopes.

Abstract

A ground-testing method and an algorithm are presented to demonstrate how to predict the image quality for an orbiting space telescope under 0g. By using a mathematical model of the ground-testing system, one has to change the gravity conditions to get a 0g wavefront. For the Hinode space telescope, one can change the gravity acceleration from +1 g to -1 g by rotating the telescope upside down, which is difficult for larger optical space telescopes. In this paper we introduce a new algorithm to get 0g results by measuring the wavefronts under different gravity acceleration sets. In this way we can predict the 0g image quality under any gravity acceleration value. With the algorithm, we propose to use oscillation in the vertical direction to mimic the change of the telescope figure under acceleration caused by gravity. Criteria of tests and estimates of errors are discussed in this paper. A test experiment is designed and performed for a scaled model of the space solar telescope (SST). The optical test system includes a 1.4-mm-thick, 101.6-mm-diameter flat mirror, and has almost the same gravity sag as the SST's 1-m-diameter primary has; thus, it can be compared to the Hinode method. As a suspending spring mass oscillation system, it provides a gravity environment varying sinusoidally around 1g. Gravity accelerations and response wavefronts are aligned in time. Test results and comparisons to Hinode's are achieved and listed. The differences between our method and Hinode's results is less than 1/20λ RMS, which is sufficient for testing a diffraction limited space optical system.