Active correction system of a deployable telescope for Earth observation

Abstract

Deployable optics can bring major cost reductions to the field of Earth Observation. One of the key challenges in the development of a deployable optical system, however, is making sure that it can meet its performance targets following its deployment. In this paper, a novel active correction system for a deployable telescope is described. The correction system co-aligns and phases the primary mirror segments and subsequently corrects remaining aberrations using a deformable mirror. A novel phasing sensor called PistonCam can bring telescope segments into phase while the telescope is staring at extended scenes. By only sampling segment boundaries, PistonCam is able to isolate piston and tip/tilt errors which allows the errors to be corrected more effectively. After phasing process has been completed, a moving scene sharpness optimization technique is used to correct the remaining aberrations with a deformable mirror, The technique does not require a constant scene, unlike existing sharpness optimization techniques. As such, the telescope does not need to track a ground scene during the correction process. The technique can also be used for continuous correction of telescope deformations. The active optics system offers robust aberration correction, is computationally inexpensive and requires limited additional optical hardware.

Review on thermal and mechanical challenges in the development of deployable space optics

“…From that, the top-down misalignment budgets were derived and presented in Ref. 97. In addition to the optical analysis, several Master of Science theses have been published by the TU Delft, [98][99][100][101][102] detailing the evolution of the mechanical design from the first iteration by Dolkens to the latest developments reported by Dolkens et al 103 The DST, as shown in Fig.…”

“…Successful operation of the system relies on three layers of increasingly strict tolerances. 97 The deployment mechanism should be accurate enough to reach a coarse alignment in the micron range. The system is then actuated to a nominal position in the order of λ∕20, with short WFE jitter disturbances kept below the order of λ∕100, with λ being the diffraction-limited wavelength of the telescope.…”

Phasing a sparse telescope

“…Figure 1 shows us the basic idea of a small deployable telescope in the Fresnel configuration [11]. It has two drawbacks: the segments' positions and shapes.…”

“…We assumed that the initial errors in the tip, tilt, and piston, were up to ±1 mm, and the working wave length was in the visible, ~0.5 µm. Now we employed a method akin to optical coherence tomography (OCT): we scanned one segment and observed the combined PSF of all segments, to see when it varied in intensity [1,11,15]. Such a variation meant that fringes ran through the image in focus.…”

“…For earthobserving and for astronomical telescopes we assumed using an unresolved star for the initial alignment stages. We have previously aligned successfully a 7-DOF system on an extended ground scene in the lab, but optimization during short fly-over time was difficult [11].…”