Optical Beam Control for Imaging Spacecraft with Large Apertures

Paper number

IAC-08.C1.8.5

Author

Dr. Brij Agrawal, Naval Postgraduate School, United States

Year

2008

Abstract

To meet demands for enhanced collection capability, many future imaging spacecraft will require mirror sizes of 5-10 m or greater in diameter.  These mirrors, like the primary mirror on the James Webb Space Telescope (JWST), will be segmented and light weight and require on orbit deployment.  Meeting mirror surface figure and alignment requirements to minimize optical beam wave front error is very challenging.  The mirror surface could also have structure and control interactions.  The coarse pointing of the optical beam is achieved by the spacecraft bus attitude control system.  However, fine pointing and correction for aberrations in the optical beam due to mirror surface error, optical train imperfections and jitter is performed by the optical payload control system.  One approach, as used on JWST, is to correct mirror surface error through the use of adaptive optics beam control techniques.  This paper discusses the control techniques for optical beam control of imaging spacecraft with large apertures using adaptive optics.

At the Spacecraft Research Design Center, Naval Postgraduate School, a test bed has been developed to test various adaptive optics control techniques for optical beam control of imaging spacecraft with large apertures.  An adaptive optics control system consists of three elements; a mirror and actuator component, a sensor component, and a computer control component.  The test bed consists of three control systems: two adaptive control and one jitter control.  The first adaptive control system consists of a primary mirror, reference beam and wave front sensor.  The aberration in the perfect reference beam is created by the imperfection in the mirror surface.  The wave front sensor measures this aberration and using adaptive optics techniques generates control inputs for surface actuators on the primary mirror.  The jitter control system is used to correct jitter in the optical beam generated by the spacecraft and is corrected by using a position sensing detector and fast steering mirror.  There is an additional adaptive optics system to correct aberrations in the image.

Several control techniques were evaluated, including indirect iterative feedback; direct iterative feedback, zonal; direct iterative feedback, modal from zonal; direct iterative feedback, modal from Zernike derivatives; iterative gradient feedback, variance minimization; iterative gradient feedback, slope minimization; and combined direct iterative and gradient feedback.  Based on the experimental results, the combined iterative and gradient feedback method provided the best performance for wave front correction.  Notch filtering was found to be effective in cancelling the effect of sinusoidal disturbances.

Abstract document

IAC-08.C1.8.5.pdf

Manuscript document

IAC-08.C1.8.5.pdf (🔒 authorized access only).

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